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Subject: Re: [PATCH] TDA1997x: enable EDID support
To:     =?UTF-8?Q?Krzysztof_Ha=c5=82asa?= <khalasa@piap.pl>,
        Tim Harvey <tharvey@gateworks.com>
Cc:     Mauro Carvalho Chehab <mchehab@kernel.org>,
        linux-media <linux-media@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>
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From:   Hans Verkuil <hverkuil@xs4all.nl>
Message-ID: <e9acc316-54c4-0387-eaaf-18dfb4dce34e@xs4all.nl>
Date:   Tue, 8 Jun 2021 09:27:24 +0200
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Hi Krzysztof,

On 08/06/2021 06:54, Krzysztof Hałasa wrote:
> Tim,
> 
> Tim Harvey <tharvey@gateworks.com> writes:
> 
>>> 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 where the source above is from (this was all so long ago) but
> 
> That's your gateworks_fslc_3.14_1.0.x_ga branch (3.14 is the kernel and
> 1.0.x_ga is IIRC some Freescale versioning) :-)
> 
>> 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.
> 
> Right. I guess the else meant it was available and &= ~83 meant no
> EDID... Anyway one could simply drop a 24c02 or a similar chip directly
> to SDA/SCL HDMI lines, that's what the monitor makers had been doing for
> a long time. Then, I guess, you would need the internal_edid = 0
> (otherwise the TDA chip would be fighting the EEPROM on the SDA line).
> Not that I know of any such design using this driver, I guess we can
> safely skip this part.
> 

OK, I think the history is clear. Can you post a v2 with a Fixes tag and
comment a bit on why this was not caught before?

Regards,

	Hans