Am 22.12.21 um 15:11 schrieb Alex Deucher:
On Wed, Dec 22, 2021 at 3:18 AM Deng, Emily <Emily.Deng@xxxxxxx> wrote:
[AMD Official Use Only]
Currently, only ampere found this issue, but it is hard to detect ampere board, especially on arm passthrough environment.
Isn't this already handled in drm_arch_can_wc_memory()?
Could be, that function controls if we are trying to setup USWC in the
first place.
Christian
Alex
Best wishes
Emily Deng
-----Original Message-----
From: amd-gfx <amd-gfx-bounces@xxxxxxxxxxxxxxxxxxxxx> On Behalf Of
Christian König
Sent: Wednesday, December 22, 2021 4:11 PM
To: Zhao, Victor <Victor.Zhao@xxxxxxx>; amd-gfx@xxxxxxxxxxxxxxxxxxxxx
Subject: Re: [PATCH] drm/ttm: add workaround for some arm hardware issue
Am 22.12.21 um 06:51 schrieb Victor Zhao:
Some Arm based platform has hardware issue which may generate
incorrect addresses when receiving writes from the CPU with a
discontiguous set of byte enables. This affects the writes with write
combine property.
Can you point out which arm platforms are that exactly?
Workaround by change PROT_NORMAL_NC to PROT_DEVICE_nGnRE on arm.
As this is an issue with some specific arm based cpu, adding a ttm
parameter to control.
Something as fundamental as this should not be made controllable by an
module parameter.
Write combining is very important for good performance and so we should
only disable it on boards where we know that this won't work correctly.
Regards,
Christian.
Signed-off-by: Victor Zhao <Victor.Zhao@xxxxxxx>
---
drivers/gpu/drm/ttm/ttm_module.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/ttm/ttm_module.c
b/drivers/gpu/drm/ttm/ttm_module.c
index e87f40674a4d..b27473cbbd52 100644
--- a/drivers/gpu/drm/ttm/ttm_module.c
+++ b/drivers/gpu/drm/ttm/ttm_module.c
@@ -41,6 +41,12 @@
#include "ttm_module.h"
+static int enable_use_wc = 1;
+
+MODULE_PARM_DESC(enable_use_wc,
+ "control write combine usage on arm platform due to hardware issue
+with write combine found on some specific arm cpu (1 =
+enable(default), 0 = disable)"); module_param(enable_use_wc, int,
+0644);
+
/**
* ttm_prot_from_caching - Modify the page protection according to the
* ttm cacing mode
@@ -63,7 +69,7 @@ pgprot_t ttm_prot_from_caching(enum ttm_caching
caching, pgprot_t tmp)
#endif
#if defined(__ia64__) || defined(__arm__) || defined(__aarch64__) || \
defined(__powerpc__) || defined(__mips__)
- if (caching == ttm_write_combined)
+ if (caching == ttm_write_combined && enable_use_wc != 0)
tmp = pgprot_writecombine(tmp);
else
tmp = pgprot_noncached(tmp);
