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

[Thomas Zimmermann]

Hi

Am 07.03.25 um 14:32 schrieb Simona Vetter:
> 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.

Right. That's roughly how far I got in this direction. The field 
import_attach, set up by dma_buf_attach(), is currently used throughout 
the DRM code and drivers. Hence this series and the other one that 
replaced some of the uses of import_attach. Once this has all been 
resolved, there will be a few users of the field left, which might be 
uncritical.

Best regards
Thomas

>
> 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
>>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)