[PATCH v4 0/3] spi: A better solution for cros_ec_spi reliability

[PATCH v4 0/3] spi: A better solution for cros_ec_spi reliability

[Douglas Anderson]

This series is a much better solution for getting the Chrome OS EC to
talk reliably.

Patch #1 in this series is the most important.  It can land any time.

Patch #2 in this series (a SPI framework patch) needs to land before
patch #3.  Note that patches #2 and #3 really just fix a corner case
and just having patch #1 is the most important.  We don't end up on
the pumping thread very often.

Note:
- If you want some history on investigation done here, feel free to
  peruse the Chrome OS bug: <https://crbug.com/948742>.

Changes in v4:
- No needless init of err in cros_ec_spi_devm_high_pri_alloc (Guenter).
- Removed blank lines near #includes (Guenter).
- Switch to kthread_create_worker() and fix error handling (Guenter).
- Cleaner devm code (Guenter).

Changes in v3:
- cros_ec realtime patch replaces revert; now patch #1
- SPI core change now like patch v1 patch #2 (with name "rt").
- Updated description and variable name since we no longer force.

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".
- Renamed variable to "force_rt_transfers".

Douglas Anderson (3):
  platform/chrome: cros_ec_spi: Move to real time priority for transfers
  spi: Allow SPI devices to request the pumping thread be realtime
  platform/chrome: cros_ec_spi: Request the SPI thread be realtime

 drivers/platform/chrome/cros_ec_spi.c | 66 ++++++++++++++++++++++-----
 drivers/spi/spi.c                     | 36 ++++++++++++---
 include/linux/spi/spi.h               |  2 +
 3 files changed, 86 insertions(+), 18 deletions(-)

-- 
2.21.0.1020.gf2820cf01a-goog

[PATCH v4 1/3] platform/chrome: cros_ec_spi: Move to real time priority for transfers

[Douglas Anderson]

In commit 37a186225a0c ("platform/chrome: cros_ec_spi: Transfer
messages at high priority") we moved transfers to a high priority
workqueue.  This helped make them much more reliable.

...but, we still saw failures.

We were actually finding ourselves competing for time with dm-crypt
which also scheduled work on HIGHPRI workqueues.  While we can
consider reverting the change that made dm-crypt run its work at
HIGHPRI, the argument in commit a1b89132dc4f ("dm crypt: use
WQ_HIGHPRI for the IO and crypt workqueues") is somewhat compelling.
It does make sense for IO to be scheduled at a priority that's higher
than the default user priority.  It also turns out that dm-crypt isn't
alone in using high priority like this.  loop_prepare_queue() does
something similar for loopback devices.

Looking in more detail, it can be seen that the high priority
workqueue isn't actually that high of a priority.  It runs at MIN_NICE
which is _fairly_ high priority but still below all real time
priority.

Should we move cros_ec_spi to real time priority to fix our problems,
or is this just escalating a priority war?  I'll argue here that
cros_ec_spi _does_ belong at real time priority.  Specifically
cros_ec_spi actually needs to run quickly for correctness.  As I
understand this is exactly what real time priority is for.

There currently doesn't appear to be any way to use the standard
workqueue APIs with a real time priority, so we'll switch over to
using using a kthread worker.  We'll match the priority that the SPI
core uses when it wants to do things on a realtime thread and just use
"MAX_RT_PRIO - 1".

This commit plus the patch ("platform/chrome: cros_ec_spi: Request the
SPI thread be realtime") are enough to get communications very close
to 100% reliable (the only known problem left is when serial console
is turned on, which isn't something that happens in shipping devices).
Specifically this test case now passes (tested on rk3288-veyron-jerry):

  dd if=/dev/zero of=/var/log/foo.txt bs=4M count=512&
  while true; do
    ectool version > /dev/null;
  done

It should be noted that "/var/log" is encrypted (and goes through
dm-crypt) and also passes through a loopback device.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
---

Changes in v4:
- No needless init of err in cros_ec_spi_devm_high_pri_alloc (Guenter).
- Removed blank lines near #includes (Guenter).
- Switch to kthread_create_worker() and fix error handling (Guenter).
- Cleaner devm code (Guenter).

