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diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt new file mode 100644 index 00000000..fe232690 --- /dev/null +++ b/Documentation/DMA-API.txt @@ -0,0 +1,673 @@ + Dynamic DMA mapping using the generic device + ============================================ + + James E.J. Bottomley <James.Bottomley@HansenPartnership.com> + +This document describes the DMA API. For a more gentle introduction +of the API (and actual examples) see +Documentation/DMA-API-HOWTO.txt. + +This API is split into two pieces. Part I describes the API. Part II +describes the extensions to the API for supporting non-consistent +memory machines. Unless you know that your driver absolutely has to +support non-consistent platforms (this is usually only legacy +platforms) you should only use the API described in part I. + +Part I - dma_ API +------------------------------------- + +To get the dma_ API, you must #include <linux/dma-mapping.h> + + +Part Ia - Using large dma-coherent buffers +------------------------------------------ + +void * +dma_alloc_coherent(struct device *dev, size_t size, + dma_addr_t *dma_handle, gfp_t flag) + +Consistent memory is memory for which a write by either the device or +the processor can immediately be read by the processor or device +without having to worry about caching effects. (You may however need +to make sure to flush the processor's write buffers before telling +devices to read that memory.) + +This routine allocates a region of <size> bytes of consistent memory. +It also returns a <dma_handle> which may be cast to an unsigned +integer the same width as the bus and used as the physical address +base of the region. + +Returns: a pointer to the allocated region (in the processor's virtual +address space) or NULL if the allocation failed. + +Note: consistent memory can be expensive on some platforms, and the +minimum allocation length may be as big as a page, so you should +consolidate your requests for consistent memory as much as possible. +The simplest way to do that is to use the dma_pool calls (see below). + +The flag parameter (dma_alloc_coherent only) allows the caller to +specify the GFP_ flags (see kmalloc) for the allocation (the +implementation may choose to ignore flags that affect the location of +the returned memory, like GFP_DMA). + +void +dma_free_coherent(struct device *dev, size_t size, void *cpu_addr, + dma_addr_t dma_handle) + +Free the region of consistent memory you previously allocated. dev, +size and dma_handle must all be the same as those passed into the +consistent allocate. cpu_addr must be the virtual address returned by +the consistent allocate. + +Note that unlike their sibling allocation calls, these routines +may only be called with IRQs enabled. + + +Part Ib - Using small dma-coherent buffers +------------------------------------------ + +To get this part of the dma_ API, you must #include <linux/dmapool.h> + +Many drivers need lots of small dma-coherent memory regions for DMA +descriptors or I/O buffers. Rather than allocating in units of a page +or more using dma_alloc_coherent(), you can use DMA pools. These work +much like a struct kmem_cache, except that they use the dma-coherent allocator, +not __get_free_pages(). Also, they understand common hardware constraints +for alignment, like queue heads needing to be aligned on N-byte boundaries. + + + struct dma_pool * + dma_pool_create(const char *name, struct device *dev, + size_t size, size_t align, size_t alloc); + +The pool create() routines initialize a pool of dma-coherent buffers +for use with a given device. It must be called in a context which +can sleep. + +The "name" is for diagnostics (like a struct kmem_cache name); dev and size +are like what you'd pass to dma_alloc_coherent(). The device's hardware +alignment requirement for this type of data is "align" (which is expressed +in bytes, and must be a power of two). If your device has no boundary +crossing restrictions, pass 0 for alloc; passing 4096 says memory allocated +from this pool must not cross 4KByte boundaries. + + + void *dma_pool_alloc(struct dma_pool *pool, gfp_t gfp_flags, + dma_addr_t *dma_handle); + +This allocates memory from the pool; the returned memory will meet the size +and alignment requirements specified at creation time. Pass GFP_ATOMIC to +prevent blocking, or if it's permitted (not in_interrupt, not holding SMP locks), +pass GFP_KERNEL to allow blocking. Like dma_alloc_coherent(), this returns +two values: an address usable by the cpu, and the dma address usable by the +pool's