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

Frans Klaver
2014-09-17T11:57:35Z

On Wed, Sep 17, 2014 at 12:34 PM, Henrique de Moraes Holschuh
<hmh@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?

Thanks,
Frans