linux-kernel - Re: [PATCH 10/13] eeepc-laptop: compare proper return values in get

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Date:	Wed, 17 Sep 2014 09:12:23 -0700
From:	Darren Hart <dvhart@...radead.org>
To:	Frans Klaver <fransklaver@...il.com>
Cc:	Henrique de Moraes Holschuh <hmh@....eng.br>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Corentin Chary <corentin.chary@...il.com>,
	Rafael Wysocki <rafael.j.wysocki@...el.com>,
	acpi4asus-user <acpi4asus-user@...ts.sourceforge.net>,
	platform-driver-x86 <platform-driver-x86@...r.kernel.org>,
	linux-kernel@...r.kernel.org,
	linux-acpi <linux-acpi@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>
Subject: Re: [PATCH 10/13] eeepc-laptop: compare proper return values in
 get_cpufv

On Wed, Sep 17, 2014 at 01:57:33PM +0200, Frans Klaver wrote:
> On Wed, Sep 17, 2014 at 12:34 PM, Henrique de Moraes Holschuh
> <hmh@....eng.br> wrote:
> > On Tue, 16 Sep 2014, Darren Hart wrote:
> >> - When reading and writing sysfs device attribute files, avoid dependency
> >>   on specific error codes wherever possible. This minimizes coupling to
> >>   the error handling implemementation within the kernel.
> >>
> >>   In general, failures to read or write sysfs device attributes shall
> >>   propogate errors wherever possible. Common errors include, but are not
> >>   limited to:
> >>
> >>   -EIO: The read or store operation is not supported, typically returned by
> >>         the sysfs system itself if the read or store pointer is NULL.
> >>
> >>   -ENXIO: The read or store operation failed
> >
> > from errno(3):
> >        EIO             Input/output error (POSIX.1)
> >        ENXIO           No such device or address (POSIX.1)
> >
> > It makes sense to retry EIO.  ENXIO means there's nobody listening at the
> > time, and isn't usually retried.
> >
> > The device-based interfaces get it right.  A typical example is the
> > cpu-based devices, where ENXIO means "no such processor", while EIO means
> > "whatever you're trying to do failed",  so a MSR read would return ENXIO if
> > the processor core is offline/doesn't exist, and EIO if the processor core
> > is there, but raised a #GP when the MSR read was attempted.
> 
> Here's something I don't quite understand. How should one then
> distinguish between sysfs's use of EIO "can't (read from|write to)
> this file", and this example's EIO "something went wrong, you might
> want to try again"? Why not use EAGAIN "Resource temporarily
> unavailable" in the case where trying again makes sense? I wouldn't
> normally retry the last operation if I was just told something
> actually went wrong.
> 
> I've only been at it for a couple of weeks, but I get the impression
> that sysfs has never really been guided regarding error codes, or has
> gone to live its own life now kept in check with "don't change the
> errors, it may break userspace". Does that make sense?

Right, this was the distinction I was trying to make with the above description.
Henrique's points are valid, but based on the sysfs subsystem already using EIO
in the way that it does, I felt the above made sense.

That said, I'm not personally tied to them, it's just what I have derived from
recent discussions on the subject and what I observed in existing usage.

-- 
Darren Hart
Intel Open Source Technology Center
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