Re: [PATCH v4 2/3] spi: Allow SPI devices to request the pumping thread be realtime

[Enric Balletbo i Serra]

Hi,

On 15/5/19 18:48, Douglas Anderson wrote:
> Right now the only way to get the SPI pumping thread bumped up to
> realtime priority is for the controller to request it.  However it may
> be that the controller works fine with the normal priority but
> communication to a particular SPI device on the bus needs realtime
> priority.
> 
> Let's add a way for devices to request realtime priority when they set
> themselves up.
> 
> NOTE: this will just affect the priority of transfers that end up on
> the SPI core's pumping thread.  In many cases transfers happen in the
> context of the caller so if you need realtime priority for all
> transfers you should ensure the calling context is also realtime
> priority.
> 
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> Reviewed-by: Guenter Roeck <groeck@xxxxxxxxxxxx>

Tested-by: Enric Balletbo i Serra <enric.balletbo@xxxxxxxxxxxxx>

Thanks,
 Enric

> ---
> 
> Changes in v4: None
> Changes in v3:
> - SPI core change now like patch v1 patch #2 (with name "rt").
> 
> Changes in v2:
> - Now only force transfers to the thread for devices that want it.
> - Squashed patch #1 and #2 together.
> - Renamed variable to "force_rt_transfers".
> 
>  drivers/spi/spi.c       | 36 ++++++++++++++++++++++++++++++------
>  include/linux/spi/spi.h |  2 ++
>  2 files changed, 32 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
> index 8eb7460dd744..466984796dd9 100644
> --- a/drivers/spi/spi.c
> +++ b/drivers/spi/spi.c
> @@ -1364,10 +1364,32 @@ static void spi_pump_messages(struct kthread_work *work)
>  	__spi_pump_messages(ctlr, true);
>  }
>  
> -static int spi_init_queue(struct spi_controller *ctlr)
> +/**
> + * spi_set_thread_rt - set the controller to pump at realtime priority
> + * @ctlr: controller to boost priority of
> + *
> + * This can be called because the controller requested realtime priority
> + * (by setting the ->rt value before calling spi_register_controller()) or
> + * because a device on the bus said that its transfers needed realtime
> + * priority.
> + *
> + * NOTE: at the moment if any device on a bus says it needs realtime then
> + * the thread will be at realtime priority for all transfers on that
> + * controller.  If this eventually becomes a problem we may see if we can
> + * find a way to boost the priority only temporarily during relevant
> + * transfers.
> + */
> +static void spi_set_thread_rt(struct spi_controller *ctlr)
>  {
>  	struct sched_param param = { .sched_priority = MAX_RT_PRIO - 1 };
>  
> +	dev_info(&ctlr->dev,
> +		"will run message pump with realtime priority\n");
> +	sched_setscheduler(ctlr->kworker_task, SCHED_FIFO, &param);
> +}
> +
> +static int spi_init_queue(struct spi_controller *ctlr)
> +{
>  	ctlr->running = false;
>  	ctlr->busy = false;
>  
> @@ -1387,11 +1409,8 @@ static int spi_init_queue(struct spi_controller *ctlr)
>  	 * request and the scheduling of the message pump thread. Without this
>  	 * setting the message pump thread will remain at default priority.
>  	 */
> -	if (ctlr->rt) {
> -		dev_info(&ctlr->dev,
> -			"will run message pump with realtime priority\n");
> -		sched_setscheduler(ctlr->kworker_task, SCHED_FIFO, &param);
> -	}
> +	if (ctlr->rt)
> +		spi_set_thread_rt(ctlr);
>  
>  	return 0;
>  }
> @@ -2982,6 +3001,11 @@ int spi_setup(struct spi_device *spi)
>  
>  	spi_set_cs(spi, false);
>  
> +	if (spi->rt && !spi->controller->rt) {
> +		spi->controller->rt = true;
> +		spi_set_thread_rt(spi->controller);
> +	}
> +
>  	dev_dbg(&spi->dev, "setup mode %d, %s%s%s%s%u bits/w, %u Hz max --> %d\n",
>  			(int) (spi->mode & (SPI_CPOL | SPI_CPHA)),
>  			(spi->mode & SPI_CS_HIGH) ? "cs_high, " : "",
> diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
> index 053abd22ad31..15505c2485d6 100644
> --- a/include/linux/spi/spi.h
> +++ b/include/linux/spi/spi.h
> @@ -109,6 +109,7 @@ void spi_statistics_add_transfer_stats(struct spi_statistics *stats,
>   *	This may be changed by the device's driver, or left at the
>   *	default (0) indicating protocol words are eight bit bytes.
>   *	The spi_transfer.bits_per_word can override this for each transfer.
> + * @rt: Make the pump thread real time priority.
>   * @irq: Negative, or the number passed to request_irq() to receive
>   *	interrupts from this device.
>   * @controller_state: Controller's runtime state
> @@ -143,6 +144,7 @@ struct spi_device {
>  	u32			max_speed_hz;
>  	u8			chip_select;
>  	u8			bits_per_word;
> +	bool			rt;
>  	u32			mode;
>  #define	SPI_CPHA	0x01			/* clock phase */
>  #define	SPI_CPOL	0x02			/* clock polarity */
>