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SubjectRe: [PATCH v3 1/6] tools: iio: iio_generic_buffer ensure alignment
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On 9/24/23 18:57, Jonathan Cameron wrote:
> On Fri, 22 Sep 2023 14:16:08 +0300
> Matti Vaittinen <mazziesaccount@gmail.com> wrote:
>
>> The iio_generic_buffer can return garbage values when the total size of
>> scan data is not a multiple of largest element in the scan. This can be
>> demonstrated by reading a scan consisting for example of one 4 byte and
>> one 2 byte element, where the 4 byte elemnt is first in the buffer.
>>
>> The IIO generic buffert code does not take into accunt the last two
>> padding bytes that are needed to ensure that the 4byte data for next
>> scan is correctly aligned.
>>
>> Add padding bytes required to align the next sample into the scan size.
>>
>> Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
>> ---
>> Please note, This one could have RFC in subject.:
>> I attempted to write the fix so that the alignment is done based on the
>> biggest channel data. This may be wrong. Maybe a fixed 8 byte alignment
>> should be used instead? This patch can be dropped from the series if the
>> fix is not correct / agreed.
>>
>> tools/iio/iio_generic_buffer.c | 15 ++++++++++++++-
>> 1 file changed, 14 insertions(+), 1 deletion(-)
>>
>> diff --git a/tools/iio/iio_generic_buffer.c b/tools/iio/iio_generic_buffer.c
>> index 44bbf80f0cfd..fc562799a109 100644
>> --- a/tools/iio/iio_generic_buffer.c
>> +++ b/tools/iio/iio_generic_buffer.c
>> @@ -54,9 +54,12 @@ enum autochan {
>> static unsigned int size_from_channelarray(struct iio_channel_info *channels, int num_channels)
>> {
>> unsigned int bytes = 0;
>> - int i = 0;
>> + int i = 0, max = 0;
>> + unsigned int misalignment;
>>
>> while (i < num_channels) {
>> + if (channels[i].bytes > max)
>> + max = channels[i].bytes;
>> if (bytes % channels[i].bytes == 0)
>> channels[i].location = bytes;
>> else
>> @@ -66,6 +69,16 @@ static unsigned int size_from_channelarray(struct iio_channel_info *channels, in
>> bytes = channels[i].location + channels[i].bytes;
>> i++;
>> }
>> + /*
>> + * We wan't the data in next sample to also be properly aligned so
>> + * we'll add padding at the end if needed. TODO: should we use fixed
>> + * 8 byte alignment instead of the size of the biggest samnple?
>> + */
>
> Should be aligned to max size seen in the scan.

Or, maybe it should be
min(max_size_in_scan, 8);
?

I think my suggestion above may yield undesirable effects should the
scan elements be greater than 8 bytes. (Don't know if this is supported
though)

>
>> + misalignment = bytes % max;
>> + if (misalignment) {
>> + printf("Misalignment %u. Adding Padding %u\n", misalignment, max - misalignment);
>
> No print statement as this is correct behaviour (well the tool is buggy but the kernel generates it
> correctly I believe). Fine to add a comment though!

Oh, indeed. The print was forgotten from my test runs. Thanks for
pointing it out!

>
>> + bytes += max - misalignment;
>> + }
>>
>> return bytes;
>> }
>

Yours,
-- Matti

--
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland

~~ When things go utterly wrong vim users can always type :help! ~~

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