device. + + + void dma_pool_free(struct dma_pool *pool, void *vaddr, + dma_addr_t addr); + +This puts memory back into the pool. The pool is what was passed to +the pool allocation routine; the cpu (vaddr) and dma addresses are what +were returned when that routine allocated the memory being freed. + + + void dma_pool_destroy(struct dma_pool *pool); + +The pool destroy() routines free the resources of the pool. They must be +called in a context which can sleep. Make sure you've freed all allocated +memory back to the pool before you destroy it. + + +Part Ic - DMA addressing limitations +------------------------------------ + +int +dma_supported(struct device *dev, u64 mask) + +Checks to see if the device can support DMA to the memory described by +mask. + +Returns: 1 if it can and 0 if it can't. + +Notes: This routine merely tests to see if the mask is possible. It +won't change the current mask settings. It is more intended as an +internal API for use by the platform than an external API for use by +driver writers. + +int +dma_set_mask(struct device *dev, u64 mask) + +Checks to see if the mask is possible and updates the device +parameters if it is. + +Returns: 0 if successful and a negative error if not. + +int +dma_set_coherent_mask(struct device *dev, u64 mask) + +Checks to see if the mask is possible and updates the device +parameters if it is. + +Returns: 0 if successful and a negative error if not. + +u64 +dma_get_required_mask(struct device *dev) + +This API returns the mask that the platform requires to +operate efficiently. Usually this means the returned mask +is the minimum required to cover all of memory. Examining the +required mask gives drivers with variable descriptor sizes the +opportunity to use smaller descriptors as necessary. + +Requesting the required mask does not alter the current mask. If you +wish to take advantage of it, you should issue a dma_set_mask() +call to set the mask to the value returned. + + +Part Id - Streaming DMA mappings +-------------------------------- + +dma_addr_t +dma_map_single(struct device *dev, void *cpu_addr, size_t size, + enum dma_data_direction direction) + +Maps a piece of processor virtual memory so it can be accessed by the +device and returns the physical handle of the memory. + +The direction for both api's may be converted freely by casting. +However the dma_ API uses a strongly typed enumerator for its +direction: + +DMA_NONE no direction (used for debugging) +DMA_TO_DEVICE data is going from the memory to the device +DMA_FROM_DEVICE data is coming from the device to the memory +DMA_BIDIRECTIONAL direction isn't known + +Notes: Not all memory regions in a machine can be mapped by this +API. Further, regions that appear to be physically contiguous in +kernel virtual space may not be contiguous as physical memory. Since +this API does not provide any scatter/gather capability, it will fail +if the user tries to map a non-physically contiguous piece of memory. +For this reason, it is recommended that memory mapped by this API be +obtained only from sources which guarantee it to be physically contiguous +(like kmalloc). + +Further, the physical address of the memory must be within the +dma_mask of the device (the dma_mask represents a bit mask of the +addressable region for the device. I.e., if the physical address of +the memory anded with the dma_mask is still equal to the physical +address, then the device can perform DMA to the memory). In order to +ensure that the memory allocated by kmalloc is within the dma_mask, +the driver may specify various platform-dependent flags to restrict +the physical memory range of the allocation (e.g. on x86, GFP_DMA +guarantees to be within the first 16Mb of available physical memory, +as required by ISA devices). + +Note also that the above constraints on physical contiguity and +dma_mask may not apply if the platform has an IOMMU (a device which +supplies a physical to virtual mapping between the I/O memory bus and +the device). However, to be portable, device driver writers may *not* +assume that such an IOMMU exists. + +Warnings: Memory coherency operates at a granularity called the cache +line width. In order for memory mapped by this API to operate +correctly, the mapped region must begin exactly on a cache line +boundary and end exactly on one (to prevent two separately mapped +regions from sharing a single cache line). Since the cache line size +may not be known at compile time, the API will not enforce this +requirement. Therefore, it is recommended that driver writers who +don't take special care