linux-kernel - Re: [PATCH v2 03/10] i2c: xiic: Switch to Xiic standard mode for i2c-read

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Date:   Wed, 29 Jun 2022 16:09:55 +0200
From:   Krzysztof Adamski <krzysztof.adamski@...ia.com>
To:     Marek Vasut <marex@...x.de>,
        Raviteja Narayanam <raviteja.narayanam@...inx.com>,
        linux-i2c@...r.kernel.org, michal.simek@...inx.com
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        git@...inx.com, joe@...ches.com
Subject: Re: [PATCH v2 03/10] i2c: xiic: Switch to Xiic standard mode for
 i2c-read

Hi Marek,

W dniu 29.06.2022 o 16:05, Marek Vasut pisze:
>> [...]
>>
>> If those two modes only differ in software complexity but we are not
>> able to support only the simpler one and we have support for the more
>> complicated (standard mode) anyways, we know that standard mode
>> can handle or the cases while dynamic mode cannot, we also know that
>> dynamic mode is broken on some versions of the core, why do we actually
>> keep support for dynamic mode?
>
> If I recall it right, the dynamic mode was supposed to handle 
> transfers longer than 255 Bytes, which the core cannot do in Standard 
> mode. It is needed e.g. by Atmel MXT touch controller. I spent a lot 
> of time debugging the race conditions in the XIIC, which I ultimately 
> fixed (the patches are upstream), but the long transfers I rather 
> fixed in the MXT driver instead.
>
> I also recall there was supposed to be some update for the XIIC core 
> coming with newer vivado, but I might be wrong about that.

It seems to be the other way around - dynamic mode is limited to 255 
bytes - when you trigger dynamic mode you first write the address of the 
slave to the FIFO, then you write the length as one byte so you can't 
request more than 255 bytes. So *standard* mode is used for those 
messages. In other words - dynamic mode is the one that is more limited 
- everything that you can do in dynamic mode you can also do in standard 
mode. So why don't we use standard mode always for everything?

Krzysztof