Changes in v3:
- cros_ec realtime patch replaces revert; now patch #1

Changes in v2: None

 drivers/platform/chrome/cros_ec_spi.c | 65 ++++++++++++++++++++++-----
 1 file changed, 53 insertions(+), 12 deletions(-)

diff --git a/drivers/platform/chrome/cros_ec_spi.c b/drivers/platform/chrome/cros_ec_spi.c
index 8e9451720e73..1e38a885c539 100644
--- a/drivers/platform/chrome/cros_ec_spi.c
+++ b/drivers/platform/chrome/cros_ec_spi.c
@@ -12,7 +12,7 @@
 #include <linux/platform_device.h>
 #include <linux/slab.h>
 #include <linux/spi/spi.h>
-
+#include <uapi/linux/sched/types.h>
 
 /* The header byte, which follows the preamble */
 #define EC_MSG_HEADER			0xec
@@ -67,12 +67,14 @@
  *      is sent when we want to turn on CS at the start of a transaction.
  * @end_of_msg_delay: used to set the delay_usecs on the spi_transfer that
  *      is sent when we want to turn off CS at the end of a transaction.
+ * @high_pri_worker: Used to schedule high priority work.
  */
 struct cros_ec_spi {
 	struct spi_device *spi;
 	s64 last_transfer_ns;
 	unsigned int start_of_msg_delay;
 	unsigned int end_of_msg_delay;
+	struct kthread_worker *high_pri_worker;
 };
 
 typedef int (*cros_ec_xfer_fn_t) (struct cros_ec_device *ec_dev,
@@ -89,7 +91,7 @@ typedef int (*cros_ec_xfer_fn_t) (struct cros_ec_device *ec_dev,
  */
 
 struct cros_ec_xfer_work_params {
-	struct work_struct work;
+	struct kthread_work work;
 	cros_ec_xfer_fn_t fn;
 	struct cros_ec_device *ec_dev;
 	struct cros_ec_command *ec_msg;
@@ -632,7 +634,7 @@ static int do_cros_ec_cmd_xfer_spi(struct cros_ec_device *ec_dev,
 	return ret;
 }
 
-static void cros_ec_xfer_high_pri_work(struct work_struct *work)
+static void cros_ec_xfer_high_pri_work(struct kthread_work *work)
 {
 	struct cros_ec_xfer_work_params *params;
 
@@ -644,12 +646,14 @@ static int cros_ec_xfer_high_pri(struct cros_ec_device *ec_dev,
 				 struct cros_ec_command *ec_msg,
 				 cros_ec_xfer_fn_t fn)
 {
-	struct cros_ec_xfer_work_params params;
-
-	INIT_WORK_ONSTACK(&params.work, cros_ec_xfer_high_pri_work);
-	params.ec_dev = ec_dev;
-	params.ec_msg = ec_msg;
-	params.fn = fn;
+	struct cros_ec_spi *ec_spi = ec_dev->priv;
+	struct cros_ec_xfer_work_params params = {
+		.work = KTHREAD_WORK_INIT(params.work,
+					  cros_ec_xfer_high_pri_work),
+		.ec_dev = ec_dev,
+		.ec_msg = ec_msg,
+		.fn = fn,
+	};
 
 	/*
 	 * This looks a bit ridiculous.  Why do the work on a
@@ -660,9 +664,8 @@ static int cros_ec_xfer_high_pri(struct cros_ec_device *ec_dev,
 	 * context switched out for too long and the EC giving up on
 	 * the transfer.
 	 */
-	queue_work(system_highpri_wq, &params.work);
-	flush_work(&params.work);
-	destroy_work_on_stack(&params.work);
+	kthread_queue_work(ec_spi->high_pri_worker, &params.work);
+	kthread_flush_work(&params.work);
 
 	return params.ret;
 }
@@ -694,6 +697,40 @@ static void cros_ec_spi_dt_probe(struct cros_ec_spi *ec_spi, struct device *dev)
 		ec_spi->end_of_msg_delay = val;
 }
 