to determine the cache line size at run time +only map virtual regions that begin and end on page boundaries (which +are guaranteed also to be cache line boundaries). + +DMA_TO_DEVICE synchronisation must be done after the last modification +of the memory region by the software and before it is handed off to +the driver. Once this primitive is used, memory covered by this +primitive should be treated as read-only by the device. If the device +may write to it at any point, it should be DMA_BIDIRECTIONAL (see +below). + +DMA_FROM_DEVICE synchronisation must be done before the driver +accesses data that may be changed by the device. This memory should +be treated as read-only by the driver. If the driver needs to write +to it at any point, it should be DMA_BIDIRECTIONAL (see below). + +DMA_BIDIRECTIONAL requires special handling: it means that the driver +isn't sure if the memory was modified before being handed off to the +device and also isn't sure if the device will also modify it. Thus, +you must always sync bidirectional memory twice: once before the +memory is handed off to the device (to make sure all memory changes +are flushed from the processor) and once before the data may be +accessed after being used by the device (to make sure any processor +cache lines are updated with data that the device may have changed). + +void +dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size, + enum dma_data_direction direction) + +Unmaps the region previously mapped. All the parameters passed in +must be identical to those passed in (and returned) by the mapping +API. + +dma_addr_t +dma_map_page(struct device *dev, struct page *page, + unsigned long offset, size_t size, + enum dma_data_direction direction) +void +dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size, + enum dma_data_direction direction) + +API for mapping and unmapping for pages. All the notes and warnings +for the other mapping APIs apply here. Also, although the <offset> +and <size> parameters are provided to do partial page mapping, it is +recommended that you never use these unless you really know what the +cache width is. + +int +dma_mapping_error(struct device *dev, dma_addr_t dma_addr) + +In some circumstances dma_map_single and dma_map_page will fail to create +a mapping. A driver can check for these errors by testing the returned +dma address with dma_mapping_error(). A non-zero return value means the mapping +could not be created and the driver should take appropriate action (e.g. +reduce current DMA mapping usage or delay and try again later). + + int + dma_map_sg(struct device *dev, struct scatterlist *sg, + int nents, enum dma_data_direction direction) + +Returns: the number of physical segments mapped (this may be shorter +than <nents> passed in if some elements of the scatter/gather list are +physically or virtually adjacent and an IOMMU maps them with a single +entry). + +Please note that the sg cannot be mapped again if it has been mapped once. +The mapping process is allowed to destroy information in the sg. + +As with the other mapping interfaces, dma_map_sg can fail. When it +does, 0 is returned and a driver must take appropriate action. It is +critical that the driver do something, in the case of a block driver +aborting the request or even oopsing is better than doing nothing and +corrupting the filesystem. + +With scatterlists, you use the resulting mapping like this: + + int i, count = dma_map_sg(dev, sglist, nents, direction); + struct scatterlist *sg; + + for_each_sg(sglist, sg, count, i) { + hw_address[i] = sg_dma_address(sg); + hw_len[i] = sg_dma_len(sg); + } + +where nents is the number of entries in the sglist. + +The implementation is free to merge several consecutive sglist entries +into one (e.g. with an IOMMU, or if several pages just happen to be +physically contiguous) and returns the actual number of sg entries it +mapped them to. On failure 0, is returned. + +Then you should loop count times (note: this can be less than nents times) +and use sg_dma_address() and sg_dma_len() macros where you previously +accessed sg->address and sg->length as shown above. + + void + dma_unmap_sg(struct device *dev, struct scatterlist *sg, + int nhwentries, enum dma_data_direction direction) + +Unmap the previously mapped scatter/gather list. All the parameters +must be the same as those and passed in to the scatter/gather mapping +API. + +Note: <nents> must be the number you passed in, *not* the number of +physical entries returned. + +void +dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle, size_t size, + enum