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    SubjectRE: [PATCH v2 03/10] i2c: xiic: Switch to Xiic standard mode for i2c-read
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    [AMD Official Use Only - General]



    > -----Original Message-----
    > From: Krzysztof Adamski <krzysztof.adamski@nokia.com>
    > Sent: Friday, July 1, 2022 12:32 PM
    > To: Datta, Shubhrajyoti <shubhrajyoti.datta@amd.com>; Marek Vasut
    > <marex@denx.de>; Raviteja Narayanam <raviteja.narayanam@xilinx.com>;
    > linux-i2c@vger.kernel.org; michal.simek@xilinx.com
    > Cc: linux-arm-kernel@lists.infradead.org; linux-kernel@vger.kernel.org;
    > git@xilinx.com; joe@perches.com
    > Subject: Re: [PATCH v2 03/10] i2c: xiic: Switch to Xiic standard mode for i2c-
    > read
    >
    > [CAUTION: External Email]
    >
    > W dniu 30.06.2022 o 10:23, Datta, Shubhrajyoti pisze:
    > > [AMD Official Use Only - General]
    > >
    > > Hi Krzysztof,
    > >
    > >> -----Original Message-----
    > >> From: Krzysztof Adamski <krzysztof.adamski@nokia.com>
    > >> Sent: Wednesday, June 29, 2022 7:40 PM
    > >> To: Marek Vasut <marex@denx.de>; Raviteja Narayanam
    > >> <raviteja.narayanam@xilinx.com>; linux-i2c@vger.kernel.org;
    > >> michal.simek@xilinx.com
    > >> Cc: linux-arm-kernel@lists.infradead.org;
    > >> linux-kernel@vger.kernel.org; git@xilinx.com; joe@perches.com
    > >> Subject: Re: [PATCH v2 03/10] i2c: xiic: Switch to Xiic standard mode
    > >> for i2c- read
    > >>
    > >> [CAUTION: External Email]
    > >>
    > >> CAUTION: This message has originated from an External Source. Please
    > >> use proper judgment and caution when opening attachments, clicking
    > >> links, or responding to this email.
    > >>
    > >>
    > >> 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?
    > > However the current mode is dynamic mode so for less than 255 we can
    > > use dynamic mode.(the current behavior will not change) Also the
    > > dynamic mode is nicer on the processor resources. We set the bytes and
    > the controller takes care of transferring.
    > >
    > > However do not have any strong views open to suggestions.
    >
    > All I'm saying is that before this patchset, the dynamic mode was used in all
    > cases and it made sense - it is easier to work with. But it turned out it has its
    > limitations and support for standard mode was added with several cases that
    > switch to that mode. The commit message suggests that the only difference is
    > in how complicated the code for handling them is, does not say why dynamic
    > mode might still be preferred. And supporting both of them complicates the
    > code noticeably.
    > My understanding now is that the code struggles to use the dynamic mode in
    > all cases that it can because that produces less interrupts and so it is slightly
    > lighter on resources. So it is a code complication vs effectiveness tradeoff.
    > Since this is I2C - a slow bus, I'm not sure it is worth it but also don't have
    > strong opinion on that. If nothing else, I think it would make sense to update
    > the commit message a little bit to better explain why it is worth keeping both
    > modes.

    Will update the commit message in the next version.

    >
    > Krzysztof
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