+static void cros_ec_spi_high_pri_release(void *worker)
+{
+	kthread_destroy_worker(worker);
+}
+
+static int cros_ec_spi_devm_high_pri_alloc(struct device *dev,
+					   struct cros_ec_spi *ec_spi)
+{
+	struct sched_param sched_priority = {
+		.sched_priority = MAX_RT_PRIO - 1,
+	};
+	int err;
+
+	ec_spi->high_pri_worker =
+		kthread_create_worker(0, "cros_ec_spi_high_pri");
+
+	if (IS_ERR(ec_spi->high_pri_worker)) {
+		err = PTR_ERR(ec_spi->high_pri_worker);
+		dev_err(dev, "Can't create cros_ec high pri worker: %d\n", err);
+		return err;
+	}
+
+	err = devm_add_action_or_reset(dev, cros_ec_spi_high_pri_release,
+				       ec_spi->high_pri_worker);
+	if (err)
+		return err;
+
+	err = sched_setscheduler_nocheck(ec_spi->high_pri_worker->task,
+					 SCHED_FIFO, &sched_priority);
+	if (err)
+		dev_err(dev, "Can't set cros_ec high pri priority: %d\n", err);
+	return err;
+}
+
 static int cros_ec_spi_probe(struct spi_device *spi)
 {
 	struct device *dev = &spi->dev;
@@ -732,6 +769,10 @@ static int cros_ec_spi_probe(struct spi_device *spi)
 
 	ec_spi->last_transfer_ns = ktime_get_ns();
 
+	err = cros_ec_spi_devm_high_pri_alloc(dev, ec_spi);
+	if (err)
+		return err;
+
 	err = cros_ec_register(ec_dev);
 	if (err) {
 		dev_err(dev, "cannot register EC\n");
-- 
2.21.0.1020.gf2820cf01a-goog

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

[Douglas Anderson]

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@chromium.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
---

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 */
-- 
2.21.0.1020.gf2820cf01a-goog

[PATCH v4 3/3] platform/chrome: cros_ec_spi: Request the SPI thread be realtime

[Douglas Anderson]

All currently known ECs in the wild are very sensitive to timing.
Specifically the ECs are known to drop a transfer if more than 8 ms
passes from the assertion of the chip select until the transfer
finishes.

Let's use the new feature introduced in the patch (spi: Allow SPI
devices to request the pumping thread be realtime") to request the SPI
pumping thread be realtime.  This means that if we get shunted off to
the SPI thread for whatever reason we won't get downgraded to low
priority.

NOTES:
- We still need to keep ourselves as high priority since the SPI core
  doesn't guarantee that all transfers end up on the pumping thread
  (in fact, it tries pretty hard to do them in the calling context).
- If future Chrome OS ECs ever fix themselves to be less sensitive
  then we could consider adding a property (or compatible string) to
  not set this property.  For now we need it across the board.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
---

Changes in v4: None
Changes in v3:
- Updated description and variable name since we no longer force.

Changes in v2:
- Renamed variable to "force_rt_transfers".

 drivers/platform/chrome/cros_ec_spi.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/platform/chrome/cros_ec_spi.c b/drivers/platform/chrome/cros_ec_spi.c
index 1e38a885c539..daf3119191c8 100644
--- a/drivers/platform/chrome/cros_ec_spi.c
+++ b/drivers/platform/chrome/cros_ec_spi.c
@@ -740,6 +740,7 @@ static int cros_ec_spi_probe(struct spi_device *spi)
 
 	spi->bits_per_word = 8;
 	spi->mode = SPI_MODE_0;
+	spi->rt = true;
 	err = spi_setup(spi);
 	if (err < 0)
 		return err;
-- 
2.21.0.1020.gf2820cf01a-goog

