Re: [PATCH v2 0/5] drm: Provide a dedicated DMA device for PRIME import

[Simona Vetter <simona.vetter@ffwll.ch>]

On Fri, Mar 07, 2025 at 09:03:58AM +0100, Thomas Zimmermann wrote:
> Importing dma-bufs via PRIME requires a DMA-capable hardware device.
> This is not the case for USB, where DMA is performed entirely by the
> USB controller instead of the USB devices.
> 
> Drivers for USB-based hardware maintain their own workarounds for this
> problem. The original idea to resolve this was to provide different
> PRIME helpers for such devices, but the dma-buf code internally assumes
> DMA functionality as well. So that ideas is not realistic.

So dma-buf without dma is doable, but you have to avoid dma_buf_attach.
And that is a lot of surgery in the current prime helpers, since those
assume that an attachment always exists. But dma-buf itself is entirely
fine with cpu-only access through either userspace mmap or kernel vmap.

I think as an interim step this is still good, since it makes the current
hacks easier to find because at least it's all common now.
-Sima

> Let's instead turn the current workaround into a feature. Patch 1 adds a
> dma_dev field to struct drm_device and makes the PRIME code use it. Patches
> 2 to 5 replace related driver code.
> 
> It will also be useful in other code. The exynos and mediatek drivers
> already maintain a dedicated DMA device for non-PRIME code. They could
> likely use dma_dev as well. GEM-DMA helpers currently allocate DMA
> memory with the regular parent device. They should support the dma_dev
> settings as well.
> 
> Tested with udl.
> 
> v2:
> - maintain reference on dma_dev (Jani)
> - improve docs (Maxime)
> - update appletbdrm
> 
> Thomas Zimmermann (5):
>   drm/prime: Support dedicated DMA device for dma-buf imports
>   drm/appletbdrm: Set struct drm_device.dma_dev
>   drm/gm12u320: Set struct drm_device.dma_dev
>   drm/gud: Set struct drm_device.dma_dev
>   drm/udl: Set struct drm_device.dma_dev
> 
>  drivers/gpu/drm/drm_drv.c          | 21 ++++++++++++++
>  drivers/gpu/drm/drm_prime.c        |  2 +-
>  drivers/gpu/drm/gud/gud_drv.c      | 33 ++++++---------------
>  drivers/gpu/drm/gud/gud_internal.h |  1 -
>  drivers/gpu/drm/tiny/appletbdrm.c  | 27 +++++++-----------
>  drivers/gpu/drm/tiny/gm12u320.c    | 46 +++++++++---------------------
>  drivers/gpu/drm/udl/udl_drv.c      | 17 -----------
>  drivers/gpu/drm/udl/udl_drv.h      |  1 -
>  drivers/gpu/drm/udl/udl_main.c     | 14 ++++-----
>  include/drm/drm_device.h           | 41 ++++++++++++++++++++++++++
>  10 files changed, 102 insertions(+), 101 deletions(-)
> 
> -- 
> 2.48.1
> 

-- 
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch