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

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  Re: [PATCH] i2c: Raise SDA for each received bit, if necessary Wolfram Sang <wsa@the-dreams.de> - 2016-05-12 11:10 +0200
    Re: [PATCH] i2c: Raise SDA for each received bit, if necessary Thomas Zimmermann <tdz@users.sourceforge.net> - 2016-05-12 18:20 +0200
      Re: [PATCH] i2c: Raise SDA for each received bit, if necessary Wolfram Sang <wsa@the-dreams.de> - 2016-05-13 12:40 +0200
        Re: [PATCH] i2c: Raise SDA for each received bit, if necessary Thomas Zimmermann <tdz@users.sourceforge.net> - 2016-05-13 18:10 +0200

#1399797 — Re: [PATCH] i2c: Raise SDA for each received bit, if necessary

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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? 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.

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

Thanks,

   Wolfram

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#1400213

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
> 


-- 
GnuPG:          http://tdz.users.sourceforge.net/tdz.asc
Fingerprint:    16FF F599 82F8 E5AA 18C6 5220 D9DA D7D4 4EF1 DF08

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#1400693

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> 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.

So, there is no upstream user yet?

> 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.

OK. Well. As this flag is potentially dangerous, I would prefer to not
apply the patch unless there is an upstream user of this. If there is
one, I'd be okay with applying it with the flag renamed to something
like "dangerous_push_pull_bus" or similar with additional comments
saying there is some (broken) HW which needs it but nobody should get
the idea to design a bus like this.

Makes sense?

Regards,

   Wolfram

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#1400885

Hi

Am 13.05.2016 um 12:33 schrieb Wolfram Sang:
> 
>> 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.
> 
> So, there is no upstream user yet?

No.

>> 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.
> 
> OK. Well. As this flag is potentially dangerous, I would prefer to not
> apply the patch unless there is an upstream user of this. If there is
> one, I'd be okay with applying it with the flag renamed to something
> like "dangerous_push_pull_bus" or similar with additional comments
> saying there is some (broken) HW which needs it but nobody should get
> the idea to design a bus like this.
> 
> Makes sense?

Sure, thanks!

Best regards
Thomas

> Regards,
> 
>    Wolfram
> 


-- 
GnuPG:          http://tdz.users.sourceforge.net/tdz.asc
Fingerprint:    16FF F599 82F8 E5AA 18C6 5220 D9DA D7D4 4EF1 DF08

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