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PCI / PM: Update runtime PM documentation for PCI devices

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Section 3.2 "Device Runtime Power Management" of pci.txt has become
outdated, so update it to correctly reflect the current code flow.

Also update the comment in local_pci_probe() to document the fact
that pm_runtime_put_noidle() is not the only runtime PM helper
function that can be used to decrement the device's runtime PM
usage counter in .probe().

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
This commit is contained in:
Rafael J. Wysocki 2015-09-18 03:08:40 +02:00
parent 1f93e4a96c
commit a8360062cc
2 changed files with 42 additions and 16 deletions
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51
Documentation/power/pci.txt
View File

@ -979,20 +979,45 @@ every time right after the runtime_resume() callback has returned
(alternatively, the runtime_suspend() callback will have to check if the
device should really be suspended and return -EAGAIN if that is not the case).
The runtime PM of PCI devices is disabled by default. It is also blocked by
pci_pm_init() that runs the pm_runtime_forbid() helper function. If a PCI
driver implements the runtime PM callbacks and intends to use the runtime PM
framework provided by the PM core and the PCI subsystem, it should enable this
feature by executing the pm_runtime_enable() helper function. However, the
driver should not call the pm_runtime_allow() helper function unblocking
the runtime PM of the device. Instead, it should allow user space or some
platform-specific code to do that (user space can do it via sysfs), although
once it has called pm_runtime_enable(), it must be prepared to handle the
The runtime PM of PCI devices is enabled by default by the PCI core. PCI
device drivers do not need to enable it and should not attempt to do so.
However, it is blocked by pci_pm_init() that runs the pm_runtime_forbid()
helper function. In addition to that, the runtime PM usage counter of
each PCI device is incremented by local_pci_probe() before executing the
probe callback provided by the device's driver.
If a PCI driver implements the runtime PM callbacks and intends to use the
runtime PM framework provided by the PM core and the PCI subsystem, it needs
to decrement the device's runtime PM usage counter in its probe callback
function. If it doesn't do that, the counter will always be different from
zero for the device and it will never be runtime-suspended. The simplest
way to do that is by calling pm_runtime_put_noidle(), but if the driver
wants to schedule an autosuspend right away, for example, it may call
pm_runtime_put_autosuspend() instead for this purpose. Generally, it
just needs to call a function that decrements the devices usage counter
from its probe routine to make runtime PM work for the device.
It is important to remember that the driver's runtime_suspend() callback
may be executed right after the usage counter has been decremented, because
user space may already have cuased the pm_runtime_allow() helper function
unblocking the runtime PM of the device to run via sysfs, so the driver must
be prepared to cope with that.
The driver itself should not call pm_runtime_allow(), though. Instead, it
should let user space or some platform-specific code do that (user space can
do it via sysfs as stated above), but it must be prepared to handle the
runtime PM of the device correctly as soon as pm_runtime_allow() is called
(which may happen at any time). [It also is possible that user space causes
pm_runtime_allow() to be called via sysfs before the driver is loaded, so in
fact the driver has to be prepared to handle the runtime PM of the device as
soon as it calls pm_runtime_enable().]
(which may happen at any time, even before the driver is loaded).
When the driver's remove callback runs, it has to balance the decrementation
of the device's runtime PM usage counter at the probe time. For this reason,
if it has decremented the counter in its probe callback, it must run
pm_runtime_get_noresume() in its remove callback. [Since the core carries
out a runtime resume of the device and bumps up the device's usage counter
before running the driver's remove callback, the runtime PM of the device
is effectively disabled for the duration of the remove execution and all
runtime PM helper functions incrementing the device's usage counter are
then effectively equivalent to pm_runtime_get_noresume().]
The runtime PM framework works by processing requests to suspend or resume
devices, or to check if they are idle (in which cases it is reasonable to

7
drivers/pci/pci-driver.c
View File

@ -299,9 +299,10 @@ static long local_pci_probe(void *_ddi)
* Unbound PCI devices are always put in D0, regardless of
* runtime PM status. During probe, the device is set to
* active and the usage count is incremented. If the driver
* supports runtime PM, it should call pm_runtime_put_noidle()
* in its probe routine and pm_runtime_get_noresume() in its
* remove routine.
* supports runtime PM, it should call pm_runtime_put_noidle(),
* or any other runtime PM helper function decrementing the usage
* count, in its probe routine and pm_runtime_get_noresume() in
* its remove routine.
*/
pm_runtime_get_sync(dev);
pci_dev->driver = pci_drv;