[PATCH v2 0/3] drm/prime: Only call dma_map_sgtable() for devices with DMA support

[Daniel Vetter]

On Mon, Feb 22, 2021 at 2:24 PM Thomas Zimmermann <tzimmermann at suse.de> wrote:
>
> Hi
>
> Am 22.02.21 um 14:09 schrieb Christian König:
> >
> >
> > Am 22.02.21 um 13:43 schrieb Thomas Zimmermann:
> >> USB-based drivers cannot use DMA, so the importing of dma-buf attachments
> >> currently fails for udl and gm12u320. This breaks joining/mirroring of
> >> displays.
> >>
> >> The fix is now a little series. To solve the issue on the importer
> >> side (i.e., the affected USB-based driver), patch 1 introduces a new
> >> PRIME callback, struct drm_driver.gem_prime_create_object, which creates
> >> an object and gives more control to the importing driver. Specifically,
> >> udl and gm12u320 can now avoid the creation of a scatter/gather table
> >> for the imported pages. Patch 1 is self-contained in the sense that it
> >> can be backported into older kernels.
> >
> > Mhm, that sounds like a little overkill to me.
> >
> > Drivers can already import the DMA-bufs all by them selves without the
> > help of the DRM functions. See amdgpu for an example.
> >
> > Daniel also already noted to me that he sees the DRM helper as a bit
> > questionable middle layer.
>
> And this bug proves that it is. :)

The trouble here is actually gem_bo->import_attach, which isn't really
part of the questionable midlayer, but fairly mandatory (only
exception is vmwgfx because not using gem) caching to make sure we
don't end up with duped imports and fun stuff like that.

And dma_buf_attach now implicitly creates the sg table already, so
we're already in game over land. I think we'd need to make
import_attach a union with import_buf or something like that, so that
you can do attachment-less importing.

> > Have you thought about doing that instead?
>
> There appears to be some useful code in drm_gem_prime_import_dev(). But
> if the general sentiment goes towards removing
> gem_prime_import_sg_table, we can work towards that as well.

I still think this part is a bit a silly midlayer for no good reason,
but I think that's orthogonal to the issue at hand here.

I'd suggest we first try to paper over the issue by using
prime_import_dev with the host controller (which hopefully is
dma-capable for most systems). And then, at leisure, try to untangle
the obj->import_attach issue.
-Daniel

>
> Best regards
> Thomas
>
> >
> > Christian.
> >
> >>
> >> Patches 2 and 3 update SHMEM and CMA helpers to use the new callback.
> >> Effectively this moves the sg table setup from the PRIME helpers into
> >> the memory managers. SHMEM now supports devices without DMA support,
> >> so custom code can be removed from udl and g12u320.
> >>
> >> Tested by joining/mirroring displays of udl and radeon under Gnome/X11.
> >>
> >> v2:
> >>     * move fix to importer side (Christian, Daniel)
> >>     * update SHMEM and CMA helpers for new PRIME callbacks
> >>
> >> Thomas Zimmermann (3):
> >>    drm: Support importing dmabufs into drivers without DMA
> >>    drm/shmem-helper: Implement struct drm_driver.gem_prime_create_object
> >>    drm/cma-helper: Implement struct drm_driver.gem_prime_create_object
> >>
> >>   drivers/gpu/drm/drm_gem_cma_helper.c    | 62 ++++++++++++++-----------
> >>   drivers/gpu/drm/drm_gem_shmem_helper.c  | 38 ++++++++++-----
> >>   drivers/gpu/drm/drm_prime.c             | 43 +++++++++++------
> >>   drivers/gpu/drm/lima/lima_drv.c         |  2 +-
> >>   drivers/gpu/drm/panfrost/panfrost_drv.c |  2 +-
> >>   drivers/gpu/drm/panfrost/panfrost_gem.c |  6 +--
> >>   drivers/gpu/drm/panfrost/panfrost_gem.h |  4 +-
> >>   drivers/gpu/drm/pl111/pl111_drv.c       |  8 ++--
> >>   drivers/gpu/drm/v3d/v3d_bo.c            |  6 +--
> >>   drivers/gpu/drm/v3d/v3d_drv.c           |  2 +-
> >>   drivers/gpu/drm/v3d/v3d_drv.h           |  5 +-
> >>   include/drm/drm_drv.h                   | 12 +++++
> >>   include/drm/drm_gem_cma_helper.h        | 12 ++---
> >>   include/drm/drm_gem_shmem_helper.h      |  6 +--
> >>   14 files changed, 120 insertions(+), 88 deletions(-)
> >>
> >> --
> >> 2.30.1
> >>
> >
>
> --
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Maxfeldstr. 5, 90409 Nürnberg, Germany
> (HRB 36809, AG Nürnberg)
> Geschäftsführer: Felix Imendörffer
>


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