platform/chrome: cros_ec_spi: Transfer messages at high priority

The software running on the Chrome OS Embedded Controller (cros_ec) handles SPI transfers in a bit of a wonky way. Specifically if the EC sees too long of a delay in a SPI transfer it will give up and the transfer will be counted as failed. Unfortunately the timeout is fairly short, though the actual number may be different for different EC codebases.

We can end up tripping the timeout pretty easily if we happen to preempt the task running the SPI transfer and don't get back to it for a little while.

Historically this hasn't been a _huge_ deal because:
1. On old devices Chrome OS used to run PREEMPT_VOLUNTARY. That meant we were pretty unlikely to take a big break from the transfer.
2. On recent devices we had faster / more processors.
3. Recent devices didn't use "cros-ec-spi-pre-delay". Using that delay makes us more likely to trip this use case.
4. For whatever reasons (I didn't dig) old kernels seem to be less likely to trip this.
5. For the most part it's kinda OK if a few transfers to the EC fail. Mostly we're just polling the battery or doing some other task where we'll try again.

Even with the above things, this issue has reared its ugly head periodically. We could solve this in a nice way by adding reliable retries to the EC protocol [1] or by re-designing the code in the EC codebase to allow it to wait longer, but that code doesn't ever seem to get changed. ...and even if it did, it wouldn't help old devices.

It's now time to finally take a crack at making this a little better. This patch isn't guaranteed to make every cros_ec SPI transfer perfect, but it should improve things by a few orders of magnitude. Specifically you can try this on a rk3288-veyron Chromebook (which is slower and also _does_ need "cros-ec-spi-pre-delay"):
md5sum /dev/zero &
md5sum /dev/zero &
md5sum /dev/zero &
md5sum /dev/zero &
while true; do

    cat /sys/class/power_supply/sbs-20-000b/charge_now > /dev/null;

done
...before this patch you'll see boatloads of errors. After this patch I don't see any in the testing I did.

The way this patch works is by effectively boosting the priority of the cros_ec transfers. As far as I know there is no simple way to just boost the priority of the current process temporarily so the way we accomplish this is by queuing the work on the system_highpri_wq.

NOTE: this patch relies on the fact that the SPI framework attempts to push the messages out on the calling context (which is the one that is boosted to high priority). As I understand from earlier (long ago) discussions with Mark Brown this should be a fine assumption. Even if it isn't true sometimes this patch will still not make things worse.

[1] https://crbug.com/678675

Signed-off-by Douglas Anderson <[email protected]>

---

Changes in v3:
- Use flush_work(), not a completion (Brian)

Changes in v2:
- Use system_highpri_wq + completion (Matthias)
- Avoid duplication by using a function pointer (Matthias)

 drivers/platform/chrome/cros_ec_spi.c | 79 +++++++++++++++++++++++++--
 1 file changed, 73 insertions(+), 6 deletions(-)

drivers/platform/chrome/cros_ec_spi.c +27 -6

index ffc38f9d4829..29d2f7d24929 100644
@@ -75,6 +75,27 @@ struct cros_ec_spi {
 	unsigned int end_of_msg_delay;
 };
 
+typedef int (*cros_ec_xfer_fn_t) (struct cros_ec_device *ec_dev,
+				  struct cros_ec_command *ec_msg);
+
+/**
+ * struct cros_ec_xfer_work_params - params for our high priority workers
+ *
+ * @work: The work_struct needed to queue work
+ * @fn: The function to use to transfer
+ * @ec_dev: ChromeOS EC device
+ * @ec_msg: Message to transfer
+ * @ret: The return value of the function
+ */
+
+struct cros_ec_xfer_work_params {
+	struct work_struct work;
+	cros_ec_xfer_fn_t fn;
+	struct cros_ec_device *ec_dev;
+	struct cros_ec_command *ec_msg;
+	int ret;
+};
+
 static void debug_packet(struct device *dev, const char *name, u8 *ptr,
 			 int len)
 {

@@ -350,13 +371,13 @@ static int cros_ec_spi_receive_response(struct cros_ec_device *ec_dev,
 }
 
 /**
- * cros_ec_pkt_xfer_spi - Transfer a packet over SPI and receive the reply
+ * do_cros_ec_pkt_xfer_spi - Transfer a packet over SPI and receive the reply
  *
  * @ec_dev: ChromeOS EC device
  * @ec_msg: Message to transfer
  */
-static int cros_ec_pkt_xfer_spi(struct cros_ec_device *ec_dev,
-				struct cros_ec_command *ec_msg)
+static int do_cros_ec_pkt_xfer_spi(struct cros_ec_device *ec_dev,
+				   struct cros_ec_command *ec_msg)
 {
 	struct ec_host_response *response;
 	struct cros_ec_spi *ec_spi = ec_dev->priv;

@@ -493,13 +514,13 @@ static int cros_ec_pkt_xfer_spi(struct cros_ec_device *ec_dev,
 }
 
