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Date:   Wed, 29 Nov 2023 14:46:09 +0100
From:   Boris Brezillon <boris.brezillon@collabora.com>
To:     Maxime Ripard <mripard@kernel.org>
Cc:     Dmitry Osipenko <dmitry.osipenko@collabora.com>,
        David Airlie <airlied@gmail.com>,
        Gerd Hoffmann <kraxel@redhat.com>,
        Gurchetan Singh <gurchetansingh@chromium.org>,
        Chia-I Wu <olvaffe@gmail.com>, Daniel Vetter <daniel@ffwll.ch>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Christian =?UTF-8?B?S8O2bmln?= <christian.koenig@amd.com>,
        Qiang Yu <yuq825@gmail.com>,
        Steven Price <steven.price@arm.com>,
        Emma Anholt <emma@anholt.net>, Melissa Wen <mwen@igalia.com>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        kernel@collabora.com, virtualization@lists.linux-foundation.org
Subject: Re: [PATCH v18 04/26] drm/shmem-helper: Refactor locked/unlocked
 functions
Message-ID: <20231129144609.7544e773@collabora.com>
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On Wed, 29 Nov 2023 14:09:47 +0100
Maxime Ripard <mripard@kernel.org> wrote:

> On Wed, Nov 29, 2023 at 08:53:30AM +0100, Boris Brezillon wrote:
> > On Wed, 29 Nov 2023 01:05:14 +0300
> > Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:
> >   
> > > On 11/28/23 15:37, Boris Brezillon wrote:  
> > > > On Tue, 28 Nov 2023 12:14:42 +0100
> > > > Maxime Ripard <mripard@kernel.org> wrote:
> > > >     
> > > >> Hi,
> > > >>
> > > >> On Fri, Nov 24, 2023 at 11:59:11AM +0100, Boris Brezillon wrote:    
> > > >>> On Fri, 24 Nov 2023 11:40:06 +0100
> > > >>> Maxime Ripard <mripard@kernel.org> wrote:
> > > >>>       
> > > >>>> On Mon, Oct 30, 2023 at 02:01:43AM +0300, Dmitry Osipenko wrote:      
> > > >>>>> Add locked and remove unlocked postfixes from drm-shmem function names,
> > > >>>>> making names consistent with the drm/gem core code.
> > > >>>>>
> > > >>>>> Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
> > > >>>>> Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
> > > >>>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>        
> > > >>>>
> > > >>>> This contradicts my earlier ack on a patch but...
> > > >>>>       
> > > >>>>> ---
> > > >>>>>  drivers/gpu/drm/drm_gem_shmem_helper.c        | 64 +++++++++----------
> > > >>>>>  drivers/gpu/drm/lima/lima_gem.c               |  8 +--
> > > >>>>>  drivers/gpu/drm/panfrost/panfrost_drv.c       |  2 +-
> > > >>>>>  drivers/gpu/drm/panfrost/panfrost_gem.c       |  6 +-
> > > >>>>>  .../gpu/drm/panfrost/panfrost_gem_shrinker.c  |  2 +-
> > > >>>>>  drivers/gpu/drm/panfrost/panfrost_mmu.c       |  2 +-
> > > >>>>>  drivers/gpu/drm/v3d/v3d_bo.c                  |  4 +-
> > > >>>>>  drivers/gpu/drm/virtio/virtgpu_object.c       |  4 +-
> > > >>>>>  include/drm/drm_gem_shmem_helper.h            | 36 +++++------
> > > >>>>>  9 files changed, 64 insertions(+), 64 deletions(-)
> > > >>>>>
> > > >>>>> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> > > >>>>> index 0d61f2b3e213..154585ddae08 100644
> > > >>>>> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> > > >>>>> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> > > >>>>> @@ -43,8 +43,8 @@ static const struct drm_gem_object_funcs drm_gem_shmem_funcs = {
> > > >>>>>  	.pin = drm_gem_shmem_object_pin,
> > > >>>>>  	.unpin = drm_gem_shmem_object_unpin,
> > > >>>>>  	.get_sg_table = drm_gem_shmem_object_get_sg_table,
> > > >>>>> -	.vmap = drm_gem_shmem_object_vmap,
> > > >>>>> -	.vunmap = drm_gem_shmem_object_vunmap,
> > > >>>>> +	.vmap = drm_gem_shmem_object_vmap_locked,
> > > >>>>> +	.vunmap = drm_gem_shmem_object_vunmap_locked,        
> > > >>>>
> > > >>>> While I think we should indeed be consistent with the names, I would
> > > >>>> also expect helpers to get the locking right by default.      
> > > >>>
> > > >>> Wait, actually I think this patch does what you suggest already. The
> > > >>> _locked() prefix tells the caller: "you should take care of the locking,
> > > >>> I expect the lock to be held when this is hook/function is called". So
> > > >>> helpers without the _locked() prefix take care of the locking (which I
> > > >>> guess matches your 'helpers get the locking right' expectation), and
> > > >>> those with the _locked() prefix don't.      
> > > >>
> > > >> What I meant by "getting the locking right" is indeed a bit ambiguous,
> > > >> sorry. What I'm trying to say I guess is that, in this particular case,
> > > >> I don't think you can expect the vmap implementation to be called with
> > > >> or without the locks held. The doc for that function will say that it's
> > > >> either one or the other, but not both.
> > > >>
> > > >> So helpers should follow what is needed to provide a default vmap/vunmap
> > > >> implementation, including what locking is expected from a vmap/vunmap
> > > >> implementation.    
> > > > 
> > > > Hm, yeah, I think that's a matter of taste. When locking is often
> > > > deferrable, like it is in DRM, I find it beneficial for funcions and
> > > > function pointers to reflect the locking scheme, rather than relying on
> > > > people properly reading the doc, especially when this is the only
> > > > outlier in the group of drm_gem_object_funcs we already have, and it's