dma_data_direction direction) +void +dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle, size_t size, + enum dma_data_direction direction) +void +dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg, int nelems, + enum dma_data_direction direction) +void +dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg, int nelems, + enum dma_data_direction direction) + +Synchronise a single contiguous or scatter/gather mapping for the cpu +and device. With the sync_sg API, all the parameters must be the same +as those passed into the single mapping API. With the sync_single API, +you can use dma_handle and size parameters that aren't identical to +those passed into the single mapping API to do a partial sync. + +Notes: You must do this: + +- Before reading values that have been written by DMA from the device + (use the DMA_FROM_DEVICE direction) +- After writing values that will be written to the device using DMA + (use the DMA_TO_DEVICE) direction +- before *and* after handing memory to the device if the memory is + DMA_BIDIRECTIONAL + +See also dma_map_single(). + +dma_addr_t +dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size, + enum dma_data_direction dir, + struct dma_attrs *attrs) + +void +dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr, + size_t size, enum dma_data_direction dir, + struct dma_attrs *attrs) + +int +dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl, + int nents, enum dma_data_direction dir, + struct dma_attrs *attrs) + +void +dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl, + int nents, enum dma_data_direction dir, + struct dma_attrs *attrs) + +The four functions above are just like the counterpart functions +without the _attrs suffixes, except that they pass an optional +struct dma_attrs*. + +struct dma_attrs encapsulates a set of "dma attributes". For the +definition of struct dma_attrs see linux/dma-attrs.h. + +The interpretation of dma attributes is architecture-specific, and +each attribute should be documented in Documentation/DMA-attributes.txt. + +If struct dma_attrs* is NULL, the semantics of each of these +functions is identical to those of the corresponding function +without the _attrs suffix. As a result dma_map_single_attrs() +can generally replace dma_map_single(), etc. + +As an example of the use of the *_attrs functions, here's how +you could pass an attribute DMA_ATTR_FOO when mapping memory +for DMA: + +#include <linux/dma-attrs.h> +/* DMA_ATTR_FOO should be defined in linux/dma-attrs.h and + * documented in Documentation/DMA-attributes.txt */ +... + + DEFINE_DMA_ATTRS(attrs); + dma_set_attr(DMA_ATTR_FOO, &attrs); + .... + n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, &attr); + .... + +Architectures that care about DMA_ATTR_FOO would check for its +presence in their implementations of the mapping and unmapping +routines, e.g.: + +void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr, + size_t size, enum dma_data_direction dir, + struct dma_attrs *attrs) +{ + .... + int foo = dma_get_attr(DMA_ATTR_FOO, attrs); + .... + if (foo) + /* twizzle the frobnozzle */ + .... + + +Part II - Advanced dma_ usage +----------------------------- + +Warning: These pieces of the DMA API should not be used in the +majority of cases, since they cater for unlikely corner cases that +don't belong in usual drivers. + +If you don't understand how cache line coherency works between a +processor and an I/O device, you should not be using this part of the +API at all. + +void * +dma_alloc_noncoherent(struct device *dev, size_t size, + dma_addr_t *dma_handle, gfp_t flag) + +Identical to dma_alloc_coherent() except that the platform will +choose to return either consistent or non-consistent memory as it sees +fit. By using this API, you are guaranteeing to the platform that you +have all the correct and necessary sync points for this memory in the +driver should it choose to return non-consistent memory. + +Note: where the platform can return consistent memory, it will +guarantee that the sync points become nops. + +Warning: Handling non-consistent memory is a real pain. You should +only ever use this API if you positively know your driver will be +required to work on one of the rare (usually non-PCI) architectures +that simply cannot make consistent memory. + +void +dma_free_noncoherent(struct device *dev, size_t size, void *cpu_addr, + dma_addr_t dma_handle) + +Free memory allocated by the nonconsistent API. All parameters must +be identical to those passed in (and returned by +dma_alloc_noncoherent()). + +int +dma_get_cache_alignment(void) + +Returns