Re: [PATCH v2 04/15] spi: Replace open coded spi_controller_xfer_timeout()

[AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>]

Il 10/07/23 17:49, Andy Shevchenko ha scritto:
> Since the new spi_controller_xfer_timeout() helper appeared,
> we may replace open coded variant in spi_transfer_wait().
> 
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> ---
>   drivers/spi/spi.c       | 25 ++-----------------------
>   include/linux/spi/spi.h |  6 +++++-
>   2 files changed, 7 insertions(+), 24 deletions(-)
> 
> diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
> index 125dea8fae00..c99ee4164f11 100644
> --- a/drivers/spi/spi.c
> +++ b/drivers/spi/spi.c
> @@ -1342,8 +1342,7 @@ static int spi_transfer_wait(struct spi_controller *ctlr,
>   {
>   	struct spi_statistics __percpu *statm = ctlr->pcpu_statistics;
>   	struct spi_statistics __percpu *stats = msg->spi->pcpu_statistics;
> -	u32 speed_hz = xfer->speed_hz;
> -	unsigned long long ms;
> +	unsigned long ms;
>   
>   	if (spi_controller_is_slave(ctlr)) {
>   		if (wait_for_completion_interruptible(&ctlr->xfer_completion)) {
> @@ -1351,29 +1350,9 @@ static int spi_transfer_wait(struct spi_controller *ctlr,
>   			return -EINTR;
>   		}
>   	} else {
> -		if (!speed_hz)
> -			speed_hz = 100000;
> -
> -		/*
> -		 * For each byte we wait for 8 cycles of the SPI clock.
> -		 * Since speed is defined in Hz and we want milliseconds,
> -		 * use respective multiplier, but before the division,
> -		 * otherwise we may get 0 for short transfers.
> -		 */
> -		ms = 8LL * MSEC_PER_SEC * xfer->len;
> -		do_div(ms, speed_hz);
> -
> -		/*
> -		 * Increase it twice and add 200 ms tolerance, use
> -		 * predefined maximum in case of overflow.
> -		 */
> -		ms += ms + 200;
> -		if (ms > UINT_MAX)
> -			ms = UINT_MAX;
> -
> +		ms = spi_controller_xfer_timeout(ctlr, xfer);

I agree on using helpers, but the logic is slightly changing here: yes it is
unlikely (and also probably useless) to get ms == UINT_MAX, but the helper is
limiting the maximum timeout value to 500mS, which may not work for some slow
controllers/devices.

This should get validated on more than a few platforms, and I'm not sure that
this kind of validation would be "fast" to get... so, probably the best thing
to do here is to add a warning in case the timeout exceeds 500mS, print the
actual value, keep it like this for a kernel version or two and check reports:
that would allow to understand what a safe maximum timeout value could be.

Aside from that, I wouldn't drop those nice comments explaining how/why the
timeout is calculated: I know how, but not everyone knows in advance.

Regards,
Angelo

>   		ms = wait_for_completion_timeout(&ctlr->xfer_completion,
>   						 msecs_to_jiffies(ms));
> -
>   		if (ms == 0) {
>   			SPI_STATISTICS_INCREMENT_FIELD(statm, timedout);
>   			SPI_STATISTICS_INCREMENT_FIELD(stats, timedout);
> diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
> index 32c94eae8926..0ce1cb18a076 100644
> --- a/include/linux/spi/spi.h
> +++ b/include/linux/spi/spi.h
> @@ -1270,12 +1270,16 @@ static inline bool spi_is_bpw_supported(struct spi_device *spi, u32 bpw)
>    * that it would take on a single data line and take twice this amount of time
>    * with a minimum of 500ms to avoid false positives on loaded systems.
>    *
> + * Assume speed to be 100 kHz if it's not defined at the time of invocation.
> + *
>    * Returns: Transfer timeout value in milliseconds.
>    */
>   static inline unsigned int spi_controller_xfer_timeout(struct spi_controller *ctlr,
>   						       struct spi_transfer *xfer)
>   {
> -	return max(xfer->len * 8 * 2 / (xfer->speed_hz / 1000), 500U);
> +	u32 speed_hz = xfer->speed_hz ?: 100000;
> +
> +	return max(xfer->len * 8 * 2 / (speed_hz / 1000), 500U);
>   }
>   
>   /*---------------------------------------------------------------------------*/