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From:   Alex Qiu <xqiu@google.com>
Date:   Thu, 1 Oct 2020 11:27:09 -0700
Message-ID: <CAA_a9xL+qP3zOy=oKHjCuR+CvsXeoU5EX9WgEhUH0Fza2Vs5DA@mail.gmail.com>
Subject: Re: [PATCH v1] i2c: npcm7xx: Support changing bus speed using debugfs.
To:     Andy Shevchenko <andy.shevchenko@gmail.com>
Cc:     Avi Fishman <avifishman70@gmail.com>,
        Tali Perry <tali.perry1@gmail.com>,
        Wolfram Sang <wsa@kernel.org>, Kun Yi <kunyi@google.com>,
        Benjamin Fair <benjaminfair@google.com>,
        Joel Stanley <joel@jms.id.au>,
        Tomer Maimon <tmaimon77@gmail.com>,
        Linux I2C <linux-i2c@vger.kernel.org>,
        OpenBMC Maillist <openbmc@lists.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Thu, Oct 1, 2020 at 10:41 AM Andy Shevchenko
<andy.shevchenko@gmail.com> wrote:
>
> On Thu, Oct 01, 2020 at 08:13:49PM +0300, Avi Fishman wrote:
> > Hi Andy,
> >
> > Customers using BMC with complex i2c topology asked us to support
> > changing bus frequency at run time, for example same device will
> > communicate with one slave at 100Kbp/s and another with 400kbp/s and
> > maybe also with smae device at different speed (for example an i2c
> > mux).
> > This is not only for debug.
>
> The above design is fragile to start with. If you have connected peripheral
> devices with different speed limitations and you try to access faster one the
> slower ones may block and break the bus which will need recovery.
>

Hi Andy,

To clarify, we are using a single read-only image to support multiple
configurations, so the supported bus rate of the devices are not known
at compile time, but at runtime. We start with 100 kHz, and go 400 kHz
if applicable. FYI, we are using 5.1 kernel, however I don't know much
about DT overlay.

Thx.

-Alex Qiu