Re: [PATCH v3] iio: buffer: align the size of scan bytes to size of the largest element

[Jonathan Cameron <jic23@kernel.org>]

On Mon, 16 Dec 2019 08:51:27 +0100
Lars-Peter Clausen <lars@metafoo.de> wrote:

> On 12/15/19 10:09 PM, Lars Möllendorf wrote:
> > -----Ursprüngliche Nachricht-----  
> >> Von: Lars Möllendorf <lars.moellendorf@plating.de>
> >> Gesendet: Freitag 13 Dezember 2019 14:58
> >> An: Jonathan Cameron <jic23@kernel.org>; Hartmut Knaack <knaack.h@gmx.de>; Lars-Peter Clausen <lars@metafoo.de>; Peter Meerwald-Stadler <pmeerw@pmeerw.net>; linux-iio@vger.kernel.org
> >> CC: Lars Möllendorf <lars.moellendorf@plating.de>
> >> Betreff: [PATCH v3] iio: buffer: align the size of scan bytes to size of the largest element
> >>
> >> Previous versions of `iio_compute_scan_bytes` only aligned each element
> >> to its own length (i.e. its own natural alignment). Because multiple
> >> consecutive sets of scan elements are buffered this does not work in
> >> case the computed scan bytes do not align with the natural alignment of
> >> the first scan element in the set.
> >>
> >> This commit fixes this by aligning the scan bytes to the natural
> >> alignment of the largest scan element in the set.  
> > 
> > 
> > 
> > After re-reading my commit message, I come to the conclusion that it really is sufficient to align the scan bytes to the natural alignment of the *first* element. This would save us the `max()` comparisons for each bit. At the moment I am not at my workstation, but I could submit a v4 next Friday.
> >   
> 
> I thought so too in the beginning, but as Jonathan pointed out, it does
> not work for all cases. Lets say you have u16,u16,u32,u16. If all
> channels are enabled the size is aligned to the first element, but the
> u32 would not be aligned in the second dataset.
> 

I'm sitting on this at the moment... Can I confirm we have consensus
that this patch is the correct fix?

Lars and Lars?

Thanks,

Jonathan


> > 
> >   
> >> Fixes: 959d2952d124 ("staging:iio: make iio_sw_buffer_preenable much more
> >> general.")
> >> Signed-off-by: Lars Möllendorf <lars.moellendorf@plating.de>
> >> ---
> >> v3:
> >>   - Fix the problem description in the commit message
> >>   - Add "Fixes" tag
> >>
> >> v2:
> >>   - Fix subject of patch which marked it the first in a set of three.
> >>   - Add a description of the problem in the commit message
> >>
> >> ---
> >>  drivers/iio/industrialio-buffer.c | 6 +++++-
> >>  1 file changed, 5 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-buffer.c
> >> index 5d05c38c4ba9..2f037cd59d53 100644
> >> --- a/drivers/iio/industrialio-buffer.c
> >> +++ b/drivers/iio/industrialio-buffer.c
> >> @@ -546,7 +546,7 @@ static int iio_compute_scan_bytes(struct iio_dev *indio_dev,
> >>  				const unsigned long *mask, bool timestamp)
> >>  {
> >>  	unsigned bytes = 0;
> >> -	int length, i;
> >> +	int length, i, largest = 0;
> >>
> >>  	/* How much space will the demuxed element take? */
> >>  	for_each_set_bit(i, mask,
> >> @@ -554,13 +554,17 @@ static int iio_compute_scan_bytes(struct iio_dev *indio_dev,
> >>  		length = iio_storage_bytes_for_si(indio_dev, i);
> >>  		bytes = ALIGN(bytes, length);
> >>  		bytes += length;
> >> +		largest = max(largest, length);
> >>  	}
> >>
> >>  	if (timestamp) {
> >>  		length = iio_storage_bytes_for_timestamp(indio_dev);
> >>  		bytes = ALIGN(bytes, length);
> >>  		bytes += length;
> >> +		largest = max(largest, length);
> >>  	}
> >> +
> >> +	bytes = ALIGN(bytes, largest);
> >>  	return bytes;
> >>  }
> >>
> >> --
> >> 2.23.0
> >>  
>