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113ccc38 LR |
1 | ============== |
2 | Firmware cache | |
3 | ============== | |
4 | ||
5 | When Linux resumes from suspend some device drivers require firmware lookups to | |
6 | re-initialize devices. During resume there may be a period of time during which | |
7 | firmware lookups are not possible, during this short period of time firmware | |
8 | requests will fail. Time is of essence though, and delaying drivers to wait for | |
9 | the root filesystem for firmware delays user experience with device | |
10 | functionality. In order to support these requirements the firmware | |
11 | infrastructure implements a firmware cache for device drivers for most API | |
12 | calls, automatically behind the scenes. | |
13 | ||
14 | The firmware cache makes using certain firmware API calls safe during a device | |
15 | driver's suspend and resume callback. Users of these API calls needn't cache | |
16 | the firmware by themselves for dealing with firmware loss during system resume. | |
17 | ||
18 | The firmware cache works by requesting for firmware prior to suspend and | |
19 | caching it in memory. Upon resume device drivers using the firmware API will | |
20 | have access to the firmware immediately, without having to wait for the root | |
21 | filesystem to mount or dealing with possible race issues with lookups as the | |
22 | root filesystem mounts. | |
23 | ||
24 | Some 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 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 | |
4fde24d4 LR |
32 | userspace for the firmware request through the sysfs fallback mechanism |
33 | if the firmware file is not found. | |
113ccc38 LR |
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 |