On 26.12.19 22:16, Lars-Peter Clausen wrote:
> On 12/23/19 6:05 PM, Jonathan Cameron wrote:
>> 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?
>
> Current version looks good to me.
I agree that the current implementation will fix the aforementioned cases.
> Reviewed-by: Lars-Peter Clausen <lars@xxxxxxxxxx>
>
>>
>> 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
>>>>>
>>>
>>
>
--
Lars Möllendorf, B. Eng.
Tel.: +49 (0) 7641 93500-425
Fax: +49 (0) 7641 93500-999
E-Mail: lars.moellendorf@xxxxxxxxxx <mailto:lars.moellendorf@xxxxxxxxxx>
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