linux-kernel - Re: [PATCH] iio: proximity: hx9023s: fix scan

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Message-ID: <f74d6542-cc4e-40dd-8ef9-2a766d0b51ef@baylibre.com>
Date: Wed, 23 Jul 2025 10:11:50 -0500
From: David Lechner <dlechner@...libre.com>
To: Andy Shevchenko <andriy.shevchenko@...el.com>
Cc: Jonathan Cameron <jic23@...nel.org>, Nuno Sá
 <nuno.sa@...log.com>, Andy Shevchenko <andy@...nel.org>,
 Yasin Lee <yasin.lee.x@...il.com>, linux-iio@...r.kernel.org,
 linux-kernel@...r.kernel.org, Jonathan Cameron <Jonathan.Cameron@...wei.com>
Subject: Re: [PATCH] iio: proximity: hx9023s: fix scan_type endianness

On 7/23/25 10:06 AM, Andy Shevchenko wrote:
> On Wed, Jul 23, 2025 at 09:57:58AM -0500, David Lechner wrote:
>> On 7/23/25 9:37 AM, Andy Shevchenko wrote:
>>> On Wed, Jul 23, 2025 at 09:29:37AM -0500, David Lechner wrote:
>>>> On 7/23/25 9:13 AM, Andy Shevchenko wrote:
>>>>> On Tue, Jul 22, 2025 at 06:08:37PM -0500, David Lechner wrote:
>>>>>> On 7/22/25 6:07 PM, David Lechner wrote:
>>>>>>> Change the scan_type endianness from IIO_BE to IIO_LE. This matches
>>>>>>> the call to cpu_to_le16() in hx9023s_trigger_handler() that formats
>>>>>>> the data before pushing it to the IIO buffer.
>>>>>
>>>>>> It is odd to have data already in CPU-endian and convert it to LE
>>>>>> before pushing to buffers. So I'm a bit tempted to do this instead
>>>>>> since it probably isn't likely anyone is using this on a big-endian
>>>>>> system:
>>>>>
>>>>> I can say that first of all, we need to consult with the datasheet for the
>>>>> actual HW endianess. And second, I do not believe that CPU endianess may be
>>>>> used, 
>>>>
>>>> Why not? Lot's of IIO drivers use IIO_CPU in their scan buffers.
>>>>
>>>>> I can't imagine when this (discrete?) component can be integrated in such
>>>>> a way. That said, I think your second approach even worse.
>>>>
>>>> hx9023s_sample() is calling get_unaligned_le16() on all of the data
>>>> read over the bus, so in the driver, all data is stored CPU-endian
>>>> already rather than passing actual raw bus data to the buffer.
>>>
>>> I see, now it makes a lot of sense. Thanks for clarifying this to me.
>>>
>>>> So it seems a waste of CPU cycles to convert it back to little-endian
>>>> to push to the buffer only for consumers to have to convert it back
>>>> to CPU-endian again. But since most systems are little-endian already
>>>> this doesn't really matter since no actual conversion is done in this
>>>> case.
>>>
>>> Right, but it's buggy on BE, isn't it?
>>>
>>
>> Right now, the driver is buggy everywhere. The scan info says that the
>> scan data is BE, but in reality, it is LE (no matter the CPU-endianness).
>>
>> With the simple patch, it fixes the scan info to reflect reality that
>> the data is LE in the buffer. This works on BE systems. They just have
>> an extra conversion from BE to LE in the kernel when pushing to the
>> buffer and userspace would have to convert back to BE to do math on it.
>>
>> With the alternate patch you didn't like, the forced conversion to LE
>> when pushing to buffers is dropped, so nothing would change on LE
>> systems but BE systems wouldn't have the extra order swapping.
> 
> But do they need that? If you supply CPU order (and it is already in a such
> after get_unaligned_*() calls) then everything would be good, no?
> 

It doesn't make sense to my why, but the existing code is changing
back to LE before pushing to buffers for some reason.


	iio_for_each_active_channel(indio_dev, bit) {
		index = indio_dev->channels[bit].channel;
		data->buffer.channels[i++] = cpu_to_le16(data->ch_data[index].diff);
	}

	iio_push_to_buffers_with_ts(indio_dev, &data->buffer,
				    sizeof(data->buffer), pf->timestamp);

I agree that it seems unnecessary which is why I suggested the
alternate patch to drop the cpu_to_le16() and just leave it
CPU-endian when pushing to the buffers.