Re: [PATCH v6 3/3] eeprom: at24: enable runtime pm support

[sakari.ailus@xxxxxx]

Hi Tomasz,

On Wed, Sep 20, 2017 at 12:56:09PM +0900, Tomasz Figa wrote:
> Thanks Raj.
> 
> Let me post my comments inline.
> 
> On Wed, Sep 20, 2017 at 12:52 PM, Mani, Rajmohan
> <rajmohan.mani@xxxxxxxxx> wrote:
> > Adding Tomasz...
> >
> >> -----Original Message-----
> >> From: Mohandass, Divagar
> >> Sent: Monday, September 04, 2017 3:29 AM
> >> To: robh+dt@xxxxxxxxxx; mark.rutland@xxxxxxx; wsa@xxxxxxxxxxxxx;
> >> sakari.ailus@xxxxxx
> >> Cc: devicetree@xxxxxxxxxxxxxxx; linux-i2c@xxxxxxxxxxxxxxx; linux-
> >> kernel@xxxxxxxxxxxxxxx; Mani, Rajmohan <rajmohan.mani@xxxxxxxxx>;
> >> Mohandass, Divagar <divagar.mohandass@xxxxxxxxx>
> >> Subject: [PATCH v6 3/3] eeprom: at24: enable runtime pm support
> >>
> >> Currently the device is kept in D0, there is an opportunity to save power by
> >> enabling runtime pm.
> >>
> >> Device can be daisy chained from PMIC and we can't rely on I2C core for auto
> >> resume/suspend. Driver will decide when to resume/suspend.
> >>
> >> Signed-off-by: Divagar Mohandass <divagar.mohandass@xxxxxxxxx>
> >> ---
> >>  drivers/misc/eeprom/at24.c | 38
> >> ++++++++++++++++++++++++++++++++++++++
> >>  1 file changed, 38 insertions(+)
> >>
> >> diff --git a/drivers/misc/eeprom/at24.c b/drivers/misc/eeprom/at24.c index
> >> 2199c42..d718a7a 100644
> >> --- a/drivers/misc/eeprom/at24.c
> >> +++ b/drivers/misc/eeprom/at24.c
> >> @@ -24,6 +24,7 @@
> >>  #include <linux/i2c.h>
> >>  #include <linux/nvmem-provider.h>
> >>  #include <linux/platform_data/at24.h>
> >> +#include <linux/pm_runtime.h>
> >>
> >>  /*
> >>   * I2C EEPROMs from most vendors are inexpensive and mostly
> >> interchangeable.
> >> @@ -501,11 +502,21 @@ static ssize_t at24_eeprom_write_i2c(struct
> >> at24_data *at24, const char *buf,  static int at24_read(void *priv, unsigned int
> >> off, void *val, size_t count)  {
> >>       struct at24_data *at24 = priv;
> >> +     struct i2c_client *client;
> >>       char *buf = val;
> >> +     int ret;
> >>
> >>       if (unlikely(!count))
> >>               return count;
> >>
> >> +     client = at24_translate_offset(at24, &off);
> >> +
> >> +     ret = pm_runtime_get_sync(&client->dev);
> >> +     if (ret < 0) {
> >> +             pm_runtime_put_noidle(&client->dev);
> >> +             return ret;
> >> +     }
> >> +
> >>       /*
> >>        * Read data from chip, protecting against concurrent updates
> >>        * from this host, but not from other I2C masters.
> >> @@ -518,6 +529,7 @@ static int at24_read(void *priv, unsigned int off, void
> >> *val, size_t count)
> >>               status = at24->read_func(at24, buf, off, count);
> >>               if (status < 0) {
> >>                       mutex_unlock(&at24->lock);
> >> +                     pm_runtime_put(&client->dev);
> >>                       return status;
> >>               }
> >>               buf += status;
> >> @@ -527,17 +539,29 @@ static int at24_read(void *priv, unsigned int off, void
> >> *val, size_t count)
> >>
> >>       mutex_unlock(&at24->lock);
> >>
> >> +     pm_runtime_put(&client->dev);
> >> +
> >>       return 0;
> >>  }
> >>
> >>  static int at24_write(void *priv, unsigned int off, void *val, size_t count)  {
> >>       struct at24_data *at24 = priv;
> >> +     struct i2c_client *client;
> >>       char *buf = val;
> >> +     int ret;
> >>
> >>       if (unlikely(!count))
> >>               return -EINVAL;
> >>
> >> +     client = at24_translate_offset(at24, &off);
> >> +
> >> +     ret = pm_runtime_get_sync(&client->dev);
> >> +     if (ret < 0) {
> >> +             pm_runtime_put_noidle(&client->dev);
> >> +             return ret;
> >> +     }
> >> +
> >>       /*
> >>        * Write data to chip, protecting against concurrent updates
> >>        * from this host, but not from other I2C masters.
> >> @@ -550,6 +574,7 @@ static int at24_write(void *priv, unsigned int off, void
> >> *val, size_t count)
> >>               status = at24->write_func(at24, buf, off, count);
> >>               if (status < 0) {
> >>                       mutex_unlock(&at24->lock);
> >> +                     pm_runtime_put(&client->dev);
> >>                       return status;
> >>               }
> >>               buf += status;
> >> @@ -559,6 +584,8 @@ static int at24_write(void *priv, unsigned int off, void
> >> *val, size_t count)
> >>
> >>       mutex_unlock(&at24->lock);
> >>
> >> +     pm_runtime_put(&client->dev);
> >> +
> >>       return 0;
> >>  }
> >>
> >> @@ -743,11 +770,17 @@ static int at24_probe(struct i2c_client *client, const
> >> struct i2c_device_id *id)
> >>
> >>       i2c_set_clientdata(client, at24);
> >>
> >> +     /* enable runtime pm */
> >> +     pm_runtime_get_noresume(&client->dev);
> >> +     pm_runtime_set_active(&client->dev);
> >> +     pm_runtime_enable(&client->dev);
> 
> Do we need this get_noresume/set_active dance? I remember it was for
> some reason needed for PCI devices, but I don't see why for I2C
> anything else than just pm_runtime_enable() would be necessary.

You specifically do not need (all) this for PCI devices, but AFAIU for I²C
devices you do. The runtime PM status of a device is disabled by default
and the use count is zero, but on ACPI based systems the device is still
powered on.

> 
> Also, we enable runtime PM, but we don't provide any callbacks. If
> there is no callback in any level of the hierarchy, NULL would be
> returned in [3], making [2] return -ENOSYS and [1] fail. The behavior
> depends on subsystem and whether the device is attached to a
> pm_domain. In our particular case I'd guess the device would be in an
> ACPI pm_domain and that would work, but the driver is generic and must
> work in any cases.

Agreed.

Cc Mika, too.

-- 
Regards,

Sakari Ailus
e-mail: sakari.ailus@xxxxxx