Re: [PATCH v4 1/3] platform/chrome: cros_ec_spi: Move to real time priority for transfers

[Guenter Roeck]

On Wed, May 15, 2019 at 9:48 AM Douglas Anderson <dianders@chromium.org> wrote:
>
> In commit 37a186225a0c ("platform/chrome: cros_ec_spi: Transfer
> messages at high priority") we moved transfers to a high priority
> workqueue.  This helped make them much more reliable.
>
> ...but, we still saw failures.
>
> We were actually finding ourselves competing for time with dm-crypt
> which also scheduled work on HIGHPRI workqueues.  While we can
> consider reverting the change that made dm-crypt run its work at
> HIGHPRI, the argument in commit a1b89132dc4f ("dm crypt: use
> WQ_HIGHPRI for the IO and crypt workqueues") is somewhat compelling.
> It does make sense for IO to be scheduled at a priority that's higher
> than the default user priority.  It also turns out that dm-crypt isn't
> alone in using high priority like this.  loop_prepare_queue() does
> something similar for loopback devices.
>
> Looking in more detail, it can be seen that the high priority
> workqueue isn't actually that high of a priority.  It runs at MIN_NICE
> which is _fairly_ high priority but still below all real time
> priority.
>
> Should we move cros_ec_spi to real time priority to fix our problems,
> or is this just escalating a priority war?  I'll argue here that
> cros_ec_spi _does_ belong at real time priority.  Specifically
> cros_ec_spi actually needs to run quickly for correctness.  As I
> understand this is exactly what real time priority is for.
>
> There currently doesn't appear to be any way to use the standard
> workqueue APIs with a real time priority, so we'll switch over to
> using using a kthread worker.  We'll match the priority that the SPI
> core uses when it wants to do things on a realtime thread and just use
> "MAX_RT_PRIO - 1".
>
> This commit plus the patch ("platform/chrome: cros_ec_spi: Request the
> SPI thread be realtime") are enough to get communications very close
> to 100% reliable (the only known problem left is when serial console
> is turned on, which isn't something that happens in shipping devices).
> Specifically this test case now passes (tested on rk3288-veyron-jerry):
>
>   dd if=/dev/zero of=/var/log/foo.txt bs=4M count=512&
>   while true; do
>     ectool version > /dev/null;
>   done
>
> It should be noted that "/var/log" is encrypted (and goes through
> dm-crypt) and also passes through a loopback device.
>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>

Reviewed-by: Guenter Roeck <groeck@chromium.org>

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@chromium.org>
> Reviewed-by: Guenter Roeck <groeck@chromium.org>

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

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 */
> 

Re: [PATCH v4 3/3] platform/chrome: cros_ec_spi: Request the SPI thread be realtime

[Enric Balletbo i Serra]

Hi,

On 15/5/19 18:48, Douglas Anderson wrote:
> All currently known ECs in the wild are very sensitive to timing.
> Specifically the ECs are known to drop a transfer if more than 8 ms
> passes from the assertion of the chip select until the transfer
> finishes.
> 
> Let's use the new feature introduced in the patch (spi: Allow SPI
> devices to request the pumping thread be realtime") to request the SPI
> pumping thread be realtime.  This means that if we get shunted off to
> the SPI thread for whatever reason we won't get downgraded to low
> priority.
> 
> NOTES:
> - We still need to keep ourselves as high priority since the SPI core
>   doesn't guarantee that all transfers end up on the pumping thread
>   (in fact, it tries pretty hard to do them in the calling context).
> - If future Chrome OS ECs ever fix themselves to be less sensitive
>   then we could consider adding a property (or compatible string) to
>   not set this property.  For now we need it across the board.
> 
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> Reviewed-by: Guenter Roeck <groeck@chromium.org>

For my own reference:

