dm: usb: Add a README for driver model

Add some documentation describing how USB is implemented with USB. This
might make things easier for people to understand.
Signed-off-by: Simon Glass <sjg@chromium.org>

doc/driver-model/usb-info.txt

+How USB works with driver model
+===============================
+
+Introduction
+------------
+
+Driver model USB support makes use of existing features but changes how
+drivers are found. This document provides some information intended to help
+understand how things work with USB in U-Boot when driver model is enabled.
+
+
+Enabling driver model for USB
+-----------------------------
+
+A new CONFIG_DM_USB option is provided to enable driver model for USB. This
+causes the USB uclass to be included, and drops the equivalent code in
+usb.c. In particular the usb_init() function is then implemented by the
+uclass.
+
+
+Support for EHCI and XHCI
+-------------------------
+
+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,
+};
+
+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 ofdata_to_platdata() method allows the controller driver to grab any
+necessary settings from the device tree.
+
+The ops here are ehci_usb_ops. All EHCI drivers will use these same ops in
+most cases, since they are all EHCI-compatible. For EHCI there are also some
+special operations that can be overridden when calling ehci_register().
+
+The driver can use priv_auto_alloc_size to set the size of its private data.
+This can hold run-time information needed by the driver for operation. It
+exists when the device is probed (not when it is bound) and is removed when
+the driver is removed.
+
+Note that usb_platdata is currently only used to deal with setting up a bus
+in USB device mode (OTG operation). It can be omitted if that is not
+supported.
+
+The driver's probe() method should do the basic controller init and then
+call ehci_register() to register itself as an EHCI device. It should call
+ehci_deregister() in the remove() method. Registering a new EHCI device
+does not by itself cause the bus to be scanned.
+
+The old ehci_hcd_init() function is no-longer used. Nor is it necessary to
+set up the USB controllers from board init code. When 'usb start' is used,
+each controller will be probed and its bus scanned.
+
+XHCI works in a similar way.
+
+
+Data structures
+---------------
+
+The following primary data structures are in use:
+
+- 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
+	dev_get_parentdata(dev). It matches the old structure except that the
+	parent and child information is not present (since driver model
+	handles that). Once the device is set up, you can find the device
+	descriptor and current configuration descriptor in this structure.
+
+- 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
+	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
+	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.
+
+Of these, only struct usb_device was used prior to driver model.
+
+
+USB buses
+---------
+
+Given a controller, you know the bus - it is the one attached to the
+controller. Each controller handles exactly one bus. Every controller has a
+root hub attached to it. This hub, which is itself a USB device, can provide
+one or more 'ports' to which additional devices can be attached. It is
+possible to power up a hub and find out which of its ports have devices
+attached.
+
+Devices are given addresses starting at 1. The root hub is always address 1,
+and from there the devices are numbered in sequence. The USB uclass takes
+care of this numbering automatically during enumeration.
+
+USB devices are enumerated by finding a device on a particular hub, and
+setting its address to the next available address. The USB bus stretches out
+in a tree structure, potentially with multiple hubs each with several ports
+and perhaps other hubs. Some hubs will have their own power since otherwise
+the 5V 500mA power supplied by the controller will not be sufficient to run
+very many devices.
+
+Enumeration in U-Boot takes a long time since devices are probed one at a
+time, and each is given sufficient time to wake up and announce itself. The
+timeouts are set for the slowest device.
+
+Up to 127 devices can be on each bus. USB has four bus speeds: low
+(1.5Mbps), full (12Mbps), high (480Mbps) which is only available with USB2
+and newer (EHCI), and super (5Gbps) which is only available with USB3 and
+newer (XHCI). If you connect a super-speed device to a high-speed hub, you
+will only get high-speed.
+
+
+USB operations
+--------------
+
+As before driver model, messages can be sent using submit_bulk_msg() and the
+like. These are now implemented by the USB uclass and route through the
+controller drivers. Note that messages are not sent to the driver of the
+device itself - i.e. they don't pass down the stack to the controller.
+U-Boot simply finds the controller to which the device is attached, and sends
+the message there with an appropriate 'pipe' value so it can be addressed
+properly. Having said that, the USB device which should receive the message
+is passed in to the driver methods, for use by sandbox. This design decision
+is open for review and the code impact of changing it is small since the
+methods are typically implemented by the EHCI and XHCI stacks.
+
+Controller drivers (in UCLASS_USB) themselves provide methods for sending
+each message type. For XHCI an additional alloc_device() method is provided
+since XHCI needs to allocate a device context before it can even read the
+device's descriptor.
+
+These methods use a 'pipe' which is a collection of bit fields used to
+describe the type of message, direction of transfer and the intended
+recipient (device number).
+
+
+USB Devices
+-----------
+
+USB devices are found using a simple algorithm which works through the
+available hubs in a depth-first search. Devices can be in any uclass, but
+are attached to a parent hub (or controller in the case of the root hub) and
