[PATCH] drm/gem-shmem: Optimize DMA mapping for exported buffers

[Jacek Lawrynowicz]

Hi,

On 3/25/2025 2:53 PM, Christian König wrote:
> Am 25.03.25 um 14:37 schrieb Jacek Lawrynowicz:
>> Use DMA_ATTR_SKIP_CPU_SYNC flag for exported buffers during DMA mapping.
>> The same flag is already used by drm_gem_map_dma_buf() for imported
>> buffers.
>>
>> Signed-off-by: Jacek Lawrynowicz <jacek.lawrynowicz at linux.intel.com>
>> ---
>>  drivers/gpu/drm/drm_gem_shmem_helper.c |  8 ++++++--
>>  include/drm/drm_gem.h                  | 12 ++++++++++++
>>  2 files changed, 18 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
>> index d99dee67353a1..8938d8e3de52f 100644
>> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
>> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
>> @@ -699,7 +699,7 @@ EXPORT_SYMBOL_GPL(drm_gem_shmem_get_sg_table);
>>  static struct sg_table *drm_gem_shmem_get_pages_sgt_locked(struct drm_gem_shmem_object *shmem)
>>  {
>>  	struct drm_gem_object *obj = &shmem->base;
>> -	int ret;
>> +	int ret, flags = 0;
>>  	struct sg_table *sgt;
>>  
>>  	if (shmem->sgt)
>> @@ -716,8 +716,12 @@ static struct sg_table *drm_gem_shmem_get_pages_sgt_locked(struct drm_gem_shmem_
>>  		ret = PTR_ERR(sgt);
>>  		goto err_put_pages;
>>  	}
>> +
>> +	if (drm_gem_is_exported())
>> +		flags |= DMA_ATTR_SKIP_CPU_SYNC;
> 
> We should probably just unconditionally set this flag or not at all.
> 
> Otherwise we could run into quite some surprises.

I see that this flag is usually set in drm_gem_map_dma_buf() and similar callbacks across drm drivers.
Shouldn't it be the default on x86?

Regards,
Jacek