Re: [PATCH] iio: sx9310: Prefer async probe

[Doug Anderson]

Hi,

On Tue, Sep 1, 2020 at 6:47 AM Jonathan Cameron
<Jonathan.Cameron@xxxxxxxxxx> wrote:
>
> On Tue, 1 Sep 2020 00:03:55 -0700
> Stephen Boyd <swboyd@xxxxxxxxxxxx> wrote:
>
> > Quoting Doug Anderson (2020-08-31 11:59:37)
> > > On Sat, Aug 29, 2020 at 10:18 AM Jonathan Cameron <jic23@xxxxxxxxxx> wrote:
> > > > Agreed. However, I'd like a comment next to the place we turn it on
> > > > saying what delays we are trying to mitigate by enabling it in this
> > > > driver.
> > >
> > > OK, I used function graph to get a trace.  Some of the time is simply
> > > i2c transfers, which we do A LOT of during initialization (and i2c
> > > transfers aren't known for being instant).  However, that's not the
> > > big problem.  Check out the regmap_read_poll_timeout() in
> > > sx9310_init_compensation().  20 ms per sleep with a 2 second timeout.
> > > This seems to eat over around 220 ms of the time on my system.
> > >
> > > So how do you want to do this?  Do you want me to re-post the patch
> > > and mention the regmap_read_poll_timeout() in the commit message?  Do
> > > you want to just add that to the commit message yourself?
> > >
> >
> > I think the suggestion is to put a comment next to the
> > PROBE_PREFER_ASYNCHRONOUS flag indicating that
> > sx9310_init_compensation() can take a while.
>
> Yup. That's what I meant. Sorry for lack of clarity.
> A comment in the code stops others copying it without making sure they
> understand whether it is relevant in their case.  No one is going
> to look in commit messages.

OK, posted.  I expect it'll show up soon at:

https://lore.kernel.org/r/20200901081920.v2.1.Id02b2f451b3eed71ddd580f4b8b44b3e33e84970@changeid


> > Should the compensation be
> > delayed even further though to some time when userspace opens the device
> > node?
>
> It's tricky because we may hit it on an individual sysfs read.
> We could spin off a thread to do it I suppose, though that would be
> more or less the same as allowing async probe.

Also: even if we delayed the compensation further we'd still end up
with a somewhat slow probe just due to the metric buttload of i2c
transfers we do at probe time.


-Doug