Acked-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>

Thanks,
 Enric

> ---
> 
> Changes in v4: None
> Changes in v3:
> - Updated description and variable name since we no longer force.
> 
> Changes in v2:
> - Renamed variable to "force_rt_transfers".
> 
>  drivers/platform/chrome/cros_ec_spi.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/platform/chrome/cros_ec_spi.c b/drivers/platform/chrome/cros_ec_spi.c
> index 1e38a885c539..daf3119191c8 100644
> --- a/drivers/platform/chrome/cros_ec_spi.c
> +++ b/drivers/platform/chrome/cros_ec_spi.c
> @@ -740,6 +740,7 @@ static int cros_ec_spi_probe(struct spi_device *spi)
>  
>  	spi->bits_per_word = 8;
>  	spi->mode = SPI_MODE_0;
> +	spi->rt = true;
>  	err = spi_setup(spi);
>  	if (err < 0)
>  		return err;
> 

Re: [PATCH v4 1/3] platform/chrome: cros_ec_spi: Move to real time priority for transfers

[Enric Balletbo i Serra]

Hi,

On 15/5/19 19:02, Guenter Roeck wrote:
> On Wed, May 15, 2019 at 9:48 AM Douglas Anderson <dianders@chromium.org> wrote:
>>
>> In commit 37a186225a0c ("platform/chrome: cros_ec_spi: Transfer
>> messages at high priority") we moved transfers to a high priority
>> workqueue.  This helped make them much more reliable.
>>
>> ...but, we still saw failures.
>>
>> We were actually finding ourselves competing for time with dm-crypt
>> which also scheduled work on HIGHPRI workqueues.  While we can
>> consider reverting the change that made dm-crypt run its work at
>> HIGHPRI, the argument in commit a1b89132dc4f ("dm crypt: use
>> WQ_HIGHPRI for the IO and crypt workqueues") is somewhat compelling.
>> It does make sense for IO to be scheduled at a priority that's higher
>> than the default user priority.  It also turns out that dm-crypt isn't
>> alone in using high priority like this.  loop_prepare_queue() does
>> something similar for loopback devices.
>>
>> Looking in more detail, it can be seen that the high priority
>> workqueue isn't actually that high of a priority.  It runs at MIN_NICE
>> which is _fairly_ high priority but still below all real time
>> priority.
>>
>> Should we move cros_ec_spi to real time priority to fix our problems,
>> or is this just escalating a priority war?  I'll argue here that
>> cros_ec_spi _does_ belong at real time priority.  Specifically
>> cros_ec_spi actually needs to run quickly for correctness.  As I
>> understand this is exactly what real time priority is for.
>>
>> There currently doesn't appear to be any way to use the standard
>> workqueue APIs with a real time priority, so we'll switch over to
>> using using a kthread worker.  We'll match the priority that the SPI
>> core uses when it wants to do things on a realtime thread and just use
>> "MAX_RT_PRIO - 1".
>>
>> This commit plus the patch ("platform/chrome: cros_ec_spi: Request the
>> SPI thread be realtime") are enough to get communications very close
>> to 100% reliable (the only known problem left is when serial console
>> is turned on, which isn't something that happens in shipping devices).
>> Specifically this test case now passes (tested on rk3288-veyron-jerry):
>>
>>   dd if=/dev/zero of=/var/log/foo.txt bs=4M count=512&
>>   while true; do
>>     ectool version > /dev/null;
>>   done
>>
>> It should be noted that "/var/log" is encrypted (and goes through
>> dm-crypt) and also passes through a loopback device.
>>
>> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> 
> Reviewed-by: Guenter Roeck <groeck@chromium.org>
> 

Added to the for-next branch for the autobuilders to play with, if all goes well
will be queued in chrome-platform-5.3

Thanks,
 Enric

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

[Mark Brown]

[-- Attachment #1: Type: text/plain, Size: 1187 bytes --]

On Wed, May 15, 2019 at 09:48:12AM -0700, 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.

