+.. SPDX-License-Identifier: GPL-2.0+
+
How USB works with driver model
===============================
So far OHCI is not supported. Both EHCI and XHCI drivers should be declared
as drivers in the USB uclass. For example:
-static const struct udevice_id ehci_usb_ids[] = {
- { .compatible = "nvidia,tegra20-ehci", .data = USB_CTLR_T20 },
- { .compatible = "nvidia,tegra30-ehci", .data = USB_CTLR_T30 },
- { .compatible = "nvidia,tegra114-ehci", .data = USB_CTLR_T114 },
- { }
-};
-
-U_BOOT_DRIVER(usb_ehci) = {
- .name = "ehci_tegra",
- .id = UCLASS_USB,
- .of_match = ehci_usb_ids,
- .ofdata_to_platdata = ehci_usb_ofdata_to_platdata,
- .probe = tegra_ehci_usb_probe,
- .remove = tegra_ehci_usb_remove,
- .ops = &ehci_usb_ops,
- .platdata_auto_alloc_size = sizeof(struct usb_platdata),
- .priv_auto_alloc_size = sizeof(struct fdt_usb),
- .flags = DM_FLAG_ALLOC_PRIV_DMA,
-};
+.. code-block:: c
+
+ static const struct udevice_id ehci_usb_ids[] = {
+ { .compatible = "nvidia,tegra20-ehci", .data = USB_CTLR_T20 },
+ { .compatible = "nvidia,tegra30-ehci", .data = USB_CTLR_T30 },
+ { .compatible = "nvidia,tegra114-ehci", .data = USB_CTLR_T114 },
+ { }
+ };
+
+ U_BOOT_DRIVER(usb_ehci) = {
+ .name = "ehci_tegra",
+ .id = UCLASS_USB,
+ .of_match = ehci_usb_ids,
+ .ofdata_to_platdata = ehci_usb_ofdata_to_platdata,
+ .probe = tegra_ehci_usb_probe,
+ .remove = tegra_ehci_usb_remove,
+ .ops = &ehci_usb_ops,
+ .platdata_auto_alloc_size = sizeof(struct usb_platdata),
+ .priv_auto_alloc_size = sizeof(struct fdt_usb),
+ .flags = DM_FLAG_ALLOC_PRIV_DMA,
+ };
Here ehci_usb_ids is used to list the controllers that the driver supports.
Each has its own data value. Controllers must be in the UCLASS_USB uclass.
The following primary data structures are in use:
-- struct usb_device
+- struct usb_device:
This holds information about a device on the bus. All devices have
this structure, even the root hub. The controller itself does not
have this structure. You can access it for a device 'dev' with
handles that). Once the device is set up, you can find the device
descriptor and current configuration descriptor in this structure.
-- struct usb_platdata
+- struct usb_platdata:
This holds platform data for a controller. So far this is only used
as a work-around for controllers which can act as USB devices in OTG
mode, since the gadget framework does not use driver model.
-- struct usb_dev_platdata
+- struct usb_dev_platdata:
This holds platform data for a device. You can access it for a
device 'dev' with dev_get_parent_platdata(dev). It holds the device
address and speed - anything that can be determined before the device
driver is actually set up. When probing the bus this structure is
used to provide essential information to the device driver.
-- struct usb_bus_priv
+- struct usb_bus_priv:
This is private information for each controller, maintained by the
controller uclass. It is mostly used to keep track of the next
device address to use.
- This calls usb_init() which works through each controller in turn
- The controller is probed(). This does no enumeration.
