[RFC PATCH] drm/ttm: force cached mappings for system RAM on ARM

[Zhang, Jerry(Junwei)]

On 1/10/19 3:28 PM, Ard Biesheuvel wrote:
> ARM systems do not permit the use of anything other than cached
> mappings for system memory, since that memory may be mapped in the
> linear region as well, and the architecture does not permit aliases
> with mismatched attributes.
>
> So short-circuit the evaluation in ttm_io_prot() if the flags include
> TTM_PL_SYSTEM when running on ARM or arm64, and just return cached
> attributes immediately.

It sounds a case for ARM system memory access from CPU only?

If that always applies to ARM memory, suppose we should do that for 
TTM_PL_TT as well.
While TTM_PL_TT | TTM_PL_FLAG_WC is likely to work as below mention.

Regards,
Jerry
> This fixes the radeon and amdgpu [TBC] drivers when running on arm64.
> Without this change, amdgpu does not start at all, and radeon only
> produces corrupt display output.
>
> Cc: Christian Koenig <christian.koenig at amd.com>
> Cc: Huang Rui <ray.huang at amd.com>
> Cc: Junwei Zhang <Jerry.Zhang at amd.com>
> Cc: David Airlie <airlied at linux.ie>
> Reported-by: Carsten Haitzler <Carsten.Haitzler at arm.com>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> ---
>   drivers/gpu/drm/ttm/ttm_bo_util.c | 5 +++++
>   1 file changed, 5 insertions(+)
>
> diff --git a/drivers/gpu/drm/ttm/ttm_bo_util.c b/drivers/gpu/drm/ttm/ttm_bo_util.c
> index 046a6dda690a..0c1eef5f7ae3 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo_util.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo_util.c
> @@ -530,6 +530,11 @@ pgprot_t ttm_io_prot(uint32_t caching_flags, pgprot_t tmp)
>   	if (caching_flags & TTM_PL_FLAG_CACHED)
>   		return tmp;
>   
> +#if defined(__arm__) || defined(__aarch64__)
> +	/* ARM only permits cached mappings of system memory */
> +	if (caching_flags & TTM_PL_SYSTEM)
> +		return tmp;
> +#endif
>   #if defined(__i386__) || defined(__x86_64__)
>   	if (caching_flags & TTM_PL_FLAG_WC)
>   		tmp = pgprot_writecombine(tmp);