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

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Date:   Sun, 31 Jul 2022 17:00:57 +0100
From:   Jonathan Cameron <jic23@...nel.org>
To:     Matti Lehtimäki <matti.lehtimaki@...il.com>
Cc:     linux-iio@...r.kernel.org, ~postmarketos/upstreaming@...ts.sr.ht,
        Lars-Peter Clausen <lars@...afoo.de>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Alexandru Ardelean <aardelean@...iqon.com>,
        Cai Huoqing <cai.huoqing@...ux.dev>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] iio: st_sensors: Retry ID verification on failure

On Sun, 24 Jul 2022 19:43:15 +0300
Matti Lehtimäki <matti.lehtimaki@...il.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@...il.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.

Jonathan

> ---
>  drivers/iio/common/st_sensors/st_sensors_core.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iio/common/st_sensors/st_sensors_core.c b/drivers/iio/common/st_sensors/st_sensors_core.c
> index 9910ba1da085..106f7953683e 100644
> --- a/drivers/iio/common/st_sensors/st_sensors_core.c
> +++ b/drivers/iio/common/st_sensors/st_sensors_core.c
> @@ -21,6 +21,8 @@
>  
>  #include "st_sensors_core.h"
>  
> +#define VERIFY_ID_RETRY_COUNT 3
> +
>  int st_sensors_write_data_with_mask(struct iio_dev *indio_dev,
>  				    u8 reg_addr, u8 mask, u8 data)
>  {
> @@ -619,11 +621,18 @@ EXPORT_SYMBOL_NS(st_sensors_get_settings_index, IIO_ST_SENSORS);
>  int st_sensors_verify_id(struct iio_dev *indio_dev)
>  {
>  	struct st_sensor_data *sdata = iio_priv(indio_dev);
> -	int wai, err;
> +	int wai, err, i;
>  
>  	if (sdata->sensor_settings->wai_addr) {
> -		err = regmap_read(sdata->regmap,
> -				  sdata->sensor_settings->wai_addr, &wai);
> +		for (i = 0; i < VERIFY_ID_RETRY_COUNT; i++) {
> +			err = regmap_read(sdata->regmap,
> +					  sdata->sensor_settings->wai_addr, &wai);
> +
> +			if (!err && sdata->sensor_settings->wai == wai)
> +				return 0;
> +
> +			msleep(20);
How do we know 60msecs is long enough for all sensors?

> +		}
>  		if (err < 0) {
>  			dev_err(&indio_dev->dev,
>  				"failed to read Who-Am-I register.\n");