Re: [RFC PATCH v3 0/3] new subsystem for compute accelerator devices

[Jeffrey Hugo]

On 11/6/2022 2:02 PM, Oded Gabbay wrote:
> This is the third version of the RFC following the comments given on the
> second version, but more importantly, following testing done by the VPU
> driver people and myself. We found out that there is a circular dependency
> between DRM and accel. DRM calls accel exported symbols during init and when
> accel devices are registering (all the minor handling), then accel calls DRM
> exported symbols. Therefore, if the two components are compiled as modules,
> there is a circular dependency.
>
> To overcome this, I have decided to compile the accel core code as part of
> the DRM kernel module (drm.ko). IMO, this is inline with the spirit of the
> design choice to have accel reuse the DRM core code and avoid code
> duplication.
>
> Another important change is that I have reverted back to use IDR for minor
> handling instead of xarray. This is because I have found that xarray doesn't
> handle well the scenario where you allocate a NULL entry and then exchange it
> with a real pointer. It appears xarray still considers that entry a "zero"
> entry. This is unfortunate because DRM works that way (first allocates a NULL
> entry and then replaces the entry with a real pointer).
>
> I decided to revert to IDR because I don't want to hold up these patches,
> as many people are blocked until the support for accel is merged. The xarray
> issue should be fixed as a separate patch by either fixing the xarray code or
> changing how DRM + ACCEL do minor id handling.

This sounds sane to me.  However, this appears to be something that 
Matthew Wilcox should be aware of (added for visibility).  Perhaps he 
has a very quick solution.  If not, at-least he might have ideas on how 
to best address in the future.

> The patches are in the following repo:
> https://git.kernel.org/pub/scm/linux/kernel/git/ogabbay/accel.git/log/?h=accel_v3
>
> As in v2, The HEAD of that branch is a commit adding a dummy driver that
> registers an accel device using the new framework. This can be served
> as a simple reference. I have checked inserting and removing the dummy driver,
> and opening and closing /dev/accel/accel0 and nothing got broken :)
>
> v1 cover letter:
> https://lkml.org/lkml/2022/10/22/544
>
> v2 cover letter:
> https://lore.kernel.org/lkml/20221102203405.1797491-1-ogabbay@xxxxxxxxxx/T/
>
> Thanks,
> Oded.
>
> Oded Gabbay (3):
>    drivers/accel: define kconfig and register a new major
>    accel: add dedicated minor for accelerator devices
>    drm: initialize accel framework
>
>   Documentation/admin-guide/devices.txt |   5 +
>   MAINTAINERS                           |   8 +
>   drivers/Kconfig                       |   2 +
>   drivers/accel/Kconfig                 |  24 ++
>   drivers/accel/drm_accel.c             | 322 ++++++++++++++++++++++++++
>   drivers/gpu/drm/Makefile              |   1 +
>   drivers/gpu/drm/drm_drv.c             | 102 +++++---
>   drivers/gpu/drm/drm_file.c            |   2 +-
>   drivers/gpu/drm/drm_sysfs.c           |  24 +-
>   include/drm/drm_accel.h               |  97 ++++++++
>   include/drm/drm_device.h              |   3 +
>   include/drm/drm_drv.h                 |   8 +
>   include/drm/drm_file.h                |  21 +-
>   13 files changed, 582 insertions(+), 37 deletions(-)
>   create mode 100644 drivers/accel/Kconfig
>   create mode 100644 drivers/accel/drm_accel.c
>   create mode 100644 include/drm/drm_accel.h
>
> --
> 2.25.1