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Introduction
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The firmware API enables kernel code to request files required
for functionality from userspace, the uses vary:

* Microcode for CPU errata
* Device driver firmware, required to be loaded onto device
  microcontrollers
* Device driver information data (calibration data, EEPROM overrides),
  some of which can be completely optional.

Types of firmware requests
==========================

There are two types of calls:

* Synchronous
* Asynchronous

Which one you use vary depending on your requirements, the rule of thumb
however is you should strive to use the asynchronous APIs unless you also
are already using asynchronous initialization mechanisms which will not
stall or delay boot. Even if loading firmware does not take a lot of time
processing firmware might, and this can still delay boot or initialization,
as such mechanisms such as asynchronous probe can help supplement drivers.