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Date:   Wed, 29 Nov 2023 08:53:30 +0100
From:   Boris Brezillon <boris.brezillon@collabora.com>
To:     Dmitry Osipenko <dmitry.osipenko@collabora.com>
Cc:     Maxime Ripard <mripard@kernel.org>,
        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: <20231129085330.7ccb35d3@collabora.com>
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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...