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

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Date:	Sun, 12 Oct 2014 10:06:37 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Octavian Purdila <octavian.purdila@...el.com>
Cc:	Mark Roszko <mark.roszko@...il.com>,
	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 Sun, Oct 12, 2014 at 12:32:56PM +0300, Octavian Purdila wrote:
> On Sat, Oct 11, 2014 at 11:14 PM, Mark Roszko <mark.roszko@...il.com> wrote:
> 
> > This seems limiting to arches with peripherals that can support a range of
> > frequencies rather than fixed numbers.
> > Also it creates some portability quirkiness between platforms when all the
> > i2c bus drivers have different supported freq lists and you have to match
> > exactly the right frequency. I.e. one guy does 60khz but another only has
> > 80khz.
> 
> Sorry, I don't understand your points here. 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.
> 
> > Another issue is in systems where you have i2c devices on the same bus as
> > the sysfs user space driver. User space could set a bus frequency that
> > prevents operation with a system i2c device.
> 
> Changing the frequency is limited to root. Also, bus drivers do not
> have to implement set_freq if it is thought not to be safe.
> 
> 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.

As two separate sysfs attributes, maybe ? sysfs is supposed to provide
one value per attribute.

For the patch itself, I would find it better if you used is_visible to
determine if the new attributes should be visible (and/or writable) instead
of returning -EOPNOTSUPP.

Guenter
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