[PATCH] drm/bridge: anx7625: Fix invalid EDID size

[Dmitry Baryshkov]

On Tue, Jul 01, 2025 at 11:56:23AM +0300, Jani Nikula wrote:
> On Tue, 01 Jul 2025, Loic Poulain <loic.poulain at oss.qualcomm.com> wrote:
> > On Mon, Jun 30, 2025 at 10:40 AM Maxime Ripard <mripard at kernel.org> wrote:
> >>
> >> On Mon, Jun 30, 2025 at 09:46:40AM +0200, Loic Poulain wrote:
> >> > Hi Maxime,
> >> >
> >> > On Mon, Jun 30, 2025 at 9:07 AM Maxime Ripard <mripard at kernel.org> wrote:
> >> > > On Sun, Jun 29, 2025 at 04:38:36AM +0200, Loic Poulain wrote:
> >> > > > DRM checks EDID block count against allocated size in drm_edid_valid
> >> > > > function. We have to allocate the right EDID size instead of the max
> >> > > > size to prevent the EDID to be reported as invalid.
> >> > > >
> >> > > > Fixes: 7c585f9a71aa ("drm/bridge: anx7625: use struct drm_edid more")
> >> > > > Signed-off-by: Loic Poulain <loic.poulain at oss.qualcomm.com>
> >> > > > ---
> >> > > >  drivers/gpu/drm/bridge/analogix/anx7625.c | 2 +-
> >> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> >> > > >
> >> > > > diff --git a/drivers/gpu/drm/bridge/analogix/anx7625.c b/drivers/gpu/drm/bridge/analogix/anx7625.c
> >> > > > index 8a9079c2ed5c..5a81d1bfc815 100644
> >> > > > --- a/drivers/gpu/drm/bridge/analogix/anx7625.c
> >> > > > +++ b/drivers/gpu/drm/bridge/analogix/anx7625.c
> >> > > > @@ -1801,7 +1801,7 @@ static const struct drm_edid *anx7625_edid_read(struct anx7625_data *ctx)
> >> > > >               return NULL;
> >> > > >       }
> >> > > >
> >> > > > -     ctx->cached_drm_edid = drm_edid_alloc(edid_buf, FOUR_BLOCK_SIZE);
> >> > > > +     ctx->cached_drm_edid = drm_edid_alloc(edid_buf, edid_num * ONE_BLOCK_SIZE);
> >> > > >       kfree(edid_buf);
> >> > >
> >> > > Do we need to cache the whole EDIDs? AFAIU, it's only ever used to get
> >> > > the manufacturer name, which fits into a u32 / 4 u8. We should probably
> >> > > just cache that.
> >> >
> >> > While the cached EDID is indeed used internally to retrieve the
> >> > product ID, its content is also returned via the DRM read_edid
> >> > callback. This value is then used by the DRM core to enumerate
> >> > available display modes, and likely also when reading EDID from sysfs.
> >>
> >> You still don't need to allocate and store a copy of the EDIDs in your
> >> driver to implement what you listed so far.
> >
> > Right, we could change how the driver behaves on callback and just
> > cache what we need for internal usage. That change was initially a
> > pure fix, do you recommend changing all of this in this change, or in
> > a follow-up one.
> 
> If there's a follow-up, I really *really* think it should be to rewrite
> EDID reading in anx7625.c altogether. The current thing is busted in
> more ways than I have time to enumerate right now. And it's not because
> I'm in a huge rush. Just look at sp_tx_edid_read() and the functions it
> calls.
> 
> The end result should be based on providing a straightforward read_block
> callback for drm_edid_read_custom().
> 
> I've actually started this a few times myself, but it's a bit much for
> someone without the hardware to test it, nor skin in the game. The
> current code is too complex to trivially refactor.

It feels like it should be dropped completely in favour of the DDC
implementation provided by drm_dp_aux_*(). I'm not sure why the driver
implements I2C reading on its own.

-- 
With best wishes
Dmitry