Re: [PATCH v6 00/16] drm/panfrost,panthor: Cached maps and explicit flushing

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi

Am 26.11.25 um 13:44 schrieb Boris Brezillon:
> This series implements cached maps and explicit flushing for both panfrost
> and panthor. To avoid code/bug duplication, the tricky guts of the cache
> flushing ioctl which walk the sg list are broken into a new common shmem
> helper which can be used by any driver.

Gem-shmem is getting more and more complicated. I think gem-shmem would 
be better off to be a simple implementation for all the drivers that use 
shadow buffering and software rendering. There are plenty of them. And 
drivers like the ones in sysfb/ are our failure-mode fallback. They 
should have non-complicated memory management wherever possible.

Therefore, could we first duplicate the existing gem-shmem code into 
gem-uma as we discussed recently on IRC? The changes are simple:

- copy the existing gem-shmem to gem-uma (plus renames)
- convert panthor and panfrost to the new interfaces

And on top of that, further improvements, such as the series at hand, 
could be done. Later we'd convert other drivers to gem-uma where it 
fits, such as lima.

Best regards
Thomas

>
> The PanVK MR to use this lives here:
>
> https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36385
>
> The questions about the DMA-API based CPU-cache-flush mechanism used
> in this patchset have been dropped. After briefly discussing it with
> Sima and Robin, it seems there's a consensus on the fact we should
> probably expose CPU cache maintenance without going through the DMA
> API (extending drm_cache? providing MM helpers for CPU cache
> flush/invalidation? It's not clear yet how, but this will be discussed
> in a separate thread). In the meantime, we can rely on dma_sync because
> that's good enough for our usecase.
>
> Changes in v2:
> - Expose the coherency so userspace can know when it should skip cache
>    maintenance
> - Hook things up at drm_gem_object_funcs level to dma-buf cpu_prep hooks
>    can be implemented generically
> - Revisit the semantics of the flags passed to gem_sync()
> - Add BO_QUERY_INFO ioctls to query BO flags on imported objects and
>    let the UMD know when cache maintenance is needed on those
>
> Changes in v3:
> - New patch to fix panthor_gpu_coherency_set()
> - No other major changes, check each patch changelog for more details
>
> Changes in v4:
> - Two trivial fixes, check each patch changelog for more details
>
> Changes in v5:
> - Add a way to overload dma_buf_ops while still relying on the drm_prime
>    boilerplate
> - Add default shmem implementation for
>    dma_buf_ops::{begin,end}_cpu_access()
> - Provide custom dma_buf_ops to deal with CPU cache flushes around CPU
>    accesses when the BO is CPU-cacheable
> - Go back to a version of drm_gem_shmem_sync() that only deals with
>    cache maintenance, and adjust the semantics to make it clear this is
>    the only thing it cares about
> - Adjust the BO_SYNC ioctls according to the new drm_gem_shmem_sync()
>    semantics
>
> Changes in v6:
> - No major changes, check the changelog in each patch for more details
>
> Boris Brezillon (10):
>    drm/prime: Simplify life of drivers needing custom dma_buf_ops
>    drm/shmem: Provide a generic {begin,end}_cpu_access() implementation
>    drm/panthor: Provide a custom dma_buf implementation
>    drm/panthor: Fix panthor_gpu_coherency_set()
>    drm/panthor: Expose the selected coherency protocol to the UMD
>    drm/panthor: Add a PANTHOR_BO_SYNC ioctl
>    drm/panthor: Add an ioctl to query BO flags
>    drm/panfrost: Provide a custom dma_buf implementation
>    drm/panfrost: Expose the selected coherency protocol to the UMD
>    drm/panfrost: Add an ioctl to query BO flags
>
> Faith Ekstrand (5):
>    drm/shmem: Add a drm_gem_shmem_sync() helper
>    drm/panthor: Bump the driver version to 1.6
>    drm/panfrost: Add a PANFROST_SYNC_BO ioctl
>    drm/panfrost: Add flag to map GEM object Write-Back Cacheable
>    drm/panfrost: Bump the driver version to 1.6
>
> Loïc Molinari (1):
>    drm/panthor: Add flag to map GEM object Write-Back Cacheable
>
>   drivers/gpu/drm/drm_gem_shmem_helper.c     | 207 +++++++++++++++++++++
>   drivers/gpu/drm/drm_prime.c                |  10 +-
>   drivers/gpu/drm/panfrost/panfrost_device.h |   1 +
>   drivers/gpu/drm/panfrost/panfrost_drv.c    | 101 +++++++++-
>   drivers/gpu/drm/panfrost/panfrost_gem.c    |  67 +++++++
>   drivers/gpu/drm/panfrost/panfrost_gem.h    |   9 +
>   drivers/gpu/drm/panfrost/panfrost_gpu.c    |  26 ++-
>   drivers/gpu/drm/panfrost/panfrost_regs.h   |  10 +-
>   drivers/gpu/drm/panthor/panthor_device.c   |  10 +-
>   drivers/gpu/drm/panthor/panthor_drv.c      |  79 +++++++-
>   drivers/gpu/drm/panthor/panthor_gem.c      |  71 ++++++-
>   drivers/gpu/drm/panthor/panthor_gem.h      |   4 +
>   drivers/gpu/drm/panthor/panthor_gpu.c      |   2 +-
>   drivers/gpu/drm/panthor/panthor_sched.c    |  18 +-
>   include/drm/drm_drv.h                      |   8 +
>   include/drm/drm_gem_shmem_helper.h         |  24 +++
>   include/uapi/drm/panfrost_drm.h            |  76 +++++++-
>   include/uapi/drm/panthor_drm.h             | 157 +++++++++++++++-
>   18 files changed, 857 insertions(+), 23 deletions(-)
>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)