On 6/16/2016 4:43 AM, Krzysztof Kozlowski wrote:

On 06/16/2016 12:13 AM, Rhyland Klein wrote:

quoted

power_supply_get_property() should ideally return -EAGAIN if it is
called while the power_supply is being registered. There was no way
previously to determine if use_cnt == 0 meant that the power_supply
wasn't fully registered yet, or if it had already been unregistered.

Add a new boolean to the power_supply struct to simply show if
registration is completed. Lastly, modify the check in
power_supply_show_property() to also ignore -EAGAIN when so it
doesn't complain about not returning the property.

Signed-off-by: Rhyland Klein <redacted>
---
v2:
 - Modify power_supply_show_property() to not complain if it
   sees a return value of -EAGAIN after calling
   power_supply_get_property().

 drivers/power/power_supply_core.c  | 6 +++++-
 drivers/power/power_supply_sysfs.c | 2 +-
 include/linux/power_supply.h       | 1 +
 3 files changed, 7 insertions(+), 2 deletions(-)

I don't like it for two reasons:
1. There is still a short window when the information will be
inaccurate. See comment below.

2. Although the code is not very complicated but it adds another field
and some checks just for differentiating EAGAIN/ENODEV. It is
unnecessary complexity.

quoted

diff --git a/drivers/power/power_supply_core.c b/drivers/power/power_supply_core.c
index b13cd074c52a..a39a47672979 100644
--- a/drivers/power/power_supply_core.c
+++ b/drivers/power/power_supply_core.c

@@ -491,8 +491,11 @@ int power_supply_get_property(struct power_supply *psy,
 			    enum power_supply_property psp,
 			    union power_supply_propval *val)
 {
-	if (atomic_read(&psy->use_cnt) <= 0)
+	if (atomic_read(&psy->use_cnt) <= 0) {
+		if (!psy->initialized)
+			return -EAGAIN;
 		return -ENODEV;
+	}
 
 	return psy->desc->get_property(psy, psp, val);
 }

@@ -775,6 +778,7 @@ __power_supply_register(struct device *parent,
 	if (rc)
 		goto create_triggers_failed;
 
+	psy->initialized = true;

If someone calls power_supply_get_property() here, then ENODEV will be
returned which is wrong.

The problem is not solved entirely... I am not convinced that introduced
complexity is worth fixing it.

Right now, without this patch, this causes the thermal function
"update_temperature" in:

thermal_zone_device_register->
  thermal_zone_device_update->
    update_temperature
      (->thermal_zone_get_temp() from the original stack)

to print a warning as it sees ret != -EAGAIN. This causes a warning
"failed to read out thermal zone". I think the idea there is if anything
other than "try again" it complains. While this doesn't cause functional
problems, I do think avoid the warning is worth while.

I think that there is an onus on the power_supply code to be accurate in
its return codes, and EAGAIN is the correct one we should be returning.
I don't see how someone would call power_supply_get_property, but I
agree there is the possibility that if someone did call there, that it
could return the wrong value.

We could wrap the setting of initialized and use_cnt inside a mutex,
which should prevent anyone calling anything in between if we wanted to
completely plug that hole. I am not fond of the idea of adding a struct
member for such a small, specific case, but as we found before, I don't
think there is another way to differentiate otherwise.

-rhyland


-- 
nvpublic