> > > > not event documented at the drm_gem_object_funcs level [1] :P.
> > > >     
> > > >>
> > > >> If that means that vmap is always called with the locks taken, then
> > > >> drm_gem_shmem_object_vmap can just assume that it will be called with
> > > >> the locks taken and there's no need to mention it in the name (and you
> > > >> can probably sprinkle a couple of lockdep assertion to make sure the
> > > >> locking is indeed consistent).    
> > > > 
> > > > Things get very confusing when you end up having drm_gem_shmem helpers
> > > > that are suffixed with _locked() to encode the fact locking is the
> > > > caller's responsibility and no suffix for the
> > > > callee-takes-care-of-the-locking semantics, while other helpers that are
> > > > not suffixed at all actually implement the
> > > > caller-should-take-care-of-the-locking semantics.
> > > >     
> > > >>    
> > > >>>> I'm not sure how reasonable it is, but I think I'd prefer to turn this
> > > >>>> around and keep the drm_gem_shmem_object_vmap/unmap helpers name, and
> > > >>>> convert whatever function needs to be converted to the unlock suffix so
> > > >>>> we get a consistent naming.      
> > > >>>
> > > >>> That would be an _unlocked() prefix if we do it the other way around. I
> > > >>> think the main confusion comes from the names of the hooks in
> > > >>> drm_gem_shmem_funcs. Some of them, like drm_gem_shmem_funcs::v[un]map()
> > > >>> are called with the GEM resv lock held, and locking is handled by the
> > > >>> core, others, like drm_gem_shmem_funcs::[un]pin() are called
> > > >>> without the GEM resv lock held, and locking is deferred to the
> > > >>> implementation. As I said, I don't mind prefixing hooks/helpers with
> > > >>> _unlocked() for those that take care of the locking, and no prefix for
> > > >>> those that expects locks to be held, as long as it's consistent, but I
> > > >>> just wanted to make sure we're on the same page :-).      
> > > >>
> > > >> What about _nolock then? It's the same number of characters than
> > > >> _locked, plus it expresses what the function is (not) doing, not what
> > > >> context it's supposed to be called in?    
> > > > 
> > > > Just did a quick
> > > > 
> > > >   git grep _nolock drivers/gpu/drm
> > > > 
> > > > and it returns zero result, where the _locked/_unlocked pattern seems
> > > > to already be widely used. Not saying we shouldn't change that, but it
> > > > doesn't feel like a change we should do as part of this series.
> > > > 
> > > > Regards,
> > > > 
> > > > Boris
> > > > 
> > > > [1]https://elixir.bootlin.com/linux/v6.7-rc3/source/include/drm/drm_gem.h#L155    
> > > 
> > > I'm fine with dropping the _locked() postfix from the common GEM helpers
> > > and documenting the locking rule in drm_gem. Thank you all for the
> > > suggestions :)  
> > 
> > Sorry to disagree, but I think a proper function name/suffix is
> > sometimes worth a few lines of doc. Not saying we should do one or the
> > other, I think we should do both. But when I see a function suffixed
> > _locked, _unlocked or _nolock, I can immediately tell if this function
> > defers the locking to the caller or not, and then go check which lock
> > in the function doc.
> > 
> > And the second thing I'm not happy with, is the fact we go back to an
> > inconsistent naming in drm_gem_shmem_helper.c, where some functions
> > deferring the locking to the caller are suffixed _locked and others are
> > not, because ultimately, you need a different name when you expose the
> > two variants...  
> 
> I guess one of the point I was trying to make was also: why do you need
> both?
> 
> If one is better than the other (whatever better means here), then all
> drivers should use it.
> 
> The counterpart being that if provided a choice, you can be sure that a
> lot of people will get it wrong. The one example I have in mind for
> example was the drm_atomic_helper_commit_tail vs
> drm_atomic_helper_commit_tail_rpm. The latter is now widely used, and
> most of it is cargo-cult.
> 
> I think you were referring to the locks being deferred vs taken right
> now before, why do we need to have the choice between the two?

Because DRM locking is complex, and you sometimes have to call some
helpers in a context where you already hold the GEM dma_resv lock.
That's not the case for _v[un]map(), because the core always takes the
lock for us if we call drm_gem_vmap_unlocked(). Now, let's assume we
drop the _locked() suffix on drm_gem_shmem_v[un]map(), but keep it on
other helpers that need both variants. This results in an inconsistent
naming scheme inside the same source file, which I find utterly
confusing.

Note that the initial reason I asked Dmitry if he could add the
_locked suffix to drm_gem_shmem_vmap() is because I started using
drm_gem_shmem_vmap() in powervr, before realizing this version wasn't
taking the lock, and I should have used drm_gem_vmap_unlocked()
instead, so this is not something I'm making up. Not saying the
confusion only comes from the naming, because the various layers of
indirection we have clearly don't help, but having a name reflecting
the fact the locking is deferred to the caller would have helped, I
think.