Re: [PATCH 2/6] iio: adc: ti-adc128s052: Fix ADC value on BE systems

[Matti Vaittinen]

On 31/03/2025 14:11, Jonathan Cameron wrote:
> On Mon, 31 Mar 2025 11:02:55 +0300
> Matti Vaittinen <mazziesaccount@xxxxxxxxx> wrote:
>
> > ADCs supported by the ti-adc128s052 driver do return the ADC data in 16
> > bits using big-endian format. The driver does unconditionally swap the
> > bytes. This leads to wrong values being reported to users on big endian
> > systems.
> >
> > Fix this by using the be16_to_cpu() instead of doing unconditional byte
> > swapping.

Appears this was one of the patches I should've never written. Nothing 
went right :) Sorry for the noise. I'll try improving for the v2

> It's not doing unconditional byte swap that I can see. The
> adc->buffer[0] << 8 | adc->buffer[1]
> will work on big or little endian systems as we are explicitly saying
> which byte represents higher bit values in a 16 bit output so on little
> endian it's a byte swap, but on big endian it's a noop (the compiler might
> noticed that and replace this code sequence with an assignment)
>
> Good cleanup, but not a fix as such unless I'm missing something.

No, you're not missing anything. I am the one who just got confused. I 
am not exactly sure what I was thinking. :rolleyes: This definitely 
isn't a fix. And, as a not a fix needing porting, I may squash this with 
some other patch. I need to take another look at this :)

> > Fixes: 913b86468674 ("iio: adc: Add TI ADC128S052")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Matti Vaittinen <mazziesaccount@xxxxxxxxx>
> > ---
> > I have no big endian machines on my hands to test this. Problem was
> > spotted by reading the code, which leaves some room for errors.
> > Careful reviewing is appreciated!
> > ---
> >   drivers/iio/adc/ti-adc128s052.c | 13 +++++++------
> >   1 file changed, 7 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/iio/adc/ti-adc128s052.c b/drivers/iio/adc/ti-adc128s052.c
> > index a456ea78462f..d1e31122ea0d 100644
> > --- a/drivers/iio/adc/ti-adc128s052.c
> > +++ b/drivers/iio/adc/ti-adc128s052.c
> > @@ -28,19 +28,20 @@ struct adc128 {
> >   	struct regulator *reg;
> >   	struct mutex lock;
> >   
> > -	u8 buffer[2] __aligned(IIO_DMA_MINALIGN);
> > +	__be16 buffer __aligned(IIO_DMA_MINALIGN);
> >   };
> >   
> >   static int adc128_adc_conversion(struct adc128 *adc, u8 channel)
> >   {
> >   	int ret;
> > +	char *msg = (char *)&adc->buffer;
> >   
> > -	mutex_lock(&adc->lock);
> > +	msg[0] = channel << 3;
> > +	msg[1] = 0;
>
> Given you are writing shared state why move this out of the lock?

Very Valid Point. I'm not 100% sure what I thought of, probably assumed 
IIO core would serialize the calls. That would've been nasty bug! I 
appreciate your sharp eyes :)

Thanks!

Yours,
	-- Matti