linux-kernel - RE: [PATCH v3 0/6] iio: Add output buffer support

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Message-ID: <CY4PR03MB26318D0C0553CB04E53DA21299939@CY4PR03MB2631.namprd03.prod.outlook.com>
Date:   Mon, 8 Mar 2021 13:17:43 +0000
From:   "Sa, Nuno" <Nuno.Sa@...log.com>
To:     "Sa, Nuno" <Nuno.Sa@...log.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Jonathan Cameron <jic23@...nel.org>
CC:     "Hennerich, Michael" <Michael.Hennerich@...log.com>,
        "zzzzArdelean, zzzzAlexandru" <alexandru.Ardelean@...log.com>,
        "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>,
        "Bogdan, Dragos" <Dragos.Bogdan@...log.com>
Subject: RE: [PATCH v3 0/6] iio: Add output buffer support



> -----Original Message-----
> From: Sa, Nuno <Nuno.Sa@...log.com>
> Sent: Monday, March 8, 2021 2:01 PM
> To: Jonathan Cameron <Jonathan.Cameron@...wei.com>; Jonathan
> Cameron <jic23@...nel.org>
> Cc: Hennerich, Michael <Michael.Hennerich@...log.com>;
> zzzzArdelean, zzzzAlexandru <alexandru.Ardelean@...log.com>;
> linux-kernel@...r.kernel.org; linux-iio@...r.kernel.org;
> lars@...afoo.de; Bogdan, Dragos <Dragos.Bogdan@...log.com>
> Subject: RE: [PATCH v3 0/6] iio: Add output buffer support
> 
> [External]
> 
> 
> 
> > -----Original Message-----
> > From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> > Sent: Monday, March 8, 2021 12:52 PM
> > To: Sa, Nuno <Nuno.Sa@...log.com>; Jonathan Cameron
> > <jic23@...nel.org>
> > Cc: Hennerich, Michael <Michael.Hennerich@...log.com>;
> > zzzzArdelean, zzzzAlexandru <alexandru.Ardelean@...log.com>;
> > linux-kernel@...r.kernel.org; linux-iio@...r.kernel.org;
> > lars@...afoo.de; Bogdan, Dragos <Dragos.Bogdan@...log.com>
> > Subject: Re: [PATCH v3 0/6] iio: Add output buffer support
> >
> > [External]
> >
> > On Mon, 8 Mar 2021 10:07:05 +0000
> > "Sa, Nuno" <Nuno.Sa@...log.com> wrote:
> >
> > > > -----Original Message-----
> > > > From: Jonathan Cameron <jic23@...nel.org>
> > > > Sent: Saturday, March 6, 2021 6:35 PM
> > > > To: Hennerich, Michael <Michael.Hennerich@...log.com>
> > > > Cc: zzzzArdelean, zzzzAlexandru
> > <alexandru.Ardelean@...log.com>;
> > > > linux-kernel@...r.kernel.org; linux-iio@...r.kernel.org;
> > > > lars@...afoo.de; Sa, Nuno <Nuno.Sa@...log.com>; Bogdan,
> > Dragos
> > > > <Dragos.Bogdan@...log.com>
> > > > Subject: Re: [PATCH v3 0/6] iio: Add output buffer support
> > > >
> > > > On Fri, 5 Mar 2021 08:57:08 +0000
> > > > "Hennerich, Michael" <Michael.Hennerich@...log.com> wrote:
> > > >
> > > > > Hi Jonathan and others,
> > > > >
> > > > > With output/dac buffer support the semantics of the
> > scan_element
> > > > type may change.
> > > > >
> > > > > Today the Format is
> > [be|le]:[s|u]bits/storagebitsXrepeat[>>shift].
> > > > >
> > > > > While shift (if specified) is the shift that needs to be applied
> prior
> > to
> > > > masking out unused bits.
> > > > >
> > > > > So far so good and it sounds universal.
> > > > >
> > > > > However, we use the right shift (operator) for that, which
> makes
> > > > sense for capture devices.
> > > > > For output devices the more logical operator would be the left
> > shift.
> > > > >
> > > > > I'm not proposing a new Format here. I just want to get some
> > > > agreement that for an output device
> > > > >
> > > > > le:s12/16>>4
> > > > >
> > > > > is understood as a left shift of 4, since the unused bits are then
> > on
> > > > the LSB.
> > > >
> > > > Good question. Guess I wasn't thinking ahead when I came up
> with
> > > > that :)
> > > >
> > > > I'm not sure I'd mind if we did decide to define a new format for
> > > > output
> > > > buffers. Feels like it should be easy to do.
> > > >
> > > > What do others think?
> > > >
> > >
> > > I guess the most straight forward thing would be just to add a
> 'shift_l'
> > variable
> > > to 'struct scan_type'' and make sure either 'shift_l' or 'shift' is
> defined
> > and then
> > > properly export either ">>" or "<<" to userspace?
> >
> > Given we already know it's an output channel, can we not just use
> that
> > to make the decision?
> >
> > Jonathan
> 
> I would argue that having two variables gives us more flexibility for
> whatever
> the future brings us :). But if we can sanely say that an output buffer
> will
> always use left shifts, then we could definitely use that information... I
> mean,
> we are already doing that assumption for input buffers and right
> shifts...

Hmm, giving it a bit more thought I think you can disregard the above.
Using the information that it's an output channel should be more than
enough...

- Nuno Sá