From 849369d6c66d3054688672f97d31fceb8e8230fb Mon Sep 17 00:00:00 2001
From: root <root@artemis.panaceas.org>
Date: Fri, 25 Dec 2015 04:40:36 +0000
Subject: initial_commit

---
 Documentation/firmware_class/README | 108 ++++++++++++++++++++++++++++++++++++
 1 file changed, 108 insertions(+)
 create mode 100644 Documentation/firmware_class/README

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diff --git a/Documentation/firmware_class/README b/Documentation/firmware_class/README
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+
+ request_firmware() hotplug interface:
+ ------------------------------------
+	Copyright (C) 2003 Manuel Estrada Sainz
+
+ Why:
+ ---
+
+ Today, the most extended way to use firmware in the Linux kernel is linking
+ it statically in a header file. Which has political and technical issues:
+
+  1) Some firmware is not legal to redistribute.
+  2) The firmware occupies memory permanently, even though it often is just
+     used once.
+  3) Some people, like the Debian crowd, don't consider some firmware free
+     enough and remove entire drivers (e.g.: keyspan).
+
+ High level behavior (mixed):
+ ============================
+
+ kernel(driver): calls request_firmware(&fw_entry, $FIRMWARE, device)
+
+ userspace:
+ 	- /sys/class/firmware/xxx/{loading,data} appear.
+	- hotplug gets called with a firmware identifier in $FIRMWARE
+	  and the usual hotplug environment.
+		- hotplug: echo 1 > /sys/class/firmware/xxx/loading
+
+ kernel: Discard any previous partial load.
+
+ userspace:
+		- hotplug: cat appropriate_firmware_image > \
+					/sys/class/firmware/xxx/data
+
+ kernel: grows a buffer in PAGE_SIZE increments to hold the image as it
+	 comes in.
+
+ userspace:
+		- hotplug: echo 0 > /sys/class/firmware/xxx/loading
+
+ kernel: request_firmware() returns and the driver has the firmware
+	 image in fw_entry->{data,size}. If something went wrong
+	 request_firmware() returns non-zero and fw_entry is set to
+	 NULL.
+
+ kernel(driver): Driver code calls release_firmware(fw_entry) releasing
+		 the firmware image and any related resource.
+
+ High level behavior (driver code):
+ ==================================
+
+	 if(request_firmware(&fw_entry, $FIRMWARE, device) == 0)
+	 	copy_fw_to_device(fw_entry->data, fw_entry->size);
+	 release(fw_entry);
+
+ Sample/simple hotplug script:
+ ============================
+
+	# Both $DEVPATH and $FIRMWARE are already provided in the environment.
+
+	HOTPLUG_FW_DIR=/usr/lib/hotplug/firmware/
+
+	echo 1 > /sys/$DEVPATH/loading
+	cat $HOTPLUG_FW_DIR/$FIRMWARE > /sysfs/$DEVPATH/data
+	echo 0 > /sys/$DEVPATH/loading
+
+ Random notes:
+ ============
+
+ - "echo -1 > /sys/class/firmware/xxx/loading" will cancel the load at
+   once and make request_firmware() return with error.
+
+ - firmware_data_read() and firmware_loading_show() are just provided
+   for testing and completeness, they are not called in normal use.
+
+ - There is also /sys/class/firmware/timeout which holds a timeout in
+   seconds for the whole load operation.
+
+ - request_firmware_nowait() is also provided for convenience in
+   user contexts to request firmware asynchronously, but can't be called
+   in atomic contexts.
+
+
+ about in-kernel persistence:
+ ---------------------------
+ Under some circumstances, as explained below, it would be interesting to keep
+ firmware images in non-swappable kernel memory or even in the kernel image
+ (probably within initramfs).
+
+ Note that this functionality has not been implemented.
+
+ - Why OPTIONAL in-kernel persistence may be a good idea sometimes:
+ 
+	- If the device that needs the firmware is needed to access the
+	  filesystem. When upon some error the device has to be reset and the
+	  firmware reloaded, it won't be possible to get it from userspace.
+	  e.g.:
+		- A diskless client with a network card that needs firmware.
+		- The filesystem is stored in a disk behind an scsi device
+		  that needs firmware.
+	- Replacing buggy DSDT/SSDT ACPI tables on boot.
+	  Note: this would require the persistent objects to be included
+	  within the kernel image, probably within initramfs.
+	  
+   And the same device can be needed to access the filesystem or not depending
+   on the setup, so I think that the choice on what firmware to make
+   persistent should be left to userspace.
+
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