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    SubjectRe: [PATCH v2] platform/chrome: cros_ec_spi: Transfer messages at high priority
    On Wed, Apr 03, 2019 at 10:04:16AM -0700, Brian Norris wrote:
    > I know some of this was hashed out last night, but I wasn't reading my
    > email then to interject ;)
    >
    > On Wed, Apr 3, 2019 at 9:05 AM Douglas Anderson <dianders@chromium.org> wrote:
    > > +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);
    > > + params.ec_dev = ec_dev;
    > > + params.ec_msg = ec_msg;
    > > + params.fn = fn;
    > > + init_completion(&params.completion);
    > > +
    > > + /*
    > > + * 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);
    >
    > Does anybody know what the definition of "too long" is for the phrase
    > "Don't queue works which can run for too long" in the documentation?
    >
    > > + wait_for_completion(&params.completion);
    >
    > I think flush_workqueue() was discussed and rejected, but what about
    > flush_work()? Then you don't have to worry about the rest of the
    > contents of the workqueue -- just your own work--and I think you could
    > avoid the 'completion'.

    Indeed, flush_work() seems the right thing to do.

    I thought to remember that there is a function to wait for a work to
    complete and scanned through workqueue.h for it, but somehow missed it.

    > You might also have a tiny race in the current implementation, since
    > (a) you can't queue the same work item twice and
    > (b) technically, the complete() call is still while the work item is
    > running -- you don't really guarantee the work item has finished
    > before you continue.
    > So the combination of (a) and (b) means that moving from one xfer to
    > the next, you might not successfully queue your work at all. You could
    > probably test this by checking the return value of queue_work() under
    > a heavy EC workload. Avoiding the completion would also avoid this
    > race.

    Each transfer has it's own work struct (allocated on the stack), hence
    a) does not occur. b) is still true, but shouldn't be a problem on
    its own.

    Anyway, using flush_work() as you suggested is the better solution :)

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