[PATCH] MIPS: Introduce cmdline argument writecombine=

[Tiezhu Yang <yangtiezhu@loongson.cn>]

Loongson processors have a writecombine issue that maybe failed to
write back framebuffer used with ATI Radeon or AMD GPU at times,
after commit 8a08e50cee66 ("drm: Permit video-buffers writecombine
mapping for MIPS"), there exists some errors such as blurred screen
and lockup, and so on.

With this patch, disable writecombine by default for Loongson64 to
work well with ATI Radeon or AMD GPU, and it has no influence on the
other platforms due to writecombine is enabled by default.

Additionally, if it is necessary, writecombine=on can be set manually
in the cmdline to enhance the performance for Loongson LS7A integrated
graphics in the future.

[   60.958721] radeon 0000:03:00.0: ring 0 stalled for more than 10079msec
[   60.965315] radeon 0000:03:00.0: GPU lockup (current fence id 0x0000000000000112 last fence id 0x000000000000011d on ring 0)
[   60.976525] radeon 0000:03:00.0: ring 3 stalled for more than 10086msec
[   60.983156] radeon 0000:03:00.0: GPU lockup (current fence id 0x0000000000000374 last fence id 0x00000000000003a8 on ring 3)

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
---
 arch/mips/include/asm/pgtable.h |  4 ++++
 arch/mips/kernel/cpu-probe.c    | 19 +++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/arch/mips/include/asm/pgtable.h b/arch/mips/include/asm/pgtable.h
index dd7a0f5..34869f7 100644
--- a/arch/mips/include/asm/pgtable.h
+++ b/arch/mips/include/asm/pgtable.h
@@ -473,6 +473,10 @@ static inline pgprot_t pgprot_noncached(pgprot_t _prot)
 static inline pgprot_t pgprot_writecombine(pgprot_t _prot)
 {
 	unsigned long prot = pgprot_val(_prot);
+	extern bool mips_writecombine;
+
+	if (!mips_writecombine)
+		return pgprot_noncached(_prot);
 
 	/* cpu_data[0].writecombine is already shifted by _CACHE_SHIFT */
 	prot = (prot & ~_CACHE_MASK) | cpu_data[0].writecombine;
diff --git a/arch/mips/kernel/cpu-probe.c b/arch/mips/kernel/cpu-probe.c
index e2955f1..98777ca 100644
--- a/arch/mips/kernel/cpu-probe.c
+++ b/arch/mips/kernel/cpu-probe.c
@@ -459,6 +459,25 @@ static int __init ftlb_disable(char *s)
 
 __setup("noftlb", ftlb_disable);
 
+#ifdef CONFIG_MACH_LOONGSON64
+bool mips_writecombine; /* initialise to false by default */
+#else
+bool mips_writecombine = true;
+#endif
+EXPORT_SYMBOL(mips_writecombine);
+
+static int __init writecombine_setup(char *str)
+{
+	if (strcmp(str, "on") == 0)
+		mips_writecombine = true;
+	else if (strcmp(str, "off") == 0)
+		mips_writecombine = false;
+
+	return 1;
+}
+
+__setup("writecombine=", writecombine_setup);
+
 /*
  * Check if the CPU has per tc perf counters
  */
-- 
2.1.0