Re: [PATCH v4 1/5] tools: iio: iio_generic_buffer ensure alignment

[Matti Vaittinen <mazziesaccount@gmail.com>]

On 9/30/23 19:34, Jonathan Cameron wrote:
> On Wed, 27 Sep 2023 11:26:07 +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 the 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 element is first in the buffer.
>>
>> The IIO generic buffer code does not take into account the last two
>> padding bytes that are needed to ensure that the 4-byte data for next
>> scan is correctly aligned.
>>
>> Add the padding bytes required to align the next sample with the scan size.
>>
>> Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
>>
>> ---
>> I think the whole alignment code could be revised here, but I am unsure
>> what kind of alignment is expected, and if it actually depends on the
>> architecture. Anyways, I'll quote myself from another mail to explain
>> how this patch handles things:
>>
>>> For non power of2 sizes, the alignment code will result strange alignments.
>>> For example, scan consisting of two 6-byte elements would be packed -
>>> meaning the second element would probably break the alignment rules by
>>> starting from address '6'. I think that on most architectures the proper
>>> access would require 2 padding bytes to be added at the end of the first
>>> sample. Current code wouldn't do that.
>>
>>> If we allow only power of 2 sizes - I would expect a scan consisting of a
>>> 8 byte element followed by a 16 byte element to be tightly packed. I'd
>>> assume that for the 16 byte data, it'd be enough to ensure 8 byte alignment.
>>> Current code would however add 8 bytes of padding at the end of the first
>>> 8 byte element to make the 16 byte scan element to be aligned at 16 byte
>>> address. To my uneducated mind this is not needed - but maybe I just don't
>>> know what I am writing about :)
>>
>> Revision history
>> v3 => v4:
>>   - drop extra print and TODO coment
>>   - add comment clarifying alignment sizes
>> ---
>>   tools/iio/iio_generic_buffer.c | 18 +++++++++++++++++-
>>   1 file changed, 17 insertions(+), 1 deletion(-)
>>
>> diff --git a/tools/iio/iio_generic_buffer.c b/tools/iio/iio_generic_buffer.c
>> index 44bbf80f0cfd..c07c49397b19 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 +69,19 @@ 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.
>> +	 *
>> +	 * Please note, this code does ensure alignment to maximum channel
>> +	 * size. It works only as long as the channel sizes are 1, 2, 4 or 8
>> +	 * bytes. Also, on 32 bit platforms it might be enough to align also
>> +	 * the 8 byte elements to 4 byte boundary - which this code is not
>> +	 * doing.
> Very much not!  We need to present same data alignment to userspace
> indpendent of what architecture is running.
> 
> It's annoyingly inconsistent how 8 byte elements are handled on 32 bit
> architectures as some have optimized aligned access routines and others
> will read as 2 32 bit fields.  Hence we just stick to 8 byte value is
> 8 byte aligned which is always fine but wastes a bit of space on x86 32
> bit - which I don't care about ;)
> 
> Please drop this last bit of the comment as we should just say what it
> does, not conjecture what it might do!

Ok. The comment was more to catch the reviewers' attention ;) I'll just 
note the alignment works for power of 2 sample sizes and aligns 
according to the max sized sample, even if it was bigger than 8.

Thanks!

-- Matti


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

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