linux-kernel - Re: [PATCH 3/3][V3] iio: Handle enumerated properties with gaps

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Date:   Fri, 5 Jul 2019 13:35:14 +0000
From:   "Ardelean, Alexandru" <alexandru.Ardelean@...log.com>
To:     "andriy.shevchenko@...ux.intel.com" 
        <andriy.shevchenko@...ux.intel.com>
CC:     "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "jic23@...nel.org" <jic23@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
        "lars@...afoo.de" <lars@...afoo.de>
Subject: Re: [PATCH 3/3][V3] iio: Handle enumerated properties with gaps

On Thu, 2019-05-09 at 10:31 +0300, Alexandru Ardelean wrote:
> On Wed, 2019-05-08 at 16:17 +0300, Andy Shevchenko wrote:
> > [External]
> > 
> > 
> > On Wed, May 08, 2019 at 02:19:13PM +0300, Alexandru Ardelean wrote:
> > > From: Lars-Peter Clausen <lars@...afoo.de>
> > > 
> > > Some enums might have gaps or reserved values in the middle of their
> > > value
> > > range. E.g. consider a 2-bit enum where the values 0, 1 and 3 have a
> > > meaning, but 2 is a reserved value and can not be used.
> > > 
> > > Add support for such enums to the IIO enum helper functions. A reserved
> > > values is marked by setting its entry in the items array to NULL rather
> > > than the normal descriptive string value.
> > > 
> > > Also, `__sysfs_match_string()` now supports NULL gaps, so that doesn't
> > > require any changes.
> > > -     for (i = 0; i < e->num_items; ++i)
> > > +     for (i = 0; i < e->num_items; ++i) {
> > > +             if (!e->items[i])
> > > +                     continue;
> > >               len += scnprintf(buf + len, PAGE_SIZE - len, "%s ", e-
> > > > items[i]);
> > > +     }
> > 
> > The problem here that the user will have no clue where the gap is
> > happened, to
> > solve this we need either bitmap of array, where set bits shows defined
> > items,
> > or use comma-separated list of values. The latter would need another node
> > since
> > we don't break user space.
> 
> Hmmm.
> I am wondering if there are cases where userspace would care about reserved
> values and/or positions of reserved bit-fields.
> Maybe you could offer examples/use-cases where this is needed.
> 
> To some extent the kernel [drivers & frameworks] should probably not need
> to expose that "string-enum-X"  == `bitfield_2` matching; otherwise it
> doesn't really add much value ; the whole point of frameworks [in general]
> is to offer some level of abstraction to HW.
> 
> The only example I can think of [atm], is when a reserved bit-field will be
> used in the future. But then, the driver should care about this, and not
> the framework. The driver should decide that "bitfield_2" will
> enable/disable something [in the future], and should be considered in a
> such a way (when being written). If the driver can't make this prediction [
> about "bitfield_2"] then a new driver must be written anyway.
> 
> But I will agree that I may not have all arguments in mind to be 100% sure
> of all this.
> 

Hey,

Is there any feedback/counter-arguments for this?

Thanks
Alex


> Thanks
> Alex
> 
> > --
> > With Best Regards,
> > Andy Shevchenko
> > 
> >