- Then usb_scan_bus() is called. This calls usb_scan_device() to scan the
-(only) device that is attached to the controller - a root hub
+ (only) device that is attached to the controller - a root hub
- usb_scan_device() sets up a fake struct usb_device and calls
-usb_setup_device(), passing the port number to be scanned, in this case port
-0
+ usb_setup_device(), passing the port number to be scanned, in this case
+ port 0
- usb_setup_device() first calls usb_prepare_device() to set the device
-address, then usb_select_config() to select the first configuration
+ address, then usb_select_config() to select the first configuration
- at this point the device is enumerated but we do not have a real struct
-udevice for it. But we do have the descriptor in struct usb_device so we can
-use this to figure out what driver to use
+ udevice for it. But we do have the descriptor in struct usb_device so we can
+ use this to figure out what driver to use
- back in usb_scan_device(), we call usb_find_child() to try to find an
-existing device which matches the one we just found on the bus. This can
-happen if the device is mentioned in the device tree, or if we previously
-scanned the bus and so the device was created before
+ existing device which matches the one we just found on the bus. This can
+ happen if the device is mentioned in the device tree, or if we previously
+ scanned the bus and so the device was created before
- if usb_find_child() does not find an existing device, we call
-usb_find_and_bind_driver() which tries to bind one
+ usb_find_and_bind_driver() which tries to bind one
- usb_find_and_bind_driver() searches all available USB drivers (declared
-with USB_DEVICE()). If it finds a match it binds that driver to create a new
-device.
+ with USB_DEVICE()). If it finds a match it binds that driver to create a
+ new device.
- If it does not, it binds a generic driver. A generic driver is good enough
-to allow access to the device (sending it packets, etc.) but all
-functionality will need to be implemented outside the driver model.
+ to allow access to the device (sending it packets, etc.) but all
+ functionality will need to be implemented outside the driver model.
- in any case, when usb_find_child() and/or usb_find_and_bind_driver() are
-done, we have a device with the correct uclass. At this point we want to
-probe the device
+ done, we have a device with the correct uclass. At this point we want to
+ probe the device
- first we store basic information about the new device (address, port,
-speed) in its parent platform data. We cannot store it its private data
-since that will not exist until the device is probed.
+ speed) in its parent platform data. We cannot store it its private data
+ since that will not exist until the device is probed.
- then we call device_probe() which probes the device
- the first probe step is actually the USB controller's (or USB hubs's)
-child_pre_probe() method. This gets called before anything else and is
-intended to set up a child device ready to be used with its parent bus. For
-USB this calls usb_child_pre_probe() which grabs the information that was
-stored in the parent platform data and stores it in the parent private data
-(which is struct usb_device, a real one this time). It then calls
-usb_select_config() again to make sure that everything about the device is
-set up
+ child_pre_probe() method. This gets called before anything else and is
+ intended to set up a child device ready to be used with its parent bus. For
+ USB this calls usb_child_pre_probe() which grabs the information that was
+ stored in the parent platform data and stores it in the parent private data
+ (which is struct usb_device, a real one this time). It then calls
+ usb_select_config() again to make sure that everything about the device is
+ set up
- note that we have called usb_select_config() twice. This is inefficient
-but the alternative is to store additional information in the platform data.
-The time taken is minimal and this way is simpler
+ but the alternative is to store additional information in the platform data.
+ The time taken is minimal and this way is simpler
- at this point the device is set up and ready for use so far as the USB
-subsystem is concerned
+ subsystem is concerned
- the device's probe() method is then called. It can send messages and do
-whatever else it wants to make the device work.
+ whatever else it wants to make the device work.
Note that the first device is always a root hub, and this must be scanned to
find any devices. The above steps will have created a hub (UCLASS_USB_HUB),
usb_hub_post_probe() is called, and the following steps take place:
- usb_hub_post_probe() calls usb_hub_scan() to scan the hub, which in turn
-calls usb_hub_configure()
+ calls usb_hub_configure()
- hub power is enabled
- we loop through each port on the hub, performing the same steps for each
- first, check if there is a device present. This happens in
-usb_hub_port_connect_change(). If so, then usb_scan_device() is called to
-scan the device, passing the appropriate port number.
+ usb_hub_port_connect_change(). If so, then usb_scan_device() is called to
+ scan the device, passing the appropriate port number.
