linux-kernel - Re: [PATCH] i2c: Raise SDA for each received bit, if necessary

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Date:	Thu, 12 May 2016 18:19:12 +0200
From:	Thomas Zimmermann <tdz@...rs.sourceforge.net>
To:	Wolfram Sang <wsa@...-dreams.de>
Cc:	linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] i2c: Raise SDA for each received bit, if necessary

Hi

Am 12.05.2016 um 11:05 schrieb Wolfram Sang:
> Hi Thomas,
> 
> On Wed, Apr 27, 2016 at 08:11:54PM +0200, Thomas Zimmermann wrote:
>> Some I2C adapters don't raise SDA by themselves when sending a bit. This
>> behavior can be seen with the DDC channel of SiS 300 graphics cards.
> 
> I think you mean 'clients' or 'devices' here, not adapters (masters),
> right?

Yes, sorry.

> So, it seems you have a bus without a pull-up resistor (or a
> broken one?) but use now a push-pull design. This is against the I2C
> standard and for most circiuts quite dangerous as it can lead to
> shortcuts.

I see.

> Do you have any hints that the bus is designed this way? In which driver
> do you want to use the new flag?

I was playing with the DRM framework and an old SiS graphics card. I
discovered this issue while trying to read the EDID from the monitor.

I have a few other SiS cards/models here and they all expose this
behavior. So I guess it's intentional(==cheaper?), although the HW docs
don't seem mention it explicitly.

Best regards
Thomas

> 
> Thanks,
> 
>    Wolfram
> 


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