Re: [PATCH v2 0/3] drm/prime: Only call dma_map_sgtable() for devices with DMA support

["Christian König" <christian.koenig@amd.com>]

Am 22.02.21 um 13:43 schrieb Thomas Zimmermann:
> USB-based drivers cannot use DMA, so the importing of dma-buf attachments
> currently fails for udl and gm12u320. This breaks joining/mirroring of
> displays.
>
> The fix is now a little series. To solve the issue on the importer
> side (i.e., the affected USB-based driver), patch 1 introduces a new
> PRIME callback, struct drm_driver.gem_prime_create_object, which creates
> an object and gives more control to the importing driver. Specifically,
> udl and gm12u320 can now avoid the creation of a scatter/gather table
> for the imported pages. Patch 1 is self-contained in the sense that it
> can be backported into older kernels.

Mhm, that sounds like a little overkill to me.

Drivers can already import the DMA-bufs all by them selves without the 
help of the DRM functions. See amdgpu for an example.

Daniel also already noted to me that he sees the DRM helper as a bit 
questionable middle layer.

Have you thought about doing that instead?

Christian.

>
> Patches 2 and 3 update SHMEM and CMA helpers to use the new callback.
> Effectively this moves the sg table setup from the PRIME helpers into
> the memory managers. SHMEM now supports devices without DMA support,
> so custom code can be removed from udl and g12u320.
>
> Tested by joining/mirroring displays of udl and radeon under Gnome/X11.
>
> v2:
> 	* move fix to importer side (Christian, Daniel)
> 	* update SHMEM and CMA helpers for new PRIME callbacks
>
> Thomas Zimmermann (3):
>    drm: Support importing dmabufs into drivers without DMA
>    drm/shmem-helper: Implement struct drm_driver.gem_prime_create_object
>    drm/cma-helper: Implement struct drm_driver.gem_prime_create_object
>
>   drivers/gpu/drm/drm_gem_cma_helper.c    | 62 ++++++++++++++-----------
>   drivers/gpu/drm/drm_gem_shmem_helper.c  | 38 ++++++++++-----
>   drivers/gpu/drm/drm_prime.c             | 43 +++++++++++------
>   drivers/gpu/drm/lima/lima_drv.c         |  2 +-
>   drivers/gpu/drm/panfrost/panfrost_drv.c |  2 +-
>   drivers/gpu/drm/panfrost/panfrost_gem.c |  6 +--
>   drivers/gpu/drm/panfrost/panfrost_gem.h |  4 +-
>   drivers/gpu/drm/pl111/pl111_drv.c       |  8 ++--
>   drivers/gpu/drm/v3d/v3d_bo.c            |  6 +--
>   drivers/gpu/drm/v3d/v3d_drv.c           |  2 +-
>   drivers/gpu/drm/v3d/v3d_drv.h           |  5 +-
>   include/drm/drm_drv.h                   | 12 +++++
>   include/drm/drm_gem_cma_helper.h        | 12 ++---
>   include/drm/drm_gem_shmem_helper.h      |  6 +--
>   14 files changed, 120 insertions(+), 88 deletions(-)
>
> --
> 2.30.1
>