cachepc-linux

Fork of AMDESE/linux with modifications for CachePC side-channel attack
git clone https://git.sinitax.com/sinitax/cachepc-linux
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firmware_cache.rst (2620B)

      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