On Fri, 11 Dec 2020 13:15:59 -0800
Sowjanya Komatineni <skomatineni@xxxxxxxxxx> 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 <skomatineni@xxxxxxxxxx>
> ---
> 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.
