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

[Thomas Zimmermann]

Hi

Am 10.03.25 um 14:56 schrieb 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.

This went away (for USB at least) with commit 6eb0233ec2d0 ("usb: don't 
inherity DMA properties for USB devices"). Buffer sharing with USB 
graphics devices failed and we worked around it in 659ab7a49cbe ("drm: 
Use USB controller's DMA mask when importing dmabufs").

Best regards
Thomas

>
> 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
>>>>

-- 
--
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)