linux-kernel - Re: [RFC PATCH 01/20] drm: Remove users of drm_format_num

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Date:   Thu, 4 Apr 2019 17:24:03 +0100
From:   Emil Velikov <emil.l.velikov@...il.com>
To:     Maxime Ripard <maxime.ripard@...tlin.com>
Cc:     Daniel Vetter <daniel.vetter@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Sean Paul <seanpaul@...omium.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
        ML dri-devel <dri-devel@...ts.freedesktop.org>,
        Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        Hans Verkuil <hans.verkuil@...co.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        linux-media@...r.kernel.org
Subject: Re: [RFC PATCH 01/20] drm: Remove users of drm_format_num_planes

On Tue, 2 Apr 2019 at 15:51, Maxime Ripard <maxime.ripard@...tlin.com> wrote:
>
> Hi Emil,
>
> On Tue, Apr 02, 2019 at 10:43:31AM +0100, Emil Velikov wrote:
> > On Tue, 19 Mar 2019 at 21:57, Maxime Ripard <maxime.ripard@...tlin.com> wrote:
> > > drm_format_num_planes() is basically a lookup in the drm_format_info table
> > > plus an access to the num_planes field of the appropriate entry.
> > >
> > > Most drivers are using this function while having access to the entry
> > > already, which means that we will perform an unnecessary lookup. Removing
> > > the call to drm_format_num_planes is therefore more efficient.
> > >
> > > Some drivers will not have access to that entry in the function, but in
> > > this case the overhead is minimal (we just have to call drm_format_info()
> > > to perform the lookup) and we can even avoid multiple, inefficient lookups
> > > in some places that need multiple fields from the drm_format_info
> > > structure.
> > >
> >
> > I'm not fan of the duplicated loop-ups either.
> >
> > > -int drm_format_num_planes(uint32_t format)
> > > -{
> > > -       const struct drm_format_info *info;
> > > -
> > > -       info = drm_format_info(format);
> > > -       return info ? info->num_planes : 1;
> > > -}
> > > -EXPORT_SYMBOL(drm_format_num_planes);
> > > -
> >
> > The existing users are not updated to cater for the num_planes != 0
> > case... Which seems non-existent scenario since all the current format
> > descriptions have 1+ planes.
> > Should we add a test (alike the ones in 6/20) to ensure, that no entry
> > has 0 planes? Is it even worth it or I'm a bit too paranoid?
> >
> > The above comments apply to 2/20.
>
> I'm not entirely sure what you mean. num_planes is returned as is in
> the drm_format_num_planes function and it doesn't check for the
> num_planes value itself.
>
> That being said, we could definitely add some more tests to check that
> we haven't falling into the situation you describe, since most of the
> drivers indeed don't check for that value themselves. But that seems
> pretty othorgonal to me?
>
Hmm I misread the old function as "return info->num_planes ?
info->num_planes : 1;"

Pardon for the noise.
-Emil