linux-kernel - Re: [PATCH v3 1/6] tools: iio: iio_generic

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Date:   Mon, 25 Sep 2023 10:01:09 +0300
From:   Matti Vaittinen <mazziesaccount@...il.com>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Angel Iglesias <ang.iglesiasg@...il.com>,
        Andreas Klinger <ak@...klinger.de>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Benjamin Bara <bbara93@...il.com>, linux-iio@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/6] tools: iio: iio_generic_buffer ensure alignment

On 9/24/23 18:57, Jonathan Cameron wrote:
> On Fri, 22 Sep 2023 14:16:08 +0300
> Matti Vaittinen <mazziesaccount@...il.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@...il.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

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