linux-kernel - Re: [PATCH v1 RFC] video/hdmi: Fix HDMI_VENDOR_INFOFRAME

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Message-ID: <Y+v+jFKiQ3Rj2aYL@intel.com>
Date:   Tue, 14 Feb 2023 23:35:08 +0200
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Cc:     dri-devel@...ts.freedesktop.org, Helge Deller <deller@....de>,
        linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Hans Verkuil <hans.verkuil@...co.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Bernard Zhao <bernard@...o.com>
Subject: Re: [PATCH v1 RFC] video/hdmi: Fix HDMI_VENDOR_INFOFRAME_SIZE

On Tue, Feb 14, 2023 at 10:26:24PM +0100, Martin Blumenstingl wrote:
> On Mon, Feb 13, 2023 at 12:11 PM Ville Syrjälä
> <ville.syrjala@...ux.intel.com> wrote:
> [...]
> > > One could use HDMI_VENDOR_INFOFRAME_SIZE with this as well:
> > >   u8 buffer[HDMI_INFOFRAME_SIZE(VENDOR)];
> > > But it would only result in an 8 byte wide buffer.
> > > Nobody uses it like this yet.
> >
> > Not sure that would make any sense since a vendor
> > specific infoframe has no defined size until you
> > figure out which vendor defined it (via the OUI).
> My understanding is that all of the existing HDMI vendor infoframe
> code is built for HDMI_IEEE_OUI.

Only because no one has bothered to implement any
others.

> 
> > I suppose the current value of 4 is also a bit nonsense
> > as well then, becasue that is a legal value for the
> > HDMI 1.4 vendor specific infoframe, but might not be
> > valid for any other infoframe.
> >
> > We should perhaps just get rid of HDMI_VENDOR_INFOFRAME_SIZE
> > entirely.
> My thought was to make it the correct size for
> drm_hdmi_vendor_infoframe_from_display_mode(). Then developers using
> this "common" vendor infoframe don't have to worry much.
> If there's another vendor infoframe implementation (which I'm not
> aware of, but it may exist - since as you point out: it's vendor
> specific) then the driver code shouldn't use
> drm_hdmi_vendor_infoframe_from_display_mode() but rather implement
> something custom. At that point the person implementing that will also
> need to know their specific infoframe maximum size.

Yes but that other infoframe will still have
type==HDMI_INFOFRAME_TYPE_VENDOR, and
HDMI_INFOFRAME_SIZE(VENDOR) would again
give the wrong answer.

-- 
Ville Syrjälä
Intel