231 lines
10 KiB
Plaintext
231 lines
10 KiB
Plaintext
Platform Devices and Drivers
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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See <linux/platform_device.h> for the driver model interface to the
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platform bus: platform_device, and platform_driver. This pseudo-bus
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is used to connect devices on busses with minimal infrastructure,
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like those used to integrate peripherals on many system-on-chip
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processors, or some "legacy" PC interconnects; as opposed to large
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formally specified ones like PCI or USB.
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Platform devices
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~~~~~~~~~~~~~~~~
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Platform devices are devices that typically appear as autonomous
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entities in the system. This includes legacy port-based devices and
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host bridges to peripheral buses, and most controllers integrated
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into system-on-chip platforms. What they usually have in common
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is direct addressing from a CPU bus. Rarely, a platform_device will
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be connected through a segment of some other kind of bus; but its
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registers will still be directly addressable.
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Platform devices are given a name, used in driver binding, and a
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list of resources such as addresses and IRQs.
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struct platform_device {
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const char *name;
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u32 id;
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struct device dev;
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u32 num_resources;
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struct resource *resource;
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};
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Platform drivers
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~~~~~~~~~~~~~~~~
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Platform drivers follow the standard driver model convention, where
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discovery/enumeration is handled outside the drivers, and drivers
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provide probe() and remove() methods. They support power management
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and shutdown notifications using the standard conventions.
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struct platform_driver {
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int (*probe)(struct platform_device *);
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int (*remove)(struct platform_device *);
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void (*shutdown)(struct platform_device *);
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int (*suspend)(struct platform_device *, pm_message_t state);
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int (*suspend_late)(struct platform_device *, pm_message_t state);
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int (*resume_early)(struct platform_device *);
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int (*resume)(struct platform_device *);
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struct device_driver driver;
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};
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Note that probe() should general verify that the specified device hardware
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actually exists; sometimes platform setup code can't be sure. The probing
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can use device resources, including clocks, and device platform_data.
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Platform drivers register themselves the normal way:
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int platform_driver_register(struct platform_driver *drv);
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Or, in common situations where the device is known not to be hot-pluggable,
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the probe() routine can live in an init section to reduce the driver's
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runtime memory footprint:
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int platform_driver_probe(struct platform_driver *drv,
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int (*probe)(struct platform_device *))
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Device Enumeration
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~~~~~~~~~~~~~~~~~~
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As a rule, platform specific (and often board-specific) setup code will
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register platform devices:
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int platform_device_register(struct platform_device *pdev);
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int platform_add_devices(struct platform_device **pdevs, int ndev);
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The general rule is to register only those devices that actually exist,
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but in some cases extra devices might be registered. For example, a kernel
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might be configured to work with an external network adapter that might not
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be populated on all boards, or likewise to work with an integrated controller
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that some boards might not hook up to any peripherals.
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In some cases, boot firmware will export tables describing the devices
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that are populated on a given board. Without such tables, often the
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only way for system setup code to set up the correct devices is to build
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a kernel for a specific target board. Such board-specific kernels are
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common with embedded and custom systems development.
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In many cases, the memory and IRQ resources associated with the platform
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device are not enough to let the device's driver work. Board setup code
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will often provide additional information using the device's platform_data
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field to hold additional information.
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Embedded systems frequently need one or more clocks for platform devices,
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which are normally kept off until they're actively needed (to save power).
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System setup also associates those clocks with the device, so that that
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calls to clk_get(&pdev->dev, clock_name) return them as needed.
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Legacy Drivers: Device Probing
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Some drivers are not fully converted to the driver model, because they take
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on a non-driver role: the driver registers its platform device, rather than
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leaving that for system infrastructure. Such drivers can't be hotplugged
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or coldplugged, since those mechanisms require device creation to be in a
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different system component than the driver.
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The only "good" reason for this is to handle older system designs which, like
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original IBM PCs, rely on error-prone "probe-the-hardware" models for hardware
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configuration. Newer systems have largely abandoned that model, in favor of
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bus-level support for dynamic configuration (PCI, USB), or device tables
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provided by the boot firmware (e.g. PNPACPI on x86). There are too many
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conflicting options about what might be where, and even educated guesses by
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an operating system will be wrong often enough to make trouble.
