Re: [PATCH 2/4] iio: ad7949: fix incorrect SPI xfer len

[Jonathan Cameron <jic23@kernel.org>]

On Mon, 16 Sep 2019 07:51:30 +0000
"Ardelean, Alexandru" <alexandru.Ardelean@analog.com> wrote:

> On Sun, 2019-09-15 at 11:36 +0100, Jonathan Cameron wrote:
> > On Fri, 13 Sep 2019 09:56:56 +0200
> > Andrea Merello <andrea.merello@gmail.com> wrote:
> >   
> > > Il giorno ven 13 set 2019 alle ore 08:46 Ardelean, Alexandru
> > > <alexandru.Ardelean@analog.com> ha scritto:  
> > > > On Thu, 2019-09-12 at 16:43 +0200, Andrea Merello wrote:    
> > > > > [External]
> > > > > 
> > > > > This driver supports 14-bits and 16-bits devices. All of them have a 14-bit
> > > > > configuration registers. All SPI trasfers, for reading AD conversion
> > > > > results and for writing the configuration register, fit in two bytes.
> > > > > 
> > > > > The driver always uses 4-bytes xfers which seems at least pointless (maybe
> > > > > even harmful). This patch trims the SPI xfer len and the buffer size to
> > > > > two bytes.
> > > > >    
> > > > 
> > > > The length reduction proposal is fine.
> > > > 
> > > > But, this patch raises a question about endianess.
> > > > I'm actually wondering here if we need to see about maybe using a __be16 vs u16.
> > > > 
> > > > I'm not that kernel-savy yet about some of these low-level things to be completely sure here.
> > > > So, I'd let someone else maybe handle it.    
> > > 
> > > Good point.. It seems that indeed not much care has been taken about
> > > endianess here.. Probably we need also some le16_to_cpu() and
> > > firends..  
> > 
> > More complexity here :)  So a lot of earlier SPI drivers didn't set bits_per_word,
> > the result of this is that a read had no way to know how to unwind the endian
> > nature of the data.  If you do a 4 byte read, is that 4x 1 byte, 2x 2 bytes or
> > 1x 4 bytes.  Thus the SPI subsystem had no way of knowing how to convert from
> > wire order of big endian to cpu endianness.  This is particularly fun as it
> > is common to have variable length registers on SPI devices (be it described
> > on the datasheet as some registers have high and low byte addresses).
> > 
> > In drivers where this can be set to one consistent value, then the SPI subsystem
> > should do the work for us. Hence this one should be fine. ( I think :)
> >   
> 
> Based on other input:
> 
> Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>

I've applied this to the togreg branch of iio.git and pushed out as testing
for the autobuilders to play with it.

Note, as we don't have a proven case in which it causes actual harm, I haven't
marked it for stable.

Thanks,

Jonathan

> 
> > > Maybe another separate patch can be made to take care about endianess later on?
> > > 
> > > BTW Also, the  ____cacheline_aligned is a bit scaring :) I don't know
> > > what is that for...
> > >   
> > > > Thanks
> > > > Alex
> > > >    
> > > > > Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
> > > > > ---
> > > > >  drivers/iio/adc/ad7949.c | 6 +++---
> > > > >  1 file changed, 3 insertions(+), 3 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/iio/adc/ad7949.c b/drivers/iio/adc/ad7949.c
> > > > > index 518044c31a73..5c2b3446fa4a 100644
> > > > > --- a/drivers/iio/adc/ad7949.c
> > > > > +++ b/drivers/iio/adc/ad7949.c
> > > > > @@ -54,7 +54,7 @@ struct ad7949_adc_chip {
> > > > >       u8 resolution;
> > > > >       u16 cfg;
> > > > >       unsigned int current_channel;
> > > > > -     u32 buffer ____cacheline_aligned;
> > > > > +     u16 buffer ____cacheline_aligned;
> > > > >  };
> > > > > 
> > > > >  static int ad7949_spi_write_cfg(struct ad7949_adc_chip *ad7949_adc, u16 val,
> > > > > @@ -67,7 +67,7 @@ static int ad7949_spi_write_cfg(struct ad7949_adc_chip *ad7949_adc, u16 val,
> > > > >       struct spi_transfer tx[] = {
> > > > >               {
> > > > >                       .tx_buf = &ad7949_adc->buffer,
> > > > > -                     .len = 4,
> > > > > +                     .len = 2,
> > > > >                       .bits_per_word = bits_per_word,
> > > > >               },
> > > > >       };
> > > > > @@ -95,7 +95,7 @@ static int ad7949_spi_read_channel(struct ad7949_adc_chip *ad7949_adc, int *val,
> > > > >       struct spi_transfer tx[] = {
> > > > >               {
> > > > >                       .rx_buf = &ad7949_adc->buffer,
> > > > > -                     .len = 4,
> > > > > +                     .len = 2,
> > > > >                       .bits_per_word = bits_per_word,
> > > > >               },
> > > > >       };