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

[Christian König]

Am 10.03.25 um 10:50 schrieb 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.

Mhm, if I remember correctly the DMA subsystem used to use the DMA mask and other parameters of the parent device when a specific device couldn't do DMA by itself.

I do remember a lot of discussion about that for the DMA-buf tee driver.

Going to read up on that once more. Could be that this patch here is not really necessary.

Regards,
Christian.

>
> 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
>>>
>