the processor cache alignment. This is the absolute minimum +alignment *and* width that you must observe when either mapping +memory or doing partial flushes. + +Notes: This API may return a number *larger* than the actual cache +line, but it will guarantee that one or more cache lines fit exactly +into the width returned by this call. It will also always be a power +of two for easy alignment. + +void +dma_cache_sync(struct device *dev, void *vaddr, size_t size, + enum dma_data_direction direction) + +Do a partial sync of memory that was allocated by +dma_alloc_noncoherent(), starting at virtual address vaddr and +continuing on for size. Again, you *must* observe the cache line +boundaries when doing this. + +int +dma_declare_coherent_memory(struct device *dev, dma_addr_t bus_addr, + dma_addr_t device_addr, size_t size, int + flags) + +Declare region of memory to be handed out by dma_alloc_coherent when +it's asked for coherent memory for this device. + +bus_addr is the physical address to which the memory is currently +assigned in the bus responding region (this will be used by the +platform to perform the mapping). + +device_addr is the physical address the device needs to be programmed +with actually to address this memory (this will be handed out as the +dma_addr_t in dma_alloc_coherent()). + +size is the size of the area (must be multiples of PAGE_SIZE). + +flags can be or'd together and are: + +DMA_MEMORY_MAP - request that the memory returned from +dma_alloc_coherent() be directly writable. + +DMA_MEMORY_IO - request that the memory returned from +dma_alloc_coherent() be addressable using read/write/memcpy_toio etc. + +One or both of these flags must be present. + +DMA_MEMORY_INCLUDES_CHILDREN - make the declared memory be allocated by +dma_alloc_coherent of any child devices of this one (for memory residing +on a bridge). + +DMA_MEMORY_EXCLUSIVE - only allocate memory from the declared regions. +Do not allow dma_alloc_coherent() to fall back to system memory when +it's out of memory in the declared region. + +The return value will be either DMA_MEMORY_MAP or DMA_MEMORY_IO and +must correspond to a passed in flag (i.e. no returning DMA_MEMORY_IO +if only DMA_MEMORY_MAP were passed in) for success or zero for +failure. + +Note, for DMA_MEMORY_IO returns, all subsequent memory returned by +dma_alloc_coherent() may no longer be accessed directly, but instead +must be accessed using the correct bus functions. If your driver +isn't prepared to handle this contingency, it should not specify +DMA_MEMORY_IO in the input flags. + +As a simplification for the platforms, only *one* such region of +memory may be declared per device. + +For reasons of efficiency, most platforms choose to track the declared +region only at the granularity of a page. For smaller allocations, +you should use the dma_pool() API. + +void +dma_release_declared_memory(struct device *dev) + +Remove the memory region previously declared from the system. This +API performs *no* in-use checking for this region and will return +unconditionally having removed all the required structures. It is the +driver's job to ensure that no parts of this memory region are +currently in use. + +void * +dma_mark_declared_memory_occupied(struct device *dev, + dma_addr_t device_addr, size_t size) + +This is used to occupy specific regions of the declared space +(dma_alloc_coherent() will hand out the first free region it finds). + +device_addr is the *device* address of the region requested. + +size is the size (and should be a page-sized multiple). + +The return value will be either a pointer to the processor virtual +address of the memory, or an error (via PTR_ERR()) if any part of the +region is occupied. + +Part III - Debug drivers use of the DMA-API +------------------------------------------- + +The DMA-API as described above as some constraints. DMA addresses must be +released with the corresponding function with the same size for example. With +the advent of hardware IOMMUs it becomes more and more important that drivers +do not violate those constraints. In the worst case such a violation can +result in data corruption up to destroyed filesystems. + +To debug drivers and find bugs in the usage of the DMA-API checking code can +be compiled into the kernel which will tell the developer about those +violations. If your architecture supports it you can select the "Enable +debugging of DMA-API usage" option in your kernel configuration. Enabling this +option has a performance impact. Do not enable it in production kernels. + +If you boot the