Re: [PATCH 1/2] iio: st_sensors: Retry ID verification on failure

[Jonathan Cameron <jic23@kernel.org>]

On Sun, 31 Jul 2022 21:51:55 +0300
Matti Lehtimäki <matti.lehtimaki@gmail.com> wrote:

> On 31.7.2022 19.00, Jonathan Cameron wrote:
> > On Sun, 24 Jul 2022 19:43:15 +0300
> > Matti Lehtimäki <matti.lehtimaki@gmail.com> wrote:
> >   
> >> Some sensors do not always start fast enough to read a valid ID from
> >> registers at first attempt. Let's retry at most 3 times with short sleep
> >> in between to fix random timing issues.
> >>
> >> Signed-off-by: Matti Lehtimäki <matti.lehtimaki@gmail.com>  
> > Hi Matti,
> > 
> > My gut feeling is this isn't in a fast path, so why not just wait
> > for whatever the documented power up time of the sensor is?
> > 
> > I'd expect to see a sleep in st_sensors_power_enable() if one is
> > required.  
> 
> In the specification for the sensor (lis2hh12) I have on my device I
> found that the maximum boot time of the sensor (starting from Vdd power
> on) is defined as 20 ms. Not sure if the other sensors supported by the
> driver have different values but based on checking a couple of
> specifications I didn't find any bigger values so far.
> 
> >> +			msleep(20);  
> > How do we know 60msecs is long enough for all sensors?  
> 
> Based on the specification for the sensor I have and also driver used in
> Android kernel for my device (it uses a 3 x 20 ms loop) I think 20 ms is
> a good value but to be sure a slightly longer might make sense. As
> suggested in the other review comment by changing the regmap_read to
> regmap_read_poll_timeout the function doesn't always need to wait at
> least 20 ms in case first read doesn't provide the correct value, if a
> suitable shorter poll interval is used (something like 1-10 ms).
> 
> However testing on my device has shown that I still need to have a loop
> or at least a retry possibility because I have noticed a rare random
> read error (-6, happens after some time not at first read) when reading
> the id from the hardware. This could be due to for example internal
> init failure of the sensor chip causing an internal reset. Because of
> this read error regmap_read_poll_timeout returns with an error and
> without retrying to read the id the sensor probe fails.

Nasty. If you can get a confirmation that it's a possible failure on startup
from the manufacturer then I'd be happier with that justification to retry
rather than just sleep for say 30msec after power on.

Jonathan

> 
> -Matti