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

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Message-ID: <aID6jfjULn2kvvQJ@smile.fi.intel.com>
Date: Wed, 23 Jul 2025 18:06:53 +0300
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: David Lechner <dlechner@...libre.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 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?

-- 
With Best Regards,
Andy Shevchenko