resulting kernel will contain code which does some bookkeeping +about what DMA memory was allocated for which device. If this code detects an +error it prints a warning message with some details into your kernel log. An +example warning message may look like this: + +------------[ cut here ]------------ +WARNING: at /data2/repos/linux-2.6-iommu/lib/dma-debug.c:448 + check_unmap+0x203/0x490() +Hardware name: +forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong + function [device address=0x00000000640444be] [size=66 bytes] [mapped as +single] [unmapped as page] +Modules linked in: nfsd exportfs bridge stp llc r8169 +Pid: 0, comm: swapper Tainted: G W 2.6.28-dmatest-09289-g8bb99c0 #1 +Call Trace: + <IRQ> [<ffffffff80240b22>] warn_slowpath+0xf2/0x130 + [<ffffffff80647b70>] _spin_unlock+0x10/0x30 + [<ffffffff80537e75>] usb_hcd_link_urb_to_ep+0x75/0xc0 + [<ffffffff80647c22>] _spin_unlock_irqrestore+0x12/0x40 + [<ffffffff8055347f>] ohci_urb_enqueue+0x19f/0x7c0 + [<ffffffff80252f96>] queue_work+0x56/0x60 + [<ffffffff80237e10>] enqueue_task_fair+0x20/0x50 + [<ffffffff80539279>] usb_hcd_submit_urb+0x379/0xbc0 + [<ffffffff803b78c3>] cpumask_next_and+0x23/0x40 + [<ffffffff80235177>] find_busiest_group+0x207/0x8a0 + [<ffffffff8064784f>] _spin_lock_irqsave+0x1f/0x50 + [<ffffffff803c7ea3>] check_unmap+0x203/0x490 + [<ffffffff803c8259>] debug_dma_unmap_page+0x49/0x50 + [<ffffffff80485f26>] nv_tx_done_optimized+0xc6/0x2c0 + [<ffffffff80486c13>] nv_nic_irq_optimized+0x73/0x2b0 + [<ffffffff8026df84>] handle_IRQ_event+0x34/0x70 + [<ffffffff8026ffe9>] handle_edge_irq+0xc9/0x150 + [<ffffffff8020e3ab>] do_IRQ+0xcb/0x1c0 + [<ffffffff8020c093>] ret_from_intr+0x0/0xa + <EOI> <4>---[ end trace f6435a98e2a38c0e ]--- + +The driver developer can find the driver and the device including a stacktrace +of the DMA-API call which caused this warning. + +Per default only the first error will result in a warning message. All other +errors will only silently counted. This limitation exist to prevent the code +from flooding your kernel log. To support debugging a device driver this can +be disabled via debugfs. See the debugfs interface documentation below for +details. + +The debugfs directory for the DMA-API debugging code is called dma-api/. In +this directory the following files can currently be found: + + dma-api/all_errors This file contains a numeric value. If this + value is not equal to zero the debugging code + will print a warning for every error it finds + into the kernel log. Be careful with this + option, as it can easily flood your logs. + + dma-api/disabled This read-only file contains the character 'Y' + if the debugging code is disabled. This can + happen when it runs out of memory or if it was + disabled at boot time + + dma-api/error_count This file is read-only and shows the total + numbers of errors found. + + dma-api/num_errors The number in this file shows how many + warnings will be printed to the kernel log + before it stops. This number is initialized to + one at system boot and be set by writing into + this file + + dma-api/min_free_entries + This read-only file can be read to get the + minimum number of free dma_debug_entries the + allocator has ever seen. If this value goes + down to zero the code will disable itself + because it is not longer reliable. + + dma-api/num_free_entries + The current number of free dma_debug_entries + in the allocator. + + dma-api/driver-filter + You can write a name of a driver into this file + to limit the debug output to requests from that + particular driver. Write an empty string to + that file to disable the filter and see + all errors again. + +If you have this code compiled into your kernel it will be enabled by default. +If you want to boot without the bookkeeping anyway you can provide +'dma_debug=off' as a boot parameter. This will disable DMA-API debugging. +Notice that you can not enable it again at runtime. You have to reboot to do +so. + +If you want to see debug messages only for a special device driver you can +specify the dma_debug_driver=<drivername> parameter. This will enable the +driver filter at boot time. The debug code will only print errors for that +driver afterwards. This filter can be disabled or changed later using debugfs. + +When the code disables itself at runtime this is most likely because it ran +out of dma_debug_entries. These entries are preallocated at boot. The number +of preallocated entries is defined per architecture. If it is too low for you +boot with 'dma_debug_entries=<your_desired_number>' to overwrite the +architectural default. |