 /**
- * cros_ec_cmd_xfer_spi - Transfer a message over SPI and receive the reply
+ * do_cros_ec_cmd_xfer_spi - Transfer a message over SPI and receive the reply
  *
  * @ec_dev: ChromeOS EC device
  * @ec_msg: Message to transfer
  */
-static int cros_ec_cmd_xfer_spi(struct cros_ec_device *ec_dev,
-				struct cros_ec_command *ec_msg)
+static int do_cros_ec_cmd_xfer_spi(struct cros_ec_device *ec_dev,
+				   struct cros_ec_command *ec_msg)
 {
 	struct cros_ec_spi *ec_spi = ec_dev->priv;
 	struct spi_transfer trans;

@@ -611,6 +632,52 @@ static int cros_ec_cmd_xfer_spi(struct cros_ec_device *ec_dev,
 	return ret;
 }

drivers/platform/chrome/cros_ec_spi.c +30 -0

+				 struct cros_ec_command *ec_msg,
+				 cros_ec_xfer_fn_t fn)
+{
+	struct cros_ec_xfer_work_params params;
+
+	INIT_WORK(&params.work, cros_ec_xfer_high_pri_work);
+	params.ec_dev = ec_dev;
+	params.ec_msg = ec_msg;
+	params.fn = fn;
+
+	/*
+	 * This looks a bit ridiculous.  Why do the work on a
+	 * different thread if we're just going to block waiting for
+	 * the thread to finish?  The key here is that the thread is
+	 * running at high priority but the calling context might not
+	 * be.  We need to be at high priority to avoid getting
+	 * 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);
+
+	return params.ret;
+}
+
+static int cros_ec_pkt_xfer_spi(struct cros_ec_device *ec_dev,
+				struct cros_ec_command *ec_msg)
+{
+	return cros_ec_xfer_high_pri(ec_dev, ec_msg, do_cros_ec_pkt_xfer_spi);
+}

Quoted 1 line

+static int cros_ec_xfer_high_pri(struct cros_ec_device *ec_dev,

nit: the fact that a high priority workqueue is used is an implementation detail, since the driver has no function to perform a transfer with 'normal'/low priority there is no need to distinguish between the two cases. In this sense I'd be inclined to remove the 'high_pri' from the function names.

Sorry for not mentioning this earlier, I focussed on other details, anyway it's just a nit.

Hi,

Quoted 8 lines · Matthias Kaehlcke

Quoted 1 line

+static int cros_ec_xfer_high_pri(struct cros_ec_device *ec_dev,

nit: the fact that a high priority workqueue is used is an implementation detail, since the driver has no function to perform a transfer with 'normal'/low priority there is no need to distinguish between the two cases. In this sense I'd be inclined to remove the 'high_pri' from the function names.

Quoted 2 lines

Sorry for not mentioning this earlier, I focussed on other details, anyway it's just a nit.

I still kinda like having the "high_pri" in there since the point of this function is to transfer the work onto the high priority workqueue. It's not an exported function so having the implementation detail leak into the name isn't a bad thing, is it?

...so unless someone else thinks the name should change or you feel strongly about it I won't plan to change the name.

Quoted 11 lines · Doug Anderson

Hi,

Quoted 8 lines · Matthias Kaehlcke

Quoted 1 line

+static int cros_ec_xfer_high_pri(struct cros_ec_device *ec_dev,

nit: the fact that a high priority workqueue is used is an implementation detail, since the driver has no function to perform a transfer with 'normal'/low priority there is no need to distinguish between the two cases. In this sense I'd be inclined to remove the 'high_pri' from the function names.

Quoted 7 lines

Quoted 2 lines

Sorry for not mentioning this earlier, I focussed on other details, anyway it's just a nit.

I still kinda like having the "high_pri" in there since the point of this function is to transfer the work onto the high priority workqueue. It's not an exported function so having the implementation detail leak into the name isn't a bad thing, is it?

IMO the long name with details mostly irrelevant to the caller (they want to do a 'normal' transfer, the function should do the right thing to get that done) is more distracting than helpful. But yeah, this is definitely 'nit/bikeshed' territory ;-)

Quoted 2 lines

...so unless someone else thinks the name should change or you feel strongly about it I won't plan to change the name.

no strong feelings on my side, just wanted to mention it.

drivers/platform/chrome/cros_ec_spi.c static int cros_ec_cmd_xfer_spi(struct cros_ec_device *ec_dev, · line 649

Quoted 7 lines · Douglas Anderson

drivers/platform/chrome/cros_ec_spi.c +7 -0

+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(&params.work, cros_ec_xfer_high_pri_work);

Sorry for the late review, but this should have been INIT_WORK_ONSTACK(). Should it be a new patch, or is this in a non-rebasing tree yet?

Otherwise, looks fine to me:

Reviewed-by Brian Norris <[email protected]>