linux-kernel - Re: [PATCH v3 1/3] iio: mpl3115: use get_unaligned

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Message-ID: <20251110155932.o2oipfzuxhgq4vn4@antoni-VivoBook-ASUSLaptop-X512FAY-K512FA>
Date: Mon, 10 Nov 2025 16:59:32 +0100
From: Antoni Pokusinski <apokusinski01@...il.com>
To: Jonathan Cameron <jic23@...nel.org>, marcelo.schmitt1@...il.com
Cc: andy@...nel.org, nuno.sa@...log.com, dlechner@...libre.com,
	linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/3] iio: mpl3115: use get_unaligned_be24 to retrieve
 pressure data

On Sun, Nov 09, 2025 at 04:38:40PM +0000, Jonathan Cameron wrote:
> On Thu, 6 Nov 2025 22:33:49 -0300
> Marcelo Schmitt <marcelo.schmitt1@...il.com> wrote:
> 
> > On 11/05, Antoni Pokusinski wrote:
> > > The pressure measurement result is arranged as 20-bit unsigned value
> > > residing in three 8-bit registers. Hence, it can be retrieved using
> > > get_unaligned_be24 and by applying 4-bit shift.
> > > 
> > > Signed-off-by: Antoni Pokusinski <apokusinski01@...il.com>
> > > ---
> > >  drivers/iio/pressure/mpl3115.c | 7 ++++---
> > >  1 file changed, 4 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/drivers/iio/pressure/mpl3115.c b/drivers/iio/pressure/mpl3115.c  
> > ...
> > >  
> > > -		*val = be32_to_cpu(tmp) >> chan->scan_type.shift;
> > > +		*val = get_unaligned_be24(tmp) >> 4;  
> > hmm, now the number of bits shifted is dissociated from the channel characteristics.
> > We can do
> > 		*val = get_unaligned_be24(tmp) >> (24 - chan->scan_type.realbits);
> This encodes that the field is always aligned to the maximum bit. Whilst it might
> be true, there is nothing inherent that says it must be.
> 
> I'm not sure why we aren't using chan->scan_type.shift though.
The chan->scan_type.shift is 12 for the pressure channel, because
.realbits is 32. In order to better reflect the actual data format,
the pressure .shift and .realbits should be changed to 4 and 24 respectively
and the we could use the chan->scan_type.shift in here indeed.

But then the `iio_generic_buffer` tool should also be updated so that it
can manage the scan_data with realbits not being in the form 2^n.
Currently it supports only scan sizes of 1,2,4,8 bytes [1].

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/jic23/iio.git/tree/tools/iio/iio_generic_buffer.c#n189
> 
> > or maybe
> > 		*val = get_unaligned_be24(tmp) >> (sizeof(tmp) - chan->scan_type.realbits);
> 
> That one needs a BYTES_TO_BITS factor too.
> 
> > but it starts becoming too long IMO. Even longer if `tmp` gets a more meaningful
> > name. Ah well, any of the three forms should work the same at the end of day so
> > no strong opinion.
> > 
> > Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@...il.com>
> > 
> > >  		return IIO_VAL_INT;
> > >  	}
> > >  	case IIO_TEMP: { /* in 0.0625 celsius / LSB */
> > > -- 
> > > 2.25.1
> > >   
>
Kind regards,
Antoni Pokusinski