Re: [PATCH] drm: Permit video-buffers writecombine mapping for MIPS

["Koenig, Christian" <Christian.Koenig@amd.com>]

Am 23.04.19 um 14:31 schrieb Serge Semin:
> Since commit 4b050ba7a66c ("MIPS: pgtable.h: Implement the
> pgprot_writecombine function for MIPS") and commit c4687b15a848 ("MIPS: Fix
> definition of pgprot_writecombine()") write-combine vma mapping is
> available to be used by kernel subsystems for MIPS. In particular the
> uncached accelerated attribute is requested to be set by ioremap_wc()
> method and by generic PCI memory pages/ranges mapping methods. The same
> is done by the drm_io_prot()/ttm_io_prot() functions in case if
> write-combine flag is set for vma's passed for mapping. But for some
> reason the pgprot_writecombine() method calling is ifdefed to be a
> platform-specific with MIPS system being marked as lacking of one. At the
> very least it doesn't reflect the current MIPS platform implementation.
> So in order to improve the DRM subsystem performance on MIPS with UCA
> mapping enabled, we need to have pgprot_writecombine() called for buffers,
> which need store operations being combined. In case if particular MIPS
> chip doesn't support the UCA attribute, the mapping will fall back to
> noncached.
>
> Cc: Ralf Baechle <ralf@linux-mips.org>
> Cc: Paul Burton <paul.burton@mips.com>
> Cc: James Hogan <jhogan@kernel.org>
> Signed-off-by: Vadim V. Vlasov <vadim.vlasov@t-platforms.ru>
> Signed-off-by: Serge Semin <fancer.lancer@gmail.com>

Not sure about all the details, but at least from the two mile high view 
that seems to make sense.

Acked-by: Christian König <christian.koenig@amd.com>

> ---
>   drivers/gpu/drm/drm_vm.c          | 5 +++--
>   drivers/gpu/drm/ttm/ttm_bo_util.c | 4 ++--
>   2 files changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_vm.c b/drivers/gpu/drm/drm_vm.c
> index c3301046dfaa..50178dc64060 100644
> --- a/drivers/gpu/drm/drm_vm.c
> +++ b/drivers/gpu/drm/drm_vm.c
> @@ -62,7 +62,8 @@ static pgprot_t drm_io_prot(struct drm_local_map *map,
>   	/* We don't want graphics memory to be mapped encrypted */
>   	tmp = pgprot_decrypted(tmp);
>   
> -#if defined(__i386__) || defined(__x86_64__) || defined(__powerpc__)
> +#if defined(__i386__) || defined(__x86_64__) || defined(__powerpc__) || \
> +    defined(__mips__)
>   	if (map->type == _DRM_REGISTERS && !(map->flags & _DRM_WRITE_COMBINING))
>   		tmp = pgprot_noncached(tmp);
>   	else
> @@ -73,7 +74,7 @@ static pgprot_t drm_io_prot(struct drm_local_map *map,
>   		tmp = pgprot_writecombine(tmp);
>   	else
>   		tmp = pgprot_noncached(tmp);
> -#elif defined(__sparc__) || defined(__arm__) || defined(__mips__)
> +#elif defined(__sparc__) || defined(__arm__)
>   	tmp = pgprot_noncached(tmp);
>   #endif
>   	return tmp;
> diff --git a/drivers/gpu/drm/ttm/ttm_bo_util.c b/drivers/gpu/drm/ttm/ttm_bo_util.c
> index 895d77d799e4..9f918b992f7e 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo_util.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo_util.c
> @@ -539,13 +539,13 @@ pgprot_t ttm_io_prot(uint32_t caching_flags, pgprot_t tmp)
>   		tmp = pgprot_noncached(tmp);
>   #endif
>   #if defined(__ia64__) || defined(__arm__) || defined(__aarch64__) || \
> -    defined(__powerpc__)
> +    defined(__powerpc__) || defined(__mips__)
>   	if (caching_flags & TTM_PL_FLAG_WC)
>   		tmp = pgprot_writecombine(tmp);
>   	else
>   		tmp = pgprot_noncached(tmp);
>   #endif
> -#if defined(__sparc__) || defined(__mips__)
> +#if defined(__sparc__)
>   	tmp = pgprot_noncached(tmp);
>   #endif
>   	return tmp;