Re: [PATCH 1/2] spi: support inter-word delay requirement for devices

[Jonas Bonn]

Hi,

On 25/01/2019 12:53, Baolin Wang wrote:
> Hi,
> On Fri, 25 Jan 2019 at 19:44, Jonas Bonn <jonas@xxxxxxxxxxx> wrote:
> > Some devices are slow and cannot keep up with the SPI bus and therefore
> > require a short delay between words of the SPI transfer.
> >
> > The example of this that I'm looking at is a SAMA5D2 with a minimum SPI
> > clock of 400kHz talking to an AVR-based SPI slave.  The AVR cannot put
> > bytes on the bus fast enough to keep up with the SoC's SPI controller
> > even at the lowest bus speed.
> >
> > This patch introduces the ability to specify a required inter-word
> > delay for SPI devices.  It is up to the controller driver to configure
> > itself accordingly in order to introduce the requested delay.
>
> Can we configure it at runtime by the device rather than at DT time by
> the controller? If yes, we already have a patch for this, please
> check:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=eeaceb8b7d1fb64b6030249ca0dd1d902ef3069e

It's a characteristic of the SPI slave, in the same sense as CPOL/CPHA 
are, and therefore it makes sense to specify it in the device tree.

Having this as device property rather than a transfer property allows 
this to be configured one time in setup() rather than having to fiddle 
with the configuration register for every transfer.

The two approaches are complementary.

/Jonas

> > Signed-off-by: Jonas Bonn <jonas@xxxxxxxxxxx>
> > CC: Mark Brown <broonie@xxxxxxxxxx>
> > CC: Rob Herring <robh+dt@xxxxxxxxxx>
> > CC: Mark Rutland <mark.rutland@xxxxxxx>
> > CC: linux-spi@xxxxxxxxxxxxxxx
> > CC: devicetree@xxxxxxxxxxxxxxx
> > ---
> >   Documentation/devicetree/bindings/spi/spi-bus.txt | 1 +
> >   drivers/spi/spi.c                                 | 4 ++++
> >   include/linux/spi/spi.h                           | 1 +
> >   3 files changed, 6 insertions(+)
> >
> > diff --git a/Documentation/devicetree/bindings/spi/spi-bus.txt b/Documentation/devicetree/bindings/spi/spi-bus.txt
> > index 1f6e86f787ef..a5f20060676d 100644
> > --- a/Documentation/devicetree/bindings/spi/spi-bus.txt
> > +++ b/Documentation/devicetree/bindings/spi/spi-bus.txt
> > @@ -77,6 +77,7 @@ All slave nodes can contain the following optional properties:
> >                      Defaults to 1 if not present.
> >   - spi-rx-delay-us - Microsecond delay after a read transfer.
> >   - spi-tx-delay-us - Microsecond delay after a write transfer.
> > +- spi-word-delay-us - Microsecond delay between individual words of a transfer
> >
> >   Some SPI controllers and devices support Dual and Quad SPI transfer mode.
> >   It allows data in the SPI system to be transferred using 2 wires (DUAL) or 4
> > diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
> > index 9a7def7c3237..cd4d4065eca2 100644
> > --- a/drivers/spi/spi.c
> > +++ b/drivers/spi/spi.c
> > @@ -1692,6 +1692,10 @@ static int of_spi_parse_dt(struct spi_controller *ctlr, struct spi_device *spi,
> >          }
> >          spi->max_speed_hz = value;
> >
> > +       if (!of_property_read_u32(nc, "spi-word-delay-us", &value)) {
> > +               spi->word_delay = value;
> > +       }
> > +
> >          return 0;
> >   }
> >
> > diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
> > index 314d922ca607..e5200dd9d750 100644
> > --- a/include/linux/spi/spi.h
> > +++ b/include/linux/spi/spi.h
> > @@ -164,6 +164,7 @@ struct spi_device {
> >          char                    modalias[SPI_NAME_SIZE];
> >          const char              *driver_override;
> >          int                     cs_gpio;        /* chip select gpio */
> > +       uint16_t                word_delay;     /* inter-word delay (us) */
> >
> >          /* the statistics */
> >          struct spi_statistics   statistics;
> > --
> > 2.19.1