- you will recognise usb_scan_device() from the steps above. It sets up the
-device ready for use. If it is a hub, it will scan that hub before it
-continues here (recursively, depth-first)
+ device ready for use. If it is a hub, it will scan that hub before it
+ continues here (recursively, depth-first)
- once all hub ports are scanned in this way, the hub is ready for use and
-all of its downstream devices also
+ all of its downstream devices also
- additional controllers are scanned in the same way
The above method has some nice properties:
- the bus enumeration happens by virtue of driver model's natural device flow
- most logic is in the USB controller and hub uclasses; the actual device
-drivers do not need to know they are on a USB bus, at least so far as
-enumeration goes
+ drivers do not need to know they are on a USB bus, at least so far as
+ enumeration goes
- hub scanning happens automatically after a hub is probed
uclass, they share some common elements with controllers:
- they both attach private data to their children (struct usb_device,
-accessible for a child with dev_get_parent_priv(child))
+ accessible for a child with dev_get_parent_priv(child))
- they both use usb_child_pre_probe() to set up their children as proper USB
-devices
+ devices
Example - Mass Storage
As an example of a USB device driver, see usb_storage.c. It uses its own
uclass and declares itself as follows:
-U_BOOT_DRIVER(usb_mass_storage) = {
- .name = "usb_mass_storage",
- .id = UCLASS_MASS_STORAGE,
- .of_match = usb_mass_storage_ids,
- .probe = usb_mass_storage_probe,
-};
+.. code-block:: c
-static const struct usb_device_id mass_storage_id_table[] = {
- { .match_flags = USB_DEVICE_ID_MATCH_INT_CLASS,
- .bInterfaceClass = USB_CLASS_MASS_STORAGE},
- { } /* Terminating entry */
-};
+ U_BOOT_DRIVER(usb_mass_storage) = {
+ .name = "usb_mass_storage",
+ .id = UCLASS_MASS_STORAGE,
+ .of_match = usb_mass_storage_ids,
+ .probe = usb_mass_storage_probe,
+ };
+
+ static const struct usb_device_id mass_storage_id_table[] = {
+ { .match_flags = USB_DEVICE_ID_MATCH_INT_CLASS,
+ .bInterfaceClass = USB_CLASS_MASS_STORAGE},
+ { } /* Terminating entry */
+ };
-USB_DEVICE(usb_mass_storage, mass_storage_id_table);
+ USB_DEVICE(usb_mass_storage, mass_storage_id_table);
The USB_DEVICE() macro attaches the given table of matching information to
the given driver. Note that the driver is declared in U_BOOT_DRIVER() as
Here is an example device tree fragment:
+.. code-block:: none
+
usb@1 {
compatible = "sandbox,usb";
hub {
The hub is emulated by a hub emulator, and the emulated hub has a single
flash stick to emulate on one of its ports.
-When 'usb start' is used, the following 'dm tree' output will be available:
+When 'usb start' is used, the following 'dm tree' output will be available::
- usb [ + ] `-- usb@1
- usb_hub [ + ] `-- hub
- usb_emul [ + ] |-- hub-emul
- usb_emul [ + ] | `-- flash-stick
- usb_mass_st [ + ] `-- usb_mass_storage
+ usb [ + ] `-- usb@1
+ usb_hub [ + ] `-- hub
+ usb_emul [ + ] |-- hub-emul
+ usb_emul [ + ] | `-- flash-stick
+ usb_mass_st [ + ] `-- usb_mass_storage
This may look confusing. Most of it mirrors the device tree, but the
it would be possible to speed up enumeration in two ways:
- breadth-first search would allow devices to be reset and probed in
-parallel to some extent
+ parallel to some extent
- enumeration could be lazy, in the sense that we could enumerate just the
-root hub at first, then only progress to the next 'level' when a device is
-used that we cannot find. This could be made easier if the devices were
-statically declared in the device tree (which is acceptable for production
-boards where the same, known, things are on each bus).
+ root hub at first, then only progress to the next 'level' when a device is
+ used that we cannot find. This could be made easier if the devices were
+ statically declared in the device tree (which is acceptable for production
+ boards where the same, known, things are on each bus).
But in common cases the current algorithm is sufficient.
- Implement USB PHYs in driver model
- Work out a clever way to provide lazy init for USB devices
---
-Simon Glass <sjg@chromium.org>
-23-Mar-15
+
+.. Simon Glass <sjg@chromium.org>
+.. 23-Mar-15
projects / u-boot.git / commitdiff