1==============
2Firmware cache
3==============
4
5When Linux resumes from suspend some device drivers require firmware lookups to
6re-initialize devices. During resume there may be a period of time during which
7firmware lookups are not possible, during this short period of time firmware
8requests will fail. Time is of essence though, and delaying drivers to wait for
9the root filesystem for firmware delays user experience with device
10functionality. In order to support these requirements the firmware
11infrastructure implements a firmware cache for device drivers for most API
12calls, automatically behind the scenes.
13
14The firmware cache makes using certain firmware API calls safe during a device
15driver's suspend and resume callback.  Users of these API calls needn't cache
16the firmware by themselves for dealing with firmware loss during system resume.
17
18The firmware cache works by requesting for firmware prior to suspend and
19caching it in memory. Upon resume device drivers using the firmware API will
20have access to the firmware immediately, without having to wait for the root
21filesystem to mount or dealing with possible race issues with lookups as the
22root filesystem mounts.
23
24Some implementation details about the firmware cache setup:
25
26* The firmware cache is setup by adding a devres entry for each device that
27  uses all synchronous call except :c:func:`request_firmware_into_buf`.
28
29* If an asynchronous call is used the firmware cache is only set up for a
30  device if the second argument (uevent) to request_firmware_nowait() is
31  true. When uevent is true it requests that a kobject uevent be sent to
32  userspace for the firmware request through the sysfs fallback mechanism
33  if the firmware file is not found.
34
35* If the firmware cache is determined to be needed as per the above two
36  criteria the firmware cache is setup by adding a devres entry for the
37  device making the firmware request.
38
39* The firmware devres entry is maintained throughout the lifetime of the
40  device. This means that even if you release_firmware() the firmware cache
41  will still be used on resume from suspend.
42
43* The timeout for the fallback mechanism is temporarily reduced to 10 seconds
44  as the firmware cache is set up during suspend, the timeout is set back to
45  the old value you had configured after the cache is set up.
46
47* Upon suspend any pending non-uevent firmware requests are killed to avoid
48  stalling the kernel, this is done with kill_requests_without_uevent(). Kernel
49  calls requiring the non-uevent therefore need to implement their own firmware
50  cache mechanism but must not use the firmware API on suspend.
51
52