linux-kernel - Re: [PATCH 2/5] drivers: spi: core: Add optional stall delay between cs

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Date:   Wed, 26 Jun 2019 19:34:38 +0100
From:   Jonathan Cameron <jic23@...23.retrosnub.co.uk>
To:     Alexandru Ardelean <alexandru.ardelean@...log.com>
Cc:     <linux-iio@...r.kernel.org>, <linux-spi@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Michael Hennerich <michael.hennerich@...log.com>,
        Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH 2/5] drivers: spi: core: Add optional stall delay
 between cs_change transfers

On Tue, 25 Jun 2019 16:13:25 +0300
Alexandru Ardelean <alexandru.ardelean@...log.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@...log.com>
> Signed-off-by: Alexandru Ardelean <alexandru.ardelean@...log.com>

General principle seems fine to me, though naming may need some work.

cs_low_time or something more specific than stall perhaps?

+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;