Re: [PATCH 3/3] media: i2c: ak7375: Add regulator management

[Jacopo Mondi <jacopo@jmondi.org>]

Hi Yassine

On Mon, Jul 11, 2022 at 06:08:53PM +0400, Yassine Oudjana wrote:
>
> On Mon, Jul 11 2022 at 15:34:23 +0200, Jacopo Mondi <jacopo@jmondi.org>
> wrote:
> > Hello Yassine
> >
> > On Mon, Jul 11, 2022 at 08:28:39AM +0400, Yassine Oudjana wrote:
> > >  From: Yassine Oudjana <y.oudjana@protonmail.com>
> > >
> > >  Make the driver get needed regulators on probe and enable/disable
> > >  them on runtime PM callbacks.
> > >
> > >  Signed-off-by: Yassine Oudjana <y.oudjana@protonmail.com>
> > >  ---
> > >   drivers/media/i2c/ak7375.c | 39
> > > ++++++++++++++++++++++++++++++++++++++
> > >   1 file changed, 39 insertions(+)
> > >
> > >  diff --git a/drivers/media/i2c/ak7375.c b/drivers/media/i2c/ak7375.c
> > >  index 40b1a4aa846c..59d5cb00e3ba 100644
> > >  --- a/drivers/media/i2c/ak7375.c
> > >  +++ b/drivers/media/i2c/ak7375.c
> > >  @@ -6,6 +6,7 @@
> > >   #include <linux/i2c.h>
> > >   #include <linux/module.h>
> > >   #include <linux/pm_runtime.h>
> > >  +#include <linux/regulator/consumer.h>
> > >   #include <media/v4l2-ctrls.h>
> > >   #include <media/v4l2-device.h>
> > >
> > >  @@ -23,17 +24,32 @@
> > >    */
> > >   #define AK7375_CTRL_STEPS	64
> > >   #define AK7375_CTRL_DELAY_US	1000
> > >  +/*
> > >  + * The vcm takes around 3 ms to power on and start taking
> > >  + * I2C messages. This value was found experimentally due to
> > >  + * lack of documentation. 2 ms is added as a safety margin.
> > >  + */
> > >  +#define AK7375_POWER_DELAY_US	5000
> > >
> > >   #define AK7375_REG_POSITION	0x0
> > >   #define AK7375_REG_CONT		0x2
> > >   #define AK7375_MODE_ACTIVE	0x0
> > >   #define AK7375_MODE_STANDBY	0x40
> > >
> > >  +static const char * const ak7375_supply_names[] = {
> > >  +	"vdd",
> > >  +	"vio",
> > >  +};
> > >  +
> > >  +#define AK7375_NUM_SUPPLIES ARRAY_SIZE(ak7375_supply_names)
> > >  +
> > >   /* ak7375 device structure */
> > >   struct ak7375_device {
> > >   	struct v4l2_ctrl_handler ctrls_vcm;
> > >   	struct v4l2_subdev sd;
> > >   	struct v4l2_ctrl *focus;
> > >  +	struct regulator_bulk_data supplies[AK7375_NUM_SUPPLIES];
> > >  +
> > >   	/* active or standby mode */
> > >   	bool active;
> > >   };
> > >  @@ -132,6 +148,7 @@ static int ak7375_init_controls(struct
> > > ak7375_device *dev_vcm)
> > >   static int ak7375_probe(struct i2c_client *client)
> > >   {
> > >   	struct ak7375_device *ak7375_dev;
> > >  +	int i;
> >
> > I would have moved this one down to maintain variable declaration
> > in the in-famous reverse-xmas-tree ordering. Up to you.
>
> I'm used to declaring variables in the order of first use,
> but I don't really mind it either way. I'll move it down.
>
> >
> > >   	int ret;
> > >
> > >   	ak7375_dev = devm_kzalloc(&client->dev, sizeof(*ak7375_dev),
> > >  @@ -139,6 +156,17 @@ static int ak7375_probe(struct i2c_client
> > > *client)
> > >   	if (!ak7375_dev)
> > >   		return -ENOMEM;
> > >
> > >  +	for (i = 0; i < AK7375_NUM_SUPPLIES; i++)
> > >  +		ak7375_dev->supplies[i].supply = ak7375_supply_names[i];
> > >  +
> > >  +	ret = devm_regulator_bulk_get(&client->dev, AK7375_NUM_SUPPLIES,
> > >  +				      ak7375_dev->supplies);
> > >  +	if (ret) {
> > >  +		dev_err(&client->dev, "Failed to get regulators: %pe",
> > >  +			ERR_PTR(ret));
> > >  +		return ret;
> > >  +	}
> > >  +
> > >   	v4l2_i2c_subdev_init(&ak7375_dev->sd, client, &ak7375_ops);
> > >   	ak7375_dev->sd.flags |= V4L2_SUBDEV_FL_HAS_DEVNODE;
> > >   	ak7375_dev->sd.internal_ops = &ak7375_int_ops;
> > >  @@ -210,6 +238,10 @@ static int __maybe_unused
> > > ak7375_vcm_suspend(struct device *dev)
> > >   	if (ret)
> > >   		dev_err(dev, "%s I2C failure: %d\n", __func__, ret);
> > >
> > >  +	ret = regulator_bulk_disable(AK7375_NUM_SUPPLIES,
> > > ak7375_dev->supplies);
> > >  +	if (ret)
> > >  +		return ret;
> > >  +
> > >   	ak7375_dev->active = false;
> > >
> > >   	return 0;
> > >  @@ -230,6 +262,13 @@ static int __maybe_unused
> > > ak7375_vcm_resume(struct device *dev)
> > >   	if (ak7375_dev->active)
> > >   		return 0;
> > >
> > >  +	ret = regulator_bulk_enable(AK7375_NUM_SUPPLIES,
> > > ak7375_dev->supplies);
> > >  +	if (ret)
> > >  +		return ret;
> > >  +
> > >  +	/* Wait for vcm to become ready */
> > >  +	usleep_range(AK7375_POWER_DELAY_US, AK7375_POWER_DELAY_US + 10);
> > >  +
> >
> > Isn't 10usec a very small delay to be given to usleep_range() for a
> > delay of at least 3msec ? Also assuming 5msec just to be safe seems a
> > little arbitrary. Adding 2 milliseconds in the wakeup path introduces
> > a non-negligible delay.
>
> I must admit that I didn't give it too much thought. I just
> did it similar to the other delay used in this driver
> (AK7375_CTRL_DELAY_US). As for adding 2ms, I don't know what
> the worst case wake-up time is since I don't have a datasheet
> on hand, so I just wanted to stay safe. Also, this driver

