linux-kernel - Re: [PATCH v2 8/8] ARM: bcm2835: Disable i2c2 in the Device Tree

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Date:   Tue, 27 Sep 2016 20:53:21 +0200
From:   Jan Kandziora <jjj@....de>
To:     Stefan Wahren <stefan.wahren@...e.com>, eric@...olt.net,
        wsa@...-dreams.de, swarren@...dotorg.org,
        Noralf Trønnes <noralf@...nnes.org>
Cc:     linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org,
        linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 8/8] ARM: bcm2835: Disable i2c2 in the Device Tree

Am 27.09.2016 um 19:25 schrieb Stefan Wahren:
> 
>> Noralf Trønnes <noralf@...nnes.org> hat am 27. September 2016 um 13:57
>> geschrieben:
>>
>>
>> i2c2 is connected to the HDMI connector and is controlled by the
>> firmware. Disable it to stay out of harms way.
> 
> Until this point the commit message is okay, the rest is more confusing.
> 
> Btw this should avoid a warning about missing clock frequency.
> 
>>
>> From the downstream commit:
>> i2c-bcm2708/BCM270X_DT: Add support for I2C2
>>
>> The third I2C bus (I2C2) is normally reserved for HDMI use. Careless
>> use of this bus can break an attached display - use with caution.
>>
>> It is recommended to disable accesses by VideoCore by setting
>> hdmi_ignore_edid=1 or hdmi_edid_file=1 in config.txt.
>>
>> Signed-off-by: Noralf Trønnes <noralf@...nnes.org>
>> ---
>>  arch/arm/boot/dts/bcm2835-rpi.dtsi | 4 ----
>>  1 file changed, 4 deletions(-)
>>
>> diff --git a/arch/arm/boot/dts/bcm2835-rpi.dtsi
>> b/arch/arm/boot/dts/bcm2835-rpi.dtsi
>> index e9b47b2..8bffbee 100644
>> --- a/arch/arm/boot/dts/bcm2835-rpi.dtsi
>> +++ b/arch/arm/boot/dts/bcm2835-rpi.dtsi
>> @@ -59,10 +59,6 @@
>>  	clock-frequency = <100000>;
>>  };
>>  
>> -&i2c2 {
>> -	status = "okay";
>> -};
>> -
> 
> I'm not sure if this the right fix. According to bcm283x.dtsi the 3 i2c busses
> have the same compatible string "brcm,bcm2835-i2c", but the changelog suggests
> that this bus is "special".
> 
> Shouldn't we use a different compatible string? Our intention isn't to disable
> i2c2 but avoid any claims of the usual i2c driver.
> 
i2c2 should not be generally disabled.

There's dtparam=i2c2_iknowwhatimdoing which enables CPU access to this
bus. It's useful for reading out the monitor EDID by the CPU, and for
accessing controls (backlight, volume) on certain monitors.

And I have a I2C touchscreen controller on HDMI DDC.

>>  &sdhci {
>>  	status = "okay";
>>  	bus-width = <4>;
>> -- 
>> 2.8.2
>>
>>
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