On Fri, 11 Dec 2020 13:15:59 -0800
Sowjanya Komatineni <skomatin...@nvidia.com> wrote:

> This patch adds a flag SPI_MASTER_USES_HW_DUMMY_CYCLES for the controllers
> that support transfer of dummy cycles by the hardware directly.

Hm, not sure this is a good idea. I mean, if we expect regular SPI
devices to use this feature, then why not, but if it's just for
spi-mem, I'd recommend implementing a driver-specific exec_op() instead
of using the default one.

If we go for those core changes, we should at least add a
ctrl->max_dummy_cycles field so the core can fallback to regular writes
when the number of dummy cycles in the spi_mem_op exceeds what the
controller can do.

> 
> For controller with this flag set, spi-mem driver will skip dummy bytes
> transfer in the spi message.
> 
> Controller drivers can get the number of dummy cycles from spi_message.
> 
> Signed-off-by: Sowjanya Komatineni <skomatin...@nvidia.com>
> ---
>  drivers/spi/spi-mem.c   | 18 +++++++++++-------
>  include/linux/spi/spi.h |  8 ++++++++
>  2 files changed, 19 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/spi/spi-mem.c b/drivers/spi/spi-mem.c
> index f3a3f19..38a523b 100644
> --- a/drivers/spi/spi-mem.c
> +++ b/drivers/spi/spi-mem.c
> @@ -350,13 +350,17 @@ int spi_mem_exec_op(struct spi_mem *mem, const struct 
> spi_mem_op *op)
>       }
>  
>       if (op->dummy.nbytes) {
> -             memset(tmpbuf + op->addr.nbytes + 1, 0xff, op->dummy.nbytes);
> -             xfers[xferpos].tx_buf = tmpbuf + op->addr.nbytes + 1;
> -             xfers[xferpos].len = op->dummy.nbytes;
> -             xfers[xferpos].tx_nbits = op->dummy.buswidth;
> -             spi_message_add_tail(&xfers[xferpos], &msg);
> -             xferpos++;
> -             totalxferlen += op->dummy.nbytes;
> +             if (ctlr->flags & SPI_MASTER_USES_HW_DUMMY_CYCLES) {
> +                     msg.dummy_cycles = (op->dummy.nbytes * 8) / 
> op->dummy.buswidth;
> +             } else {
> +                     memset(tmpbuf + op->addr.nbytes + 1, 0xff, 
> op->dummy.nbytes);
> +                     xfers[xferpos].tx_buf = tmpbuf + op->addr.nbytes + 1;
> +                     xfers[xferpos].len = op->dummy.nbytes;
> +                     xfers[xferpos].tx_nbits = op->dummy.buswidth;
> +                     spi_message_add_tail(&xfers[xferpos], &msg);
> +                     xferpos++;
> +                     totalxferlen += op->dummy.nbytes;
> +             }
>       }
>  
>       if (op->data.nbytes) {
> diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
> index aa09fdc..2024149 100644
> --- a/include/linux/spi/spi.h
> +++ b/include/linux/spi/spi.h
> @@ -512,6 +512,8 @@ struct spi_controller {
>  
>  #define SPI_MASTER_GPIO_SS           BIT(5)  /* GPIO CS must select slave */
>  
> +#define SPI_MASTER_USES_HW_DUMMY_CYCLES      BIT(6)  /* HW dummy bytes 
> transfer */
> +
>       /* flag indicating this is an SPI slave controller */
>       bool                    slave;
>  
> @@ -1022,6 +1024,12 @@ struct spi_message {
>       unsigned                actual_length;
>       int                     status;
>  
> +     /*
> +      * dummy cycles in the message transfer. This is used by the controller
> +      * drivers supports transfer of dummy cycles directly by the hardware.
> +      */
> +     u8                      dummy_cycles;
> +
>       /* for optional use by whatever driver currently owns the
>        * spi_message ...  between calls to spi_async and then later
>        * complete(), that's the spi_controller controller driver.

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