linux-kernel - Re: [PATCH] TDA1997x: enable EDID support

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Message-ID: <CAJ+vNU0E_0pB-1T+VpdmjJNVirAwCUNjKVbEV4wEbqHOzURj_A@mail.gmail.com>
Date:   Mon, 7 Jun 2021 08:48:19 -0700
From:   Tim Harvey <tharvey@...eworks.com>
To:     Krzysztof Hałasa <khalasa@...p.pl>
Cc:     Hans Verkuil <hverkuil@...all.nl>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media <linux-media@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] TDA1997x: enable EDID support

On Mon, Jun 7, 2021 at 4:56 AM Krzysztof Hałasa <khalasa@...p.pl> wrote:
>
> Hi Hans,
>
> Hans Verkuil <hverkuil@...all.nl> writes:
>
> >> Without this patch, the TDA19971 chip's EDID is inactive.
> >
> > Was this wrong from the very beginning? How can this ever have been tested
> > without an EDID?
>
> It seems so. I suspect it might have worked in tests because this
> register isn't cleared on reboot. I.e., setting it once after power up
> makes it work to the next power up.
> Or, maybe, the HDMI signal source didn't need EDID.
>

Krzysztof,

Most likely it was that the HDMI signal source I tested with didn't
need EDID. I primarily used a V-SG4K HMDI signal generator in my
testing and development of the driver
(http://www.marshall-usa.com/monitors/model/V-SG4K-HDI.php) which
definitely doesn't need it. Other devices I tested with were another
Gateworks board with HDMI out (which also didn't need EDID) and
occasionally a 1st gen Google Chromecast and Amazon Fire stick (which
I'm not sure about).

> I'm looking at the previous version of this driver from Gateworks and it
> contains:
>
>      /* Configure EDID
>       *
>       * EDID_ENABLE bits:
>       *  7 - nack_off
>       *  6 - edid_only
>       *  1 - edid_b_en
>       *  0 - edid_a_en
>       */
>      reg = io_read(REG_EDID_ENABLE);
>      if (!tda1997x->internal_edid)
>          reg &= ~0x83; /* EDID Nack ON */
>      else
>          reg |= 0x83;  /* EDID Nack OFF */
>      io_write(REG_EDID_ENABLE, reg);
>
> Not sure what the "non-internal" EDID could be - a separate I2C EEPROM
> chip? I'm using this on Gateworks' GW54xx boards and I can't see any
> such EEPROM in the vicinity of the TDA19971, but I don't know how it is
> wired - perhaps Tim has some idea?

Not sure where the source above is from (this was all so long ago) but
my guess is that 'internal_edid' meant an EDID had been provided via
software and the else case meant there was no EDID available. There is
no support on that chip for an external EEPROM.

Tim