ar71xx: fix nondeterministic hangs during bootconsole/console handover

Reconfiguring the UART when the FIFO is not empty may cause the boot to
hang. This hang is extremely suspectible to timing differences; recompiling
the kernel with the same configuration, but different UTS_VERSION
timestamps can yield images that hang more or less often. Sometimes images
are produced that hang reproducibly.

This patch should also make it into one of the next linux-stable releases,
but it's better to get this fixed as soon as possible.

Fixes #21773, #21857.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

Backport of r49156

git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@49202 3c298f89-4303-0410-b956-a3cf2f4a3e73

4 files changed, 57 insertions, 8 deletions

diff --git a/target/linux/ar71xx/patches-3.18/003-MIPS-ath79-make-bootconsole-wait-for-both-THRE-and-T.patch b/target/linux/ar71xx/patches-3.18/003-MIPS-ath79-make-bootconsole-wait-for-both-THRE-and-T.patch
new file mode 100644
index 0000000000..2eba263732
--- /dev/null
+++ b/target/linux/ar71xx/patches-3.18/003-MIPS-ath79-make-bootconsole-wait-for-both-THRE-and-T.patch
@@ -0,0 +1,49 @@
+From f1ba020af5076172c9d29006a747ccf40027fedc Mon Sep 17 00:00:00 2001
+Message-Id: <f1ba020af5076172c9d29006a747ccf40027fedc.1458840219.git.mschiffer@universe-factory.net>
+From: Matthias Schiffer <mschiffer@universe-factory.net>
+Date: Thu, 24 Mar 2016 15:34:05 +0100
+Subject: [PATCH] MIPS: ath79: make bootconsole wait for both THRE and TEMT
+
+This makes the ath79 bootconsole behave the same way as the generic 8250
+bootconsole.
+
+Also waiting for TEMT (transmit buffer is empty) instead of just THRE
+(transmit buffer is not full) ensures that all characters have been
+transmitted before the real serial driver starts reconfiguring the serial
+controller (which would sometimes result in garbage being transmitted.)
+This change does not cause a visible performance loss.
+
+In addition, this seems to fix a hang observed in certain configurations on
+many AR7xxx/AR9xxx SoCs during autoconfig of the real serial driver.
+
+A more complete follow-up patch will disable 8250 autoconfig for ath79
+altogether (the serial controller is detected as a 16550A, which is not
+fully compatible with the ath79 serial, and the autoconfig may lead to
+undefined behavior on ath79.)
+
+Cc: <stable@vger.kernel.org>
+Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
+---
+ arch/mips/ath79/early_printk.c | 6 ++++--
+ 1 file changed, 4 insertions(+), 2 deletions(-)
+
+--- a/arch/mips/ath79/early_printk.c
++++ b/arch/mips/ath79/early_printk.c
+@@ -31,13 +31,15 @@ static inline void prom_putchar_wait(voi
+ 	} while (1);
+ }
+ 
++#define BOTH_EMPTY (UART_LSR_TEMT | UART_LSR_THRE)
++
+ static void prom_putchar_ar71xx(unsigned char ch)
+ {
+ 	void __iomem *base = (void __iomem *)(KSEG1ADDR(AR71XX_UART_BASE));
+ 
+-	prom_putchar_wait(base + UART_LSR * 4, UART_LSR_THRE, UART_LSR_THRE);
++	prom_putchar_wait(base + UART_LSR * 4, BOTH_EMPTY, BOTH_EMPTY);
+ 	__raw_writel(ch, base + UART_TX * 4);
+-	prom_putchar_wait(base + UART_LSR * 4, UART_LSR_THRE, UART_LSR_THRE);
++	prom_putchar_wait(base + UART_LSR * 4, BOTH_EMPTY, BOTH_EMPTY);
+ }
+ 
+ static void prom_putchar_ar933x(unsigned char ch)
diff --git a/target/linux/ar71xx/patches-3.18/521-MIPS-ath79-enable-UART-for-early_serial.patch b/target/linux/ar71xx/patches-3.18/521-MIPS-ath79-enable-UART-for-early_serial.patch
index 3d6ddfe18c..e546ff26fe 100644
--- a/target/linux/ar71xx/patches-3.18/521-MIPS-ath79-enable-UART-for-early_serial.patch
+++ b/target/linux/ar71xx/patches-3.18/521-MIPS-ath79-enable-UART-for-early_serial.patch
@@ -1,6 +1,6 @@
 --- a/arch/mips/ath79/early_printk.c
 +++ b/arch/mips/ath79/early_printk.c
-@@ -56,6 +56,46 @@ static void prom_putchar_dummy(unsigned
+@@ -58,6 +58,46 @@ static void prom_putchar_dummy(unsigned
  	/* nothing to do */
  }
  
