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)
