[U-Boot] [PATCH 1/2 V5] spi: Add support for preamble bytes

[Simon Glass]

Hi,

On Sun, Jun 2, 2013 at 10:24 AM, Jagan Teki <jagannadh.teki at gmail.com>wrote:

> On Wed, May 29, 2013 at 11:40 AM, Rajeshwari Shinde
> <rajeshwari.s at samsung.com> wrote:
> > A SPI slave may take time to react to a request. For SPI flash devices
> > this time is defined as one bit time, or a whole byte for 'fast read'
> > mode.
> >
> > If the SPI slave is another CPU, then the time it takes to react may
> > vary. It is convenient to allow the slave device to tag the start of
> > the actual reply so that the host can determine when this 'preamble'
> > finishes and the actual message starts.
> >
> > Add a preamble flag to the available SPI flags. If supported by the
> > driver then it will ignore any received bytes before the preamble
> > on each transaction. This ensures that reliable communication with
> > the slave is possible.
> >
> > Signed-off-by: Simon Glass <sjg at chromium.org>
> > Signed-off-by: Rajeshwari Shinde <rajeshwari.s at samsung.com>
> > ---
> > Changes in V2:
> >         - None
> > Changes in V3:
> >         - None.
> > Changes in V4:
> >         - None.
> > Changes in V5:
> >         - In commit message header changed SPI to spi.
> >  include/spi.h |    5 +++++
> >  1 files changed, 5 insertions(+), 0 deletions(-)
> >
> > diff --git a/include/spi.h b/include/spi.h
> > index 3fe2e1e..1638b50 100644
> > --- a/include/spi.h
> > +++ b/include/spi.h
> > @@ -37,11 +37,16 @@
> >  #define        SPI_LSB_FIRST   0x08                    /* per-word
> bits-on-wire */
> >  #define        SPI_3WIRE       0x10                    /* SI/SO signals
> shared */
> >  #define        SPI_LOOP        0x20                    /* loopback mode
> */
> > +#define        SPI_SLAVE       0x40                    /* slave mode */
> > +#define        SPI_PREAMBLE    0x80                    /* Skip preamble
> bytes */
> >
> >  /* SPI transfer flags */
> >  #define SPI_XFER_BEGIN 0x01                    /* Assert CS before
> transfer */
> >  #define SPI_XFER_END   0x02                    /* Deassert CS after
> transfer */
> >
> > +/* Header byte that marks the start of the message */
> > +#define SPI_PREAMBLE_END_BYTE  0xec
>
> I think this 0xec is a value of rx_data what does it indicates and
> does this value specific to hw?
>

We have a later change which generalises this using the device tree, but it
is dependent on various other patches and adjusts the generic SPI
interface. If we can get this one in then I'm sure Vadim will supply a new
patch for consideration.


> --
> Thanks,
> Jagan.
>

Regards,
Simon