@@ -47,7 +47,7 @@
  static void prom_putchar_init(void)
  {
  	void __iomem *base;
-@@ -86,8 +126,10 @@ static void prom_putchar_init(void)
+@@ -88,8 +128,10 @@ static void prom_putchar_init(void)
  
  	default:
  		_prom_putchar = prom_putchar_dummy;
diff --git a/target/linux/ar71xx/patches-3.18/707-MIPS-ath79-add-support-for-QCA953x-SoC.patch b/target/linux/ar71xx/patches-3.18/707-MIPS-ath79-add-support-for-QCA953x-SoC.patch
index 88d376cd55..e432c6f3b6 100644
--- a/target/linux/ar71xx/patches-3.18/707-MIPS-ath79-add-support-for-QCA953x-SoC.patch
+++ b/target/linux/ar71xx/patches-3.18/707-MIPS-ath79-add-support-for-QCA953x-SoC.patch
@@ -22,7 +22,7 @@ meaning of the bits CPUCLK_FROM_CPUPLL and DDRCLK_FROM_DDRPLL is reversed.
 
 --- a/arch/mips/ath79/Kconfig
 +++ b/arch/mips/ath79/Kconfig
-@@ -1257,6 +1257,10 @@ config SOC_AR934X
+@@ -1266,6 +1266,10 @@ config SOC_AR934X
  	select PCI_AR724X if PCI
  	def_bool n
  
@@ -33,7 +33,7 @@ meaning of the bits CPUCLK_FROM_CPUPLL and DDRCLK_FROM_DDRPLL is reversed.
  config SOC_QCA955X
  	select HW_HAS_PCI
  	select PCI_AR724X if PCI
-@@ -1299,7 +1303,7 @@ config ATH79_DEV_USB
+@@ -1308,7 +1312,7 @@ config ATH79_DEV_USB
  	def_bool n
  
  config ATH79_DEV_WMAC
@@ -275,7 +275,7 @@ meaning of the bits CPUCLK_FROM_CPUPLL and DDRCLK_FROM_DDRPLL is reversed.
  	else
 --- a/arch/mips/ath79/early_printk.c
 +++ b/arch/mips/ath79/early_printk.c
-@@ -114,6 +114,8 @@ static void prom_putchar_init(void)
+@@ -116,6 +116,8 @@ static void prom_putchar_init(void)
  	case REV_ID_MAJOR_AR9341:
  	case REV_ID_MAJOR_AR9342:
  	case REV_ID_MAJOR_AR9344:
diff --git a/target/linux/ar71xx/patches-3.18/735-MIPS-ath79-add-support-for-QCA956x-SoC.patch b/target/linux/ar71xx/patches-3.18/735-MIPS-ath79-add-support-for-QCA956x-SoC.patch
index 6d836adb52..b5c88e9b99 100644
--- a/target/linux/ar71xx/patches-3.18/735-MIPS-ath79-add-support-for-QCA956x-SoC.patch
+++ b/target/linux/ar71xx/patches-3.18/735-MIPS-ath79-add-support-for-QCA956x-SoC.patch
@@ -211,7 +211,7 @@
  
 --- a/arch/mips/ath79/early_printk.c
 +++ b/arch/mips/ath79/early_printk.c
-@@ -118,6 +118,8 @@ static void prom_putchar_init(void)
+@@ -120,6 +120,8 @@ static void prom_putchar_init(void)
  	case REV_ID_MAJOR_QCA9533_V2:
  	case REV_ID_MAJOR_QCA9556:
  	case REV_ID_MAJOR_QCA9558:
@@ -378,7 +378,7 @@
  }
 --- a/arch/mips/ath79/Kconfig
 +++ b/arch/mips/ath79/Kconfig
-@@ -1277,6 +1277,12 @@ config SOC_QCA955X
+@@ -1286,6 +1286,12 @@ config SOC_QCA955X
  	select PCI_AR724X if PCI
  	def_bool n
  
@@ -391,7 +391,7 @@
  config ATH79_DEV_M25P80
  	select ATH79_DEV_SPI
  	def_bool n
-@@ -1314,7 +1320,7 @@ config ATH79_DEV_USB
+@@ -1323,7 +1329,7 @@ config ATH79_DEV_USB
  	def_bool n
  
  config ATH79_DEV_WMAC