On Mon, 16 Dec 2019 08:51:27 +0100
Lars-Peter Clausen <lars@xxxxxxxxxx> wrote:
> On 12/15/19 10:09 PM, Lars Möllendorf wrote:
> > -----Ursprüngliche Nachricht-----
> >> Von: Lars Möllendorf <lars.moellendorf@xxxxxxxxxx>
> >> Gesendet: Freitag 13 Dezember 2019 14:58
> >> An: Jonathan Cameron <jic23@xxxxxxxxxx>; Hartmut Knaack <knaack.h@xxxxxx>; Lars-Peter Clausen <lars@xxxxxxxxxx>; Peter Meerwald-Stadler <pmeerw@xxxxxxxxxx>; linux-iio@xxxxxxxxxxxxxxx
> >> CC: Lars Möllendorf <lars.moellendorf@xxxxxxxxxx>
> >> 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@xxxxxxxxxx>
> >> ---
> >> 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
> >>
>