Oh sorry, I missed in the comment the value was found experimentally
an it's not documented..

> doesn't really recover if it fails to resume (which is what
> used to happen before adding a delay). Rounding up to 5ms
> felt good enough.
>
> >
> > It's likely a detail, but according to
> > Documentation/timers/timers-howto.rst
> >
> >         Since usleep_range is built on top of hrtimers, the
> >         wakeup will be very precise (ish), thus a simple
> >         usleep function would likely introduce a large number
> >         of undesired interrupts.
> >
> >         With the introduction of a range, the scheduler is
> >         free to coalesce your wakeup with any other wakeup
> >         that may have happened for other reasons, or at the
> >         worst case, fire an interrupt for your upper bound.
> >
> >         The larger a range you supply, the greater a chance
> >         that you will not trigger an interrupt; this should
> >         be balanced with what is an acceptable upper bound on
> >         delay / performance for your specific code path. Exact
> >         tolerances here are very situation specific, thus it
> >         is left to the caller to determine a reasonable range.
> >
> > If you have a min of 3msec I would try with a range of (3000, 3500).
> > What do you think ?
>
> Seems good. I haven't yet had it fail to power on within 3ms of
> turning on regulators so I guess there is no reason to worry about it.
>

Ok then :) There's anyway a comment that says the value comes from
practical experience, so if anything bad happens, it's easy to track
it down to that

Thanks
  j

> > >
> > >   	ret = ak7375_i2c_write(ak7375_dev, AK7375_REG_CONT,
> > >   		AK7375_MODE_ACTIVE, 1);
> > >   	if (ret) {
> > >  --
> > >  2.37.0
> >
>
> Thanks for the review,
> Yassine
>
>