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

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
[off-list ref] wrote:

quoted

Adding Tomasz...

quoted

-----Original Message-----
From: Mohandass, Divagar
Sent: Monday, September 04, 2017 3:29 AM
To: robh+dt@kernel.org; mark.rutland@arm.com; wsa@the-dreams.de;
sakari.ailus@iki.fi
Cc: devicetree@vger.kernel.org; linux-i2c@vger.kernel.org; linux-
kernel@vger.kernel.org; Mani, Rajmohan [off-list ref];
Mohandass, Divagar [off-list ref]
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 <redacted>
---
 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@iki.fi