[PATCH] drm/gem-shmem: Do not map s/g table by default

[Christian König]

On 30.06.25 16:34, Thomas Zimmermann wrote:
> The vast majority of drivers that use GEM-SHMEM helpers do not use
> an s/g table for imported buffers; specifically all drivers that use
> DRM_GEM_SHMEM_DRIVER_OPS. Therefore convert the initializer macro
> to DRM_GEM_SHMEM_DRIVER_OPS_NO_MAP_SGT and remove the latter. This
> helps to avoid swiotbl errors, such as seen with some Aspeed systems
> 
>   ast 0000:07:00.0: swiotlb buffer is full (sz: 3145728 bytes), total 32768 (slots), used 0 (slots)
> 
> The error is caused by the system's limited DMA capabilities and can
> happen with any GEM-SHMEM-based driver. It results in a performance
> penalty.
> 
> In the case of vgem and vkms, the devices do not support DMA at all,
> which can result in failure to map the buffer object into the kernel's
> address space. [1][2] Avoiding the s/g table fixes this problem.
> 
> The other drivers based on GEM-SHMEM, imagination, lima, panfrost,
> panthor, v3d and virtio, use the s/g table of imported buffers. Neither
> driver uses the default initializer, so they won't be affected by
> this change.
> 
> Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
> Reported-by: Zenghui Yu <zenghui.yu at linux.dev>
> Closes: https://lore.kernel.org/dri-devel/6d22bce3-4533-4cfa-96ba-64352b715741@linux.dev/ # [1]
> Reported-by: José Expósito <jose.exposito89 at gmail.com>
> Closes: https://lore.kernel.org/dri-devel/20250311172054.2903-1-jose.exposito89@gmail.com/ # [2]

Reviewed-by: Christian König <christian.koenig at amd.com>

> ---
>  drivers/gpu/drm/ast/ast_drv.c      |  2 +-
>  drivers/gpu/drm/udl/udl_drv.c      |  2 +-
>  include/drm/drm_gem_shmem_helper.h | 18 +++---------------
>  3 files changed, 5 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/gpu/drm/ast/ast_drv.c b/drivers/gpu/drm/ast/ast_drv.c
> index 054acda41909..6fbf62a99c48 100644
> --- a/drivers/gpu/drm/ast/ast_drv.c
> +++ b/drivers/gpu/drm/ast/ast_drv.c
> @@ -64,7 +64,7 @@ static const struct drm_driver ast_driver = {
>  	.minor = DRIVER_MINOR,
>  	.patchlevel = DRIVER_PATCHLEVEL,
>  
> -	DRM_GEM_SHMEM_DRIVER_OPS_NO_MAP_SGT,
> +	DRM_GEM_SHMEM_DRIVER_OPS,
>  	DRM_FBDEV_SHMEM_DRIVER_OPS,
>  };
>  
> diff --git a/drivers/gpu/drm/udl/udl_drv.c b/drivers/gpu/drm/udl/udl_drv.c
> index ce5ae7cacb90..1922988625eb 100644
> --- a/drivers/gpu/drm/udl/udl_drv.c
> +++ b/drivers/gpu/drm/udl/udl_drv.c
> @@ -57,7 +57,7 @@ static const struct drm_driver driver = {
>  
>  	/* GEM hooks */
>  	.fops = &udl_driver_fops,
> -	DRM_GEM_SHMEM_DRIVER_OPS_NO_MAP_SGT,
> +	DRM_GEM_SHMEM_DRIVER_OPS,
>  	DRM_FBDEV_SHMEM_DRIVER_OPS,
>  
>  	.name = DRIVER_NAME,
> diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
> index 35f7466dca84..92f5db84b9c2 100644
> --- a/include/drm/drm_gem_shmem_helper.h
> +++ b/include/drm/drm_gem_shmem_helper.h
> @@ -293,23 +293,11 @@ struct drm_gem_object *drm_gem_shmem_prime_import_no_map(struct drm_device *dev,
>  /**
>   * DRM_GEM_SHMEM_DRIVER_OPS - Default shmem GEM operations
>   *
> - * This macro provides a shortcut for setting the shmem GEM operations in
> - * the &drm_driver structure.
> + * This macro provides a shortcut for setting the shmem GEM operations
> + * in the &drm_driver structure. Drivers that do not require an s/g table
> + * for imported buffers should use this.
>   */
>  #define DRM_GEM_SHMEM_DRIVER_OPS \
> -	.gem_prime_import_sg_table = drm_gem_shmem_prime_import_sg_table, \
> -	.dumb_create		   = drm_gem_shmem_dumb_create
> -
> -/**
> - * DRM_GEM_SHMEM_DRIVER_OPS_NO_MAP_SGT - shmem GEM operations
> - *                                       without mapping sg_table on
> - *                                       imported buffer.
> - *
> - * This macro provides a shortcut for setting the shmem GEM operations in
> - * the &drm_driver structure for drivers that do not require a sg_table on
> - * imported buffers.
> - */
> -#define DRM_GEM_SHMEM_DRIVER_OPS_NO_MAP_SGT \
>  	.gem_prime_import       = drm_gem_shmem_prime_import_no_map, \
>  	.dumb_create            = drm_gem_shmem_dumb_create
>