On Mon, Feb 22, 2021 at 05:25:46PM +0100, Thomas Zimmermann wrote:

Hi

Am 22.02.21 um 17:10 schrieb Daniel Vetter:

...

On Mon, Feb 22, 2021 at 2:24 PM Thomas Zimmermann tzimmermann@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.

Creating the sg table is not the problem; mapping it is. So dma_buf_attach shouldn't be a problem.

dma_buf_attach will create a cached sg-mapping for you if the exporter is dynamic. Currently that's only the case for amdgpu, I guess you didn't test with that.

So yeah dma_buf_attach is a problem already. And if we can't attach, the entire obj->import_attach logic in drm_prime.c falls over, and we get all kinds of fun with double import and re-export.

...

...

...

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.

I really don't want to do this. My time is also limited, and I''ll spend time papering over the thing. And then more time for the real fix. I'd rather pull drm_gem_prime_import_dev() in to USB drivers and avoid the dma_buf_map().

Yeah I understand, it's just (as usual :-/) more complex than it seems ... -Daniel

Best regard Thomas

...

-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

-- Thomas Zimmermann Graphics Driver Developer SUSE Software Solutions Germany GmbH Maxfeldstr. 5, 90409 Nürnberg, Germany (HRB 36809, AG Nürnberg) Geschäftsführer: Felix Imendörffer