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

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Date:   Mon, 4 Jul 2022 05:45:11 +0000
From:   "Datta, Shubhrajyoti" <shubhrajyoti.datta@....com>
To:     Krzysztof Adamski <krzysztof.adamski@...ia.com>,
        Marek Vasut <marex@...x.de>,
        Raviteja Narayanam <raviteja.narayanam@...inx.com>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "michal.simek@...inx.com" <michal.simek@...inx.com>
CC:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "git@...inx.com" <git@...inx.com>,
        "joe@...ches.com" <joe@...ches.com>
Subject: RE: [PATCH v2 03/10] i2c: xiic: Switch to Xiic standard mode for
 i2c-read

[AMD Official Use Only - General]



> -----Original Message-----
> From: Krzysztof Adamski <krzysztof.adamski@...ia.com>
> Sent: Friday, July 1, 2022 12:32 PM
> To: Datta, Shubhrajyoti <shubhrajyoti.datta@....com>; 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
> 
> [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@...ia.com>
> >> Sent: Wednesday, June 29, 2022 7:40 PM
> >> 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
> >>
> >> [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