linux-kernel - Re: [RFC PATCH 3/3] i2c: show and change bus frequency via sysfs

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Date:	Tue, 14 Oct 2014 12:24:51 +0300
From:	Octavian Purdila <octavian.purdila@...el.com>
To:	Mark Roszko <mark.roszko@...il.com>
Cc:	linux-i2c <linux-i2c@...r.kernel.org>, linux-api@...r.kernel.org,
	lkml <linux-kernel@...r.kernel.org>,
	Johan Hovold <johan@...nel.org>,
	Wolfram Sang <wsa@...-dreams.de>
Subject: Re: [RFC PATCH 3/3] i2c: show and change bus frequency via sysfs

On Tue, Oct 14, 2014 at 4:53 AM, Mark Roszko <mark.roszko@...il.com> wrote:
>
> > If this limitations exists
> >they are not introduced by this patch. This patch just exposes the
> >frequency so that it can be read or changed in userspace.
>
> Ah, well right now you can have an i2c bus with driver 1 and 2. Say
> the i2c bus is configured for 60khz in kernel space which normally
> can't be changed. Driver 1 talks to a slave that cannot go above
> 100khz. Now the userspace interface is added to the i2c bus. Some
> userspace application decides to reconfigure the bus for 400khz and do
> its communication to some slave device. Now the kernel tries to do
> some background talking to the drive 1 slave and suddenly finds it can
> no longer communicate with it. Right now with the kernel space only
> configuration, the system is safe from being messed up easily. It's
> more of a sanity of configuration issue.

You need privileges to change the bus frequency, so this is a
configuration issue. Which you still have today, since can still set
the i2c frequency on some busses via a module parameter.

>
>
> >On a different not, I have noticed that a fixed set of frequencies
> >might not be the best API, since multiple drivers rather support a
> >rather large set of frequencies in a range. A better API might be to
> >expose a min-max range and let the bus driver adjust the requested
> >frequency. I will follow up with a second version that does that.
>
> I was actually thinking you could eliminate the table of supported
> frequencies and just have the bus driver handle the set frequency
> decision itself and just return an error code if it's invalid. There
> are legitiamate drivers that cannot do more than a list of frequencies
> already as well. One example is here:
> http://lxr.free-electrons.com/source/drivers/i2c/busses/i2c-bcm-kona.c#L717

Yes, I will do that for v2 and in addition I will also add min and max
attributes to make it easier to determine if a bus supports fast mode,
fast plus, high, etc.
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