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

From: Marek Vasut
Date: Wed Jun 29 2022 - 10:34:43 EST


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 ?