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

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Message-ID: <49884f95-5d22-ad65-6ea2-69b0277b096a@denx.de>
Date:   Wed, 29 Jun 2022 16:34:31 +0200
From:   Marek Vasut <marex@...x.de>
To:     Krzysztof Adamski <krzysztof.adamski@...ia.com>,
        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

On 6/29/22 16:09, Krzysztof Adamski wrote:
> 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?

Sigh, it's been a year since I looked into this, sorry.

One of the modes is maybe not supported on all the XIIC core instances ?