On Fri, 28 Aug 2020 17:01:18 -0700
Douglas Anderson <dianders@xxxxxxxxxxxx> wrote:
> On one board I found that:
> probe of 5-0028 returned 1 after 259547 usecs
>
> There's no reason to block probe of all other devices on our probe.
> Turn on async probe.
>
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> ---
> NOTE: I haven't done any analysis of the driver to see _why_ it's so
> slow, only that I have measured it to be slow. Someone could
> certainly take the time to profile / optimize it, but in any case it
> still won't hurt to be async.
Hmm. It is vanishingly rare to use that flag so I'm not particularly
keen on starting to deploy it when we don't know why a particular
driver is taking so long. I agree it should be safe but I don't
like oddities I don't understand!
There are some sleeps in there but they are all of the order of a few
msecs.
Could it be there is a regulator that is coming up very slowly?
Any other ideas?
Jonathan
>
> This is a very safe flag to turn on since:
>
> 1. It's not like our probe order was defined by anything anyway. When
> we probe is at the whim of when our i2c controller probes and that can
> be any time.
>
> 2. If some other driver needs us then they have to handle the fact
> that we might not have probed yet anyway.
>
> 3. There may be other drivers probing at the same time as us anyway
> because _they_ used async probe.
>
> While I won't say that it's impossible to tickle a bug by turning on
> async probe, I would assert that in almost all cases the bug was
> already there and needed to be fixed anyway.
>
> ALSO NOTE: measurement / testing was done on the downstream Chrome OS
> 5.4 tree. I confirmed compiling on mainline.
>
> drivers/iio/proximity/sx9310.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/drivers/iio/proximity/sx9310.c b/drivers/iio/proximity/sx9310.c
> index dc2e11b43431..444cafc53408 100644
> --- a/drivers/iio/proximity/sx9310.c
> +++ b/drivers/iio/proximity/sx9310.c
> @@ -1054,6 +1054,7 @@ static struct i2c_driver sx9310_driver = {
> .acpi_match_table = ACPI_PTR(sx9310_acpi_match),
> .of_match_table = of_match_ptr(sx9310_of_match),
> .pm = &sx9310_pm_ops,
> + .probe_type = PROBE_PREFER_ASYNCHRONOUS,
> },
> .probe = sx9310_probe,
> .id_table = sx9310_id,
