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 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. For details refer to the Fackback 33 mechanism documented below. 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