52 lines
2.6 KiB
ReStructuredText
52 lines
2.6 KiB
ReStructuredText
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==============
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Firmware cache
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==============
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When Linux resumes from suspend some device drivers require firmware lookups to
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re-initialize devices. During resume there may be a period of time during which
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firmware lookups are not possible, during this short period of time firmware
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requests will fail. Time is of essence though, and delaying drivers to wait for
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the root filesystem for firmware delays user experience with device
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functionality. In order to support these requirements the firmware
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infrastructure implements a firmware cache for device drivers for most API
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calls, automatically behind the scenes.
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The firmware cache makes using certain firmware API calls safe during a device
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driver's suspend and resume callback. Users of these API calls needn't cache
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the firmware by themselves for dealing with firmware loss during system resume.
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The firmware cache works by requesting for firmware prior to suspend and
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caching it in memory. Upon resume device drivers using the firmware API will
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have access to the firmware immediately, without having to wait for the root
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filesystem to mount or dealing with possible race issues with lookups as the
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root filesystem mounts.
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Some implementation details about the firmware cache setup:
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* The firmware cache is setup by adding a devres entry for each device that
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uses all synchronous call except :c:func:`request_firmware_into_buf`.
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* If an asynchronous call is used the firmware cache is only set up for a
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device if if the second argument (uevent) to request_firmware_nowait() is
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true. When uevent is true it requests that a kobject uevent be sent to
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userspace for the firmware request through the sysfs fallback mechanism
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if the firmware file is not found.
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* If the firmware cache is determined to be needed as per the above two
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criteria the firmware cache is setup by adding a devres entry for the
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device making the firmware request.
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* The firmware devres entry is maintained throughout the lifetime of the
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device. This means that even if you release_firmware() the firmware cache
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will still be used on resume from suspend.
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* The timeout for the fallback mechanism is temporarily reduced to 10 seconds
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as the firmware cache is set up during suspend, the timeout is set back to
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the old value you had configured after the cache is set up.
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* Upon suspend any pending non-uevent firmware requests are killed to avoid
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stalling the kernel, this is done with kill_requests_without_uevent(). Kernel
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calls requiring the non-uevent therefore need to implement their own firmware
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cache mechanism but must not use the firmware API on suspend.
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