mirror of
https://github.com/edk2-porting/linux-next.git
synced 2024-12-23 12:43:55 +08:00
20b33f84f9
Drop the doubled word "if". Signed-off-by: Randy Dunlap <rdunlap@infradead.org> Cc: Jonathan Corbet <corbet@lwn.net> Cc: linux-doc@vger.kernel.org Acked-by: Luis Chamberlain <mcgrof@kernel.org> Link: https://lore.kernel.org/r/20200704034502.17199-4-rdunlap@infradead.org Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
52 lines
2.6 KiB
ReStructuredText
52 lines
2.6 KiB
ReStructuredText
==============
|
|
Firmware cache
|
|
==============
|
|
|
|
When Linux resumes from suspend some device drivers require firmware lookups to
|
|
re-initialize devices. During resume there may be a period of time during which
|
|
firmware lookups are not possible, during this short period of time firmware
|
|
requests will fail. Time is of essence though, and delaying drivers to wait for
|
|
the root filesystem for firmware delays user experience with device
|
|
functionality. In order to support these requirements the firmware
|
|
infrastructure implements a firmware cache for device drivers for most API
|
|
calls, automatically behind the scenes.
|
|
|
|
The firmware cache makes using certain firmware API calls safe during a device
|
|
driver's suspend and resume callback. Users of these API calls needn't cache
|
|
the firmware by themselves for dealing with firmware loss during system resume.
|
|
|
|
The firmware cache works by requesting for firmware prior to suspend and
|
|
caching it in memory. Upon resume device drivers using the firmware API will
|
|
have access to the firmware immediately, without having to wait for the root
|
|
filesystem to mount or dealing with possible race issues with lookups as the
|
|
root filesystem mounts.
|
|
|
|
Some implementation details about the firmware cache setup:
|
|
|
|
* The firmware cache is setup by adding a devres entry for each device that
|
|
uses all synchronous call except :c:func:`request_firmware_into_buf`.
|
|
|
|
* If an asynchronous call is used the firmware cache is only set up for a
|
|
device if the second argument (uevent) to request_firmware_nowait() is
|
|
true. When uevent is true it requests that a kobject uevent be sent to
|
|
userspace for the firmware request through the sysfs fallback mechanism
|
|
if the firmware file is not found.
|
|
|
|
* If the firmware cache is determined to be needed as per the above two
|
|
criteria the firmware cache is setup by adding a devres entry for the
|
|
device making the firmware request.
|
|
|
|
* The firmware devres entry is maintained throughout the lifetime of the
|
|
device. This means that even if you release_firmware() the firmware cache
|
|
will still be used on resume from suspend.
|
|
|
|
* The timeout for the fallback mechanism is temporarily reduced to 10 seconds
|
|
as the firmware cache is set up during suspend, the timeout is set back to
|
|
the old value you had configured after the cache is set up.
|
|
|
|
* Upon suspend any pending non-uevent firmware requests are killed to avoid
|
|
stalling the kernel, this is done with kill_requests_without_uevent(). Kernel
|
|
calls requiring the non-uevent therefore need to implement their own firmware
|
|
cache mechanism but must not use the firmware API on suspend.
|
|
|