Messages in this thread | | | Date | Mon, 25 Sep 2023 10:01:09 +0300 | Subject | Re: [PATCH v3 1/6] tools: iio: iio_generic_buffer ensure alignment | From | Matti Vaittinen <> |
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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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