The following changes since commit a188339ca5a396acc588e5851ed7e19f66b0ebd9:

  Linux 5.2-rc1 (2019-05-19 15:47:09 -0700)

are available in the Git repository at:

  https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git tags/spi-rt-pump

for you to fetch changes up to 924b5867e7bd6a6a98014f0517b747465b108011:

  spi: Allow SPI devices to request the pumping thread be realtime (2019-05-23 14:44:02 +0100)

----------------------------------------------------------------
spi: Allow setting pump to RT priority

----------------------------------------------------------------
Douglas Anderson (1):
      spi: Allow SPI devices to request the pumping thread be realtime

 drivers/spi/spi.c       | 36 ++++++++++++++++++++++++++++++------
 include/linux/spi/spi.h |  2 ++
 2 files changed, 32 insertions(+), 6 deletions(-)

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 488 bytes --]

Applied "spi: Allow SPI devices to request the pumping thread be realtime" to the spi tree

[Mark Brown]

The patch

   spi: Allow SPI devices to request the pumping thread be realtime

has been applied to the spi tree at

   https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git for-5.3

All being well this means that it will be integrated into the linux-next
tree (usually sometime in the next 24 hours) and sent to Linus during
the next merge window (or sooner if it is a bug fix), however if
problems are discovered then the patch may be dropped or reverted.  

You may get further e-mails resulting from automated or manual testing
and review of the tree, please engage with people reporting problems and
send followup patches addressing any issues that are reported if needed.

If any updates are required or you are submitting further changes they
should be sent as incremental updates against current git, existing
patches will not be replaced.

Please add any relevant lists and maintainers to the CCs when replying
to this mail.

Thanks,
Mark

From 924b5867e7bd6a6a98014f0517b747465b108011 Mon Sep 17 00:00:00 2001
From: Douglas Anderson <dianders@chromium.org>
Date: Wed, 15 May 2019 09:48:12 -0700
Subject: [PATCH] spi: Allow SPI devices to request the pumping thread be
 realtime

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@chromium.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
---
 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 5e75944ad5d1..18f70e4bbb31 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 */
-- 
2.20.1

Re: [PATCH v4 3/3] platform/chrome: cros_ec_spi: Request the SPI thread be realtime

[Enric Balletbo i Serra]

Hi,

On 15/5/19 18:48, Douglas Anderson wrote:
> All currently known ECs in the wild are very sensitive to timing.
> Specifically the ECs are known to drop a transfer if more than 8 ms
> passes from the assertion of the chip select until the transfer
> finishes.
> 
> Let's use the new feature introduced in the patch (spi: Allow SPI
> devices to request the pumping thread be realtime") to request the SPI
> pumping thread be realtime.  This means that if we get shunted off to
> the SPI thread for whatever reason we won't get downgraded to low
> priority.
> 
> NOTES:
> - We still need to keep ourselves as high priority since the SPI core
>   doesn't guarantee that all transfers end up on the pumping thread
>   (in fact, it tries pretty hard to do them in the calling context).
> - If future Chrome OS ECs ever fix themselves to be less sensitive
>   then we could consider adding a property (or compatible string) to
>   not set this property.  For now we need it across the board.
> 
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> Reviewed-by: Guenter Roeck <groeck@chromium.org>
> ---

queued for 5.3

Thanks,
 Enric

> 
> Changes in v4: None
> Changes in v3:
> - Updated description and variable name since we no longer force.
> 
> Changes in v2:
> - Renamed variable to "force_rt_transfers".
> 
>  drivers/platform/chrome/cros_ec_spi.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/platform/chrome/cros_ec_spi.c b/drivers/platform/chrome/cros_ec_spi.c
> index 1e38a885c539..daf3119191c8 100644
> --- a/drivers/platform/chrome/cros_ec_spi.c
> +++ b/drivers/platform/chrome/cros_ec_spi.c
> @@ -740,6 +740,7 @@ static int cros_ec_spi_probe(struct spi_device *spi)
>  
>  	spi->bits_per_word = 8;
>  	spi->mode = SPI_MODE_0;
> +	spi->rt = true;
>  	err = spi_setup(spi);
>  	if (err < 0)
>  		return err;
>