[PATCH 4/4] spi: bcm2835aux: fix CPOL/CPHA setting

[Stephan Olbrich]

Martin,

Am Thursday 11 February 2016, 16:25:43 schrieb Martin Sperl:
> > On 10.02.2016, at 01:13, Eric Anholt <eric at anholt.net> wrote:
> > 
> > stephanolbrich at gmx.de writes:
> >> From: Stephan Olbrich <stephanolbrich at gmx.de>
> >> 
> >> The auxiliary spi supports only CPHA=0 modes as the first bit is
> >> always output to the pin before the first clock cycle. In CPHA=1
> >> modes the first clock edge outputs the second bit hence the slave
> >> can never read the first bit.
> >> 
> >> Also the CPHA registers switch between clocking data in/out on
> >> rising/falling edge hence depend on the CPOL setting.
> >> 
> >> Signed-off-by: Stephan Olbrich <stephanolbrich at gmx.de>
> >> ---
> >> drivers/spi/spi-bcm2835aux.c | 10 +++++-----
> >> 1 file changed, 5 insertions(+), 5 deletions(-)
> >> 
> >> diff --git a/drivers/spi/spi-bcm2835aux.c b/drivers/spi/spi-bcm2835aux.c
> >> index b90aa34..169f521 100644
> >> --- a/drivers/spi/spi-bcm2835aux.c
> >> +++ b/drivers/spi/spi-bcm2835aux.c
> >> @@ -386,12 +386,12 @@ static int bcm2835aux_spi_prepare_message(struct
> >> spi_master *master,>> 
> >> 	bs->cntl[1] = BCM2835_AUX_SPI_CNTL1_MSBF_IN;
> >> 	
> >> 	/* handle all the modes */
> >> 
> >> -	if (spi->mode & SPI_CPOL)
> >> +	if (spi->mode & SPI_CPOL) {
> >> 
> >> 		bs->cntl[0] |= BCM2835_AUX_SPI_CNTL0_CPOL;
> >> 
> >> -	if (spi->mode & SPI_CPHA)
> >> -		bs->cntl[0] |= BCM2835_AUX_SPI_CNTL0_CPHA_OUT |
> >> -			       BCM2835_AUX_SPI_CNTL0_CPHA_IN;
> >> -
> >> +		bs->cntl[0] |= BCM2835_AUX_SPI_CNTL0_CPHA_OUT;
> >> +	} else {
> >> +		bs->cntl[0] |= BCM2835_AUX_SPI_CNTL0_CPHA_IN;
> >> +	}
> >> 
> >> 	bcm2835aux_wr(bs, BCM2835_AUX_SPI_CNTL1, bs->cntl[1]);
> >> 	bcm2835aux_wr(bs, BCM2835_AUX_SPI_CNTL0, bs->cntl[0]);
> > 
> > (Note for other readers: A better name for CNTL0_CPHA_* would be
> > CNTL0_*_RISING).
> > 
> > I think you're right about not actually supporting CPHA.  I don't see
> > wany way to keep bit 1 out lasting through the first full clock cycle.
> > I think Stefan's right that we should drop CPHA from MODE_BITS
> > (actually, MODE_BITS would be nicer if we just merged it into its one
> > user, I think).
> > 
> > However, this hunk appears to be correct and would fix the timing of our
> > data going out in the CPOL=0 case and of sampling IN data for CPOL=1.
> 
> Are you sure that CPHA is not working?
> 
> I have used the following to test all combinations:
>  for C in "" "-C"; do
>   for O in "" "-O"; do
>    for H in "" "-H"; do
>     spidev_test -vD /dev/*.1 $H $O -p "\x00\x82\x00" -s 1000000;
>    done
>   done
>  done
> 
> I have tested with the 4.5-rc3 kernel without any of your patches.
> 
> And looked at the logic-analyzer: I see distinct waveform pattern
> for clk and data for all events!

The problem I see, is not that there is no waveform but that it has the wrong 
timing.
I'll make an example:
Data is 0xAA,0xFF, CPOL=1, CPHA=1
What happens is this:
CS goes active and MOSI goes high (first bit in 1)
After a short time, SCLK goes from low to high and MOSI goes to low (second 
bit is 0)
Then SCLK goes low (sampling data)
Then SCLK goes high and MOSI goes high (third bit is 1) 
and so on.
At the end with the last rising edge of SCLK MOSI goes to low and stays there 
while SCLK goes to low and then CS to inactive.

So the slave has no chance to get the first bit but gets an additional 0 at the 
end.
It may not be easy visible with 0x00,0x82,0x00 as data, so could you repeat 
your test with 0xAA,0xFF if you see the same as I do or not?

Thanks,
Stephan