Re: [Linaro-mm-sig] [PATCH] drm/todo: Add entry for moving to dma_resv_lock

[Daniel Vetter <daniel.vetter@ffwll.ch>]

On Fri, Jan 22, 2021 at 2:40 PM Christian König
<ckoenig.leichtzumerken@gmail.com> wrote:
>
> Am 22.01.21 um 14:36 schrieb Daniel Vetter:
> > Requested by Thomas. I think it justifies a new level, since I tried
> > to make some forward progress on this last summer, and gave up (for
> > now). This is very tricky.
> >
> > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > Cc: Maxime Ripard <mripard@kernel.org>
> > Cc: Thomas Zimmermann <tzimmermann@suse.de>
> > Cc: David Airlie <airlied@linux.ie>
> > Cc: Daniel Vetter <daniel@ffwll.ch>
> > Cc: Sumit Semwal <sumit.semwal@linaro.org>
> > Cc: "Christian König" <christian.koenig@amd.com>
> > Cc: linux-media@vger.kernel.org
> > Cc: linaro-mm-sig@lists.linaro.org
> > ---
> >   Documentation/gpu/todo.rst | 19 +++++++++++++++++++
> >   1 file changed, 19 insertions(+)
> >
> > diff --git a/Documentation/gpu/todo.rst b/Documentation/gpu/todo.rst
> > index dea9082c0e5f..f872d3d33218 100644
> > --- a/Documentation/gpu/todo.rst
> > +++ b/Documentation/gpu/todo.rst
> > @@ -23,6 +23,9 @@ Advanced: Tricky tasks that need fairly good understanding of the DRM subsystem
> >   and graphics topics. Generally need the relevant hardware for development and
> >   testing.
> >
> > +Expert: Only attempt these if you've successfully completed some tricky
> > +refactorings already and are an expert in the specific area
> > +
> >   Subsystem-wide refactorings
> >   ===========================
> >
> > @@ -168,6 +171,22 @@ Contact: Daniel Vetter, respective driver maintainers
> >
> >   Level: Advanced
> >
> > +Move Buffer Object Locking to dma_resv_lock()
> > +---------------------------------------------
> > +
> > +Many drivers have their own per-object locking scheme, usually using
> > +mutex_lock(). This causes all kinds of trouble for buffer sharing, since
> > +depending which driver is the exporter and importer, the locking hierarchy is
> > +reversed.
> > +
> > +To solve this we need one standard per-object locking mechanism, which is
> > +dma_resv_lock(). This lock needs to be called as the outermost lock, with all
> > +other driver specific per-object locks removed. The problem is tha rolling out
> > +the actual change to the locking contract is a flag day, due to struct dma_buf
> > +buffer sharing.
> > +
> > +Level: Expert
> > +
>
> Could you name some examples of driver locks here? I'm not aware in
> anything like this in amdgpu, radeon or neveau.

ttm based drivers are all fine. It's everything else which is a
problem, and it gets worse if you mix helpers (like shmem helpers,
which have their own locks internally) with render drivers (again with
their own mutexes).

> And yes sounds like a job for the appropriate driver maintainer.

The problem is, this one you can't do driver-by-driver because of the
dma-buf contract. I mean we tried for p2p at first, it's just too
much. I tried to do it last summer just for shmem gem helpers, and you
really have to tackle all the drivers in one go (even if you ignore
dma-buf for now, where we side-stepped the problem with pinning). This
is "scares danvet" levels of nasty.
-Daniel

> Thanks,
> Christian.
>
> >   Convert logging to drm_* functions with drm_device paramater
> >   ------------------------------------------------------------
> >
>


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