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This style of driver is discouraged. If you're updating such a driver,
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please try to move the device enumeration to a more appropriate location,
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outside the driver. This will usually be cleanup, since such drivers
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tend to already have "normal" modes, such as ones using device nodes that
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were created by PNP or by platform device setup.
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None the less, there are some APIs to support such legacy drivers. Avoid
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using these calls except with such hotplug-deficient drivers.
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struct platform_device *platform_device_alloc(
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const char *name, int id);
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You can use platform_device_alloc() to dynamically allocate a device, which
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you will then initialize with resources and platform_device_register().
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A better solution is usually:
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struct platform_device *platform_device_register_simple(
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const char *name, int id,
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struct resource *res, unsigned int nres);
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You can use platform_device_register_simple() as a one-step call to allocate
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and register a device.
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Device Naming and Driver Binding
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The platform_device.dev.bus_id is the canonical name for the devices.
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It's built from two components:
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* platform_device.name ... which is also used to for driver matching.
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* platform_device.id ... the device instance number, or else "-1"
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to indicate there's only one.
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These are concatenated, so name/id "serial"/0 indicates bus_id "serial.0", and
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"serial/3" indicates bus_id "serial.3"; both would use the platform_driver
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named "serial". While "my_rtc"/-1 would be bus_id "my_rtc" (no instance id)
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and use the platform_driver called "my_rtc".
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Driver binding is performed automatically by the driver core, invoking
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driver probe() after finding a match between device and driver. If the
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probe() succeeds, the driver and device are bound as usual. There are
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three different ways to find such a match:
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- Whenever a device is registered, the drivers for that bus are
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checked for matches. Platform devices should be registered very
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early during system boot.
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- When a driver is registered using platform_driver_register(), all
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unbound devices on that bus are checked for matches. Drivers
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usually register later during booting, or by module loading.
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- Registering a driver using platform_driver_probe() works just like
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using platform_driver_register(), except that the driver won't
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be probed later if another device registers. (Which is OK, since
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this interface is only for use with non-hotpluggable devices.)
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Early Platform Devices and Drivers
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The early platform interfaces provide platform data to platform device
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drivers early on during the system boot. The code is built on top of the
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early_param() command line parsing and can be executed very early on.
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Example: "earlyprintk" class early serial console in 6 steps
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1. Registering early platform device data
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The architecture code registers platform device data using the function
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early_platform_add_devices(). In the case of early serial console this
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should be hardware configuration for the serial port. Devices registered
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at this point will later on be matched against early platform drivers.
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2. Parsing kernel command line
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The architecture code calls parse_early_param() to parse the kernel
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command line. This will execute all matching early_param() callbacks.
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User specified early platform devices will be registered at this point.
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For the early serial console case the user can specify port on the
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kernel command line as "earlyprintk=serial.0" where "earlyprintk" is
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the class string, "serial" is the name of the platform driver and
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0 is the platform device id. If the id is -1 then the dot and the
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id can be omitted.
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3. Installing early platform drivers belonging to a certain class
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The architecture code may optionally force registration of all early
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platform drivers belonging to a certain class using the function
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early_platform_driver_register_all(). User specified devices from
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step 2 have priority over these. This step is omitted by the serial
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driver example since the early serial driver code should be disabled
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unless the user has specified port on the kernel command line.
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4. Early platform driver registration
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Compiled-in platform drivers making use of early_platform_init() are
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automatically registered during step 2 or 3. The serial driver example
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should use early_platform_init("earlyprintk", &platform_driver).
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5. Probing of early platform drivers belonging to a certain class
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The architecture code calls early_platform_driver_probe() to match
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registered early platform devices associated with a certain class with
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registered early platform drivers. Matched devices will get probed().
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This step can be executed at any point during the early boot. As soon
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as possible may be good for the serial port case.
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6. Inside the early platform driver probe()
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The driver code needs to take special care during early boot, especially
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when it comes to memory allocation and interrupt registration. The code
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in the probe() function can use is_early_platform_device() to check if
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it is called at early platform device or at the regular platform device
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time. The early serial driver performs register_console() at this point.
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For further information, see <linux/platform_device.h>.
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