+so have parent data attached to them (this is struct usb_device).
+
+By the time the device's probe() method is called, it is enumerated and is
+ready to talk to the host.
+
+The enumeration process needs to work out which driver to attach to each USB
+device. It does this by examining the device class, interface class, vendor
+ID, product ID, etc. See struct usb_driver_entry for how drivers are matched
+with USB devices - you can use the USB_DEVICE() macro to declare a USB
+driver. For example, usb_storage.c defines a USB_DEVICE() to handle storage
+devices, and it will be used for all USB devices which match.
+
+
+
+Technical details on enumeration flow
+-------------------------------------
+
+It is useful to understand precisely how a USB bus is enumerating to avoid
+confusion when dealing with USB devices.
+
+Device initialisation happens roughly like this:
+
+- At some point the 'usb start' command is run
+- 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
+- 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() first calls usb_prepare_device() to set the device
+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
+- 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
+- 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() searches all available USB drivers (declared
+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.
+- 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
+- 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.
+- 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
+- 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
+- at this point the device is set up and ready for use so far as the USB
+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.
+
+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),
+given it address 1 and set the configuration.
+
+For hubs, the hub uclass has a post_probe() method. This means that after
+any hub is probed, the uclass gets to do some processing. In this case
+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()
+- 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.
+- 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)
+- once all hub ports are scanned in this way, the hub is ready for use and
+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
+- hub scanning happens automatically after a hub is probed
+
+
+Hubs
+----
+
+USB hubs are scanned as in the section above. While hubs have their own
+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_parentdata(child))
+- they both use usb_child_pre_probe() to set up their children as proper USB
+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,
+};
+
+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);
+
+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
+'usb_mass_storage' and this must match the first parameter of USB_DEVICE.
+
+When usb_find_and_bind_driver() is called on a USB device with the
+bInterfaceClass value of USB_CLASS_MASS_STORAGE, it will automatically find
+this driver and use it.
+
+
+Counter-example: USB Ethernet
+-----------------------------
+
+As an example of the old way of doing things, see usb_ether.c. When the bus
+is scanned, all Ethernet devices will be created as generic USB devices (in
+uclass UCLASS_USB_DEV_GENERIC). Then, when the scan is completed,
+usb_host_eth_scan() will be called. This looks through all the devices on
+each bus and manually figures out which are Ethernet devices in the ways of
+yore.
+
+In fact, usb_ether should be moved to driver model. Each USB Ethernet driver
+(e.g drivers/usb/eth/asix.c) should include a USB_DEVICE() declaration, so
+that it will be found as part of normal USB enumeration. Then, instead of a
+generic USB driver, a real (driver-model-aware) driver will be used. Since
+Ethernet now supports driver model, this should be fairly easy to achieve,
+and then usb_ether.c and the usb_host_eth_scan() will melt away.
+
+
+Sandbox
+-------
+
+All driver model uclasses must have tests and USB is no exception. To
+achieve this, a sandbox USB controller is provided. This can make use of
+emulation drivers which pretend to be USB devices. Emulations are provided
+for a hub and a flash stick. These are enough to create a pretend USB bus
+(defined by the sandbox device tree sandbox.dts) which can be scanned and
+used.
+
+Tests in test/dm/usb.c make use of this feature. It allows much of the USB
+stack to be tested without real hardware being needed.
+
+Here is an example device tree fragment:
+
+	usb@1 {
+		compatible = "sandbox,usb";
+		hub {
+			compatible = "usb-hub";
+			usb,device-class = <USB_CLASS_HUB>;
+			hub-emul {
+				compatible = "sandbox,usb-hub";
+				#address-cells = <1>;
+				#size-cells = <0>;
+				flash-stick {
+					reg = <0>;
+					compatible = "sandbox,usb-flash";
+					sandbox,filepath = "flash.bin";
+				};
+			};
+		};
+	};
+
+This defines a single controller, containing a root hub (which is required).
+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:
+
+ 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
+'usb_mass_storage' device is not in the device tree. This is created by
+usb_find_and_bind_driver() based on the USB_DRIVER in usb_storage.c. While
+'flash-stick' is the emulation device, 'usb_mass_storage' is the real U-Boot
+USB device driver that talks to it.
+
+
+Future work
+-----------
+
+It is pretty uncommon to have a large USB bus with lots of hubs on an
+embedded system. In fact anything other than a root hub is uncommon. Still
+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
+- 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).
+
+But in common cases the current algorithm is sufficient.
+
+Other things that need doing:
+- Convert usb_ether to use driver model as described above
+- Test that keyboards work (and convert to driver model)
+- Move the USB gadget framework to driver model
+- Implement OHCI in driver model
+- 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