linux-kernel - Re: [PATCH v8 6/6] at24: Support probing while off

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Date:   Tue, 6 Oct 2020 13:23:35 +0200
From:   Tomasz Figa <tfiga@...omium.org>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc:     linux-i2c <linux-i2c@...r.kernel.org>,
        Wolfram Sang <wsa@...-dreams.de>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Mani, Rajmohan" <rajmohan.mani@...el.com>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Bingbu Cao <bingbu.cao@...el.com>,
        Chiranjeevi Rapolu <chiranjeevi.rapolu@...el.com>,
        Hyungwoo Yang <hyungwoo.yang@...el.com>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCH v8 6/6] at24: Support probing while off

On Tue, Oct 6, 2020 at 1:20 PM Tomasz Figa <tfiga@...omium.org> wrote:
>
> Hi Sakari,
>
> On Thu, Sep 3, 2020 at 10:15 AM Sakari Ailus
> <sakari.ailus@...ux.intel.com> wrote:
> >
> > In certain use cases (where the chip is part of a camera module, and the
> > camera module is wired together with a camera privacy LED), powering on
> > the device during probe is undesirable. Add support for the at24 to
> > execute probe while being powered off. For this to happen, a hint in form
> > of a device property is required from the firmware.
> >
> > Signed-off-by: Sakari Ailus <sakari.ailus@...ux.intel.com>
> > ---
> >  drivers/misc/eeprom/at24.c | 43 +++++++++++++++++++++++---------------
> >  1 file changed, 26 insertions(+), 17 deletions(-)
> >
> > diff --git a/drivers/misc/eeprom/at24.c b/drivers/misc/eeprom/at24.c
> > index 8f5de5f10bbea..2d24e33788d7d 100644
> > --- a/drivers/misc/eeprom/at24.c
> > +++ b/drivers/misc/eeprom/at24.c
> > @@ -595,6 +595,7 @@ static int at24_probe(struct i2c_client *client)
> >         bool i2c_fn_i2c, i2c_fn_block;
> >         unsigned int i, num_addresses;
> >         struct at24_data *at24;
> > +       bool low_power;
> >         struct regmap *regmap;
> >         bool writable;
> >         u8 test_byte;
> > @@ -733,25 +734,30 @@ static int at24_probe(struct i2c_client *client)
> >
> >         i2c_set_clientdata(client, at24);
> >
> > -       err = regulator_enable(at24->vcc_reg);
> > -       if (err) {
> > -               dev_err(dev, "Failed to enable vcc regulator\n");
> > -               return err;
> > -       }
> > +       low_power = acpi_dev_state_low_power(&client->dev);
> > +       if (!low_power) {
> > +               err = regulator_enable(at24->vcc_reg);
> > +               if (err) {
> > +                       dev_err(dev, "Failed to enable vcc regulator\n");
> > +                       return err;
> > +               }
> >
> > -       /* enable runtime pm */
> > -       pm_runtime_set_active(dev);
> > +               pm_runtime_set_active(dev);
> > +       }
> >         pm_runtime_enable(dev);
> >
>
> What's the guarantee that at this point the runtime PM wouldn't
> suspend the device? Notice that the nvmem device is already exposed to
> the userspace, which could trigger pm runtime gets and puts (and thus
> idles as well).
>
> Best regards,
> Tomasz
>
> >         /*
> > -        * Perform a one-byte test read to verify that the
> > -        * chip is functional.
> > +        * Perform a one-byte test read to verify that the chip is functional,
> > +        * unless powering on the device is to be avoided during probe (i.e.
> > +        * it's powered off right now).
> >          */
> > -       err = at24_read(at24, 0, &test_byte, 1);

Actually never mind. Someone pointed out to me that at24_read() calls
pm_runtime_get_sync() internally, so we should be fine. Sorry, for the
noise.

Best regards,
Tomasz

> > -       if (err) {
> > -               pm_runtime_disable(dev);
> > -               regulator_disable(at24->vcc_reg);
> > -               return -ENODEV;
> > +       if (!low_power) {
> > +               err = at24_read(at24, 0, &test_byte, 1);
> > +               if (err) {
> > +                       pm_runtime_disable(dev);
> > +                       regulator_disable(at24->vcc_reg);
> > +                       return -ENODEV;
> > +               }
> >         }
> >
> >         pm_runtime_idle(dev);
> > @@ -771,9 +777,11 @@ static int at24_remove(struct i2c_client *client)
> >         struct at24_data *at24 = i2c_get_clientdata(client);
> >
> >         pm_runtime_disable(&client->dev);
> > -       if (!pm_runtime_status_suspended(&client->dev))
> > -               regulator_disable(at24->vcc_reg);
> > -       pm_runtime_set_suspended(&client->dev);
> > +       if (!acpi_dev_state_low_power(&client->dev)) {
> > +               if (!pm_runtime_status_suspended(&client->dev))
> > +                       regulator_disable(at24->vcc_reg);
> > +               pm_runtime_set_suspended(&client->dev);
> > +       }
> >
> >         return 0;
> >  }
> > @@ -810,6 +818,7 @@ static struct i2c_driver at24_driver = {
> >         .probe_new = at24_probe,
> >         .remove = at24_remove,
> >         .id_table = at24_ids,
> > +       .flags = I2C_DRV_FL_ALLOW_LOW_POWER_PROBE,
> >  };
> >
> >  static int __init at24_init(void)
> > --
> > 2.20.1
> >