Re: [PATCH] platform/chrome: cros_ec_spi: Transfer messages at high priority

[Matthias Kaehlcke <mka@chromium.org>]

Hi Doug,

On Tue, Apr 02, 2019 at 03:44:44PM -0700, Douglas Anderson wrote:
> 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 creating a "WQ_HIGHPRI" workqueue and doing
> the transfers there.
> 
> 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 <dianders@chromium.org>
> ---
> 
>  drivers/platform/chrome/cros_ec_spi.c | 107 ++++++++++++++++++++++++--
>  1 file changed, 101 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/platform/chrome/cros_ec_spi.c b/drivers/platform/chrome/cros_ec_spi.c
> index ffc38f9d4829..101f2deb7d3c 100644
> --- a/drivers/platform/chrome/cros_ec_spi.c
> +++ b/drivers/platform/chrome/cros_ec_spi.c
>
> ...
>
> +static int cros_ec_pkt_xfer_spi(struct cros_ec_device *ec_dev,
> +				struct cros_ec_command *ec_msg)
> +{
> +	struct cros_ec_spi *ec_spi = ec_dev->priv;
> +	struct cros_ec_xfer_work_params params;
> +
> +	INIT_WORK(&params.work, cros_ec_pkt_xfer_spi_work);
> +	params.ec_dev = ec_dev;
> +	params.ec_msg = ec_msg;
> +
> +	queue_work(ec_spi->high_pri_wq, &params.work);
> +	flush_workqueue(ec_spi->high_pri_wq);

IIRC dedicated workqueues should be avoided unless they are needed. In
this case it seems you could use system_highpri_wq + a
completion. This would add a few extra lines to deal with the
completion, in exchange the code to create the workqueue could be
removed.

> +	return params.ret;
> +}
> +
> +static void cros_ec_cmd_xfer_spi_work(struct work_struct *work)
> +{
> +	struct cros_ec_xfer_work_params *params;
> +
> +	params = container_of(work, struct cros_ec_xfer_work_params, work);
> +	params->ret = do_cros_ec_cmd_xfer_spi(params->ec_dev, params->ec_msg);
> +}
> +
> +static int 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 cros_ec_xfer_work_params params;
> +
> +	INIT_WORK(&params.work, cros_ec_cmd_xfer_spi_work);
> +	params.ec_dev = ec_dev;
> +	params.ec_msg = ec_msg;
> +
> +	queue_work(ec_spi->high_pri_wq, &params.work);
> +	flush_workqueue(ec_spi->high_pri_wq);
> +
> +	return params.ret;
> +}

This is essentially a copy of cros_ec_pkt_xfer_spi() above. You
could add a wrapper that receives the work function to avoid the
duplicate code.

Cheers

Matthias