Re: [PATCH 2/5] drivers: spi: core: Add optional stall delay between cs_change transfers

["Ardelean, Alexandru" <alexandru.Ardelean@analog.com>]

On Wed, 2019-06-26 at 19:34 +0100, Jonathan Cameron wrote:
> [External]
> 
> On Tue, 25 Jun 2019 16:13:25 +0300
> Alexandru Ardelean <alexandru.ardelean@analog.com> wrote:
> 
> > Some devices like the ADIS16460 IMU require a stall period between
> > transfers, i.e. between when the CS is de-asserted and re-asserted. The
> > default value of 10us is not enough. This change makes the delay
> > configurable for when the next CS change goes active.
> > 
> > Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
> > Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
> 
> General principle seems fine to me, though naming may need some work.
> 
> cs_low_time or something more specific than stall perhaps?

I agree the naming could do some work.
It was one of the items [in this series] for which I was expecting a bit of a discussion.

Thanks
Alex

> 
> +CC Mark.
> 
> > ---
> >  drivers/spi/spi.c       | 3 ++-
> >  include/linux/spi/spi.h | 3 +++
> >  2 files changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
> > index 5e75944ad5d1..739de0118ee1 100644
> > --- a/drivers/spi/spi.c
> > +++ b/drivers/spi/spi.c
> > @@ -1163,7 +1163,8 @@ static int spi_transfer_one_message(struct spi_controller *ctlr,
> >  				keep_cs = true;
> >  			} else {
> >  				spi_set_cs(msg->spi, false);
> > -				udelay(10);
> > +				udelay(xfer->cs_change_stall_delay_us ?
> > +				       xfer->cs_change_stall_delay_us : 10);
> >  				spi_set_cs(msg->spi, true);
> >  			}
> >  		}
> > diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
> > index 053abd22ad31..d23add3b4790 100644
> > --- a/include/linux/spi/spi.h
> > +++ b/include/linux/spi/spi.h
> > @@ -734,6 +734,8 @@ extern void spi_res_release(struct spi_controller *ctlr,
> >   *      transfer. If 0 the default (from @spi_device) is used.
> >   * @bits_per_word: select a bits_per_word other than the device default
> >   *      for this transfer. If 0 the default (from @spi_device) is used.
> > + * @cs_change_stall_delay_us: microseconds to delay between cs_change
> > + * 	transfers.
> >   * @cs_change: affects chipselect after this transfer completes
> >   * @delay_usecs: microseconds to delay after this transfer before
> >   *	(optionally) changing the chipselect status, then starting
> > @@ -823,6 +825,7 @@ struct spi_transfer {
> >  #define	SPI_NBITS_QUAD		0x04 /* 4bits transfer */
> >  	u8		bits_per_word;
> >  	u8		word_delay_usecs;
> > +	u8		cs_change_stall_delay_us;
> >  	u16		delay_usecs;
> >  	u32		speed_hz;
> >  	u16		word_delay;