[PATCH] drm/ttm: add workaround for some arm hardware issue

[Deng, Emily]

[AMD Official Use Only]

Currently, only ampere found this issue, but it is hard to detect ampere board, especially on arm passthrough environment.

Best wishes
Emily Deng



>-----Original Message-----
>From: amd-gfx <amd-gfx-bounces at lists.freedesktop.org> On Behalf Of
>Christian König
>Sent: Wednesday, December 22, 2021 4:11 PM
>To: Zhao, Victor <Victor.Zhao at amd.com>; amd-gfx at lists.freedesktop.org
>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 at amd.com>
>> ---
>>   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);