Re: [PATCH] spi: set bits_per_word based on controller's bits_per_word_mask

[Alvaro Gamez Machado <alvaro.gamez@xxxxxxxxxx>]

On Thu, Oct 24, 2019 at 12:13:00PM +0100, Mark Brown wrote:
> On Thu, Oct 24, 2019 at 01:07:57PM +0200, Alvaro Gamez Machado wrote:
> > By leaving this value unset, a default value of 8 was being set later on.
> > 
> > If it happens that the SPI master driver doesn't support this value of 8,
> > there will be an initial inconsistency between the SPI master and the device
> > itself. This isn't a problem for most devices because kernel drivers
> 
> This will break things, client devices are working on the basis that the
> default transfer width is 8 bits.  As I've repeatedly said if we have
> different parts of the system with different ideas about the word size
> we're going to end up with data corruption.  Please take this feedback
> on board.

Oh, ok. I didn't understand this cleary from previous mails, now I see what
you mean.

I think then the only way this would be feasible is to check if 8 bits is an
acceptable number for the master and, if it isn't, apply the lowest
available data width. I believe this cannot break anything, as it leaves 8
as the default unless the master can't work with that number, in which case
it really doesn't matter what client device wants because the hardware can't
provide it.

Thanks!

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 794e20e54237..4e26ac79e133 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -3079,8 +3079,12 @@ int spi_setup(struct spi_device *spi)
                return -EINVAL;
        }
 
-       if (!spi->bits_per_word)
-               spi->bits_per_word = 8;
+       if (!spi->bits_per_word) {
+               if (spi->controller->bits_per_word_mask & SPI_BPW_MASK(8))
+                       spi->bits_per_word = 8;
+               else
+                       spi->bits_per_word = ffs(spi->controller->bits_per_word_mask);
+       }
 
        status = __spi_validate_bits_per_word(spi->controller,
                                              spi->bits_per_word);





-- 
Alvaro G. M.