dm: core: Add a clarifying comment on struct udevice's seq member

[karo-tx-uboot.git] / doc / driver-model / README.txt

diff --git a/doc/driver-model/README.txt b/doc/driver-model/README.txt
index 11af35dcbaeda05e3dda2840c814b6f585281fe8..8dfcf75c3d369219b8cbc15782723013b94b0129 100644 (file)
--- a/doc/driver-model/README.txt
+++ b/doc/driver-model/README.txt
@@ -95,7 +95,7 @@ are provided in test/dm. To run them, try:
 You should see something like this:
 
     <...U-Boot banner...>
-    Running 20 driver model tests
+    Running 22 driver model tests
     Test: dm_test_autobind
     Test: dm_test_autoprobe
     Test: dm_test_bus_children
@@ -103,7 +103,9 @@ You should see something like this:
     Device 'c-test@0': seq 0 is in use by 'a-test'
     Device 'c-test@1': seq 1 is in use by 'd-test'
     Test: dm_test_bus_children_funcs
+    Test: dm_test_bus_children_iterators
     Test: dm_test_bus_parent_data
+    Test: dm_test_bus_parent_ops
     Test: dm_test_children
     Test: dm_test_fdt
     Device 'd-test': seq 3 is in use by 'b-test'
@@ -357,7 +359,9 @@ Device Sequence Numbers
 U-Boot numbers devices from 0 in many situations, such as in the command
 line for I2C and SPI buses, and the device names for serial ports (serial0,
 serial1, ...). Driver model supports this numbering and permits devices
-to be locating by their 'sequence'.
+to be locating by their 'sequence'. This numbering unique identifies a
+device in its uclass, so no two devices within a particular uclass can have
+the same sequence number.
 
 Sequence numbers start from 0 but gaps are permitted. For example, a board
 may have I2C buses 0, 1, 4, 5 but no 2 or 3. The choice of how devices are
@@ -425,6 +429,71 @@ entirely under the control of the board author so a conflict is generally
 an error.
 
 
+Bus Drivers
+-----------
+
+A common use of driver model is to implement a bus, a device which provides
+access to other devices. Example of buses include SPI and I2C. Typically
+the bus provides some sort of transport or translation that makes it
+possible to talk to the devices on the bus.
+
+Driver model provides a few useful features to help with implementing
+buses. Firstly, a bus can request that its children store some 'parent
+data' which can be used to keep track of child state. Secondly, the bus can
+define methods which are called when a child is probed or removed. This is
+similar to the methods the uclass driver provides.
+
+Here an explanation of how a bus fits with a uclass may be useful. Consider
+a USB bus with several devices attached to it, each from a different (made
+up) uclass:
+
+   xhci_usb (UCLASS_USB)
+      eth (UCLASS_ETHERNET)
+      camera (UCLASS_CAMERA)
+      flash (UCLASS_FLASH_STORAGE)
+
+Each of the devices is connected to a different address on the USB bus.
+The bus device wants to store this address and some other information such
+as the bus speed for each device.
+
+To achieve this, the bus device can use dev->parent_priv in each of its
+three children. This can be auto-allocated if the bus driver has a non-zero
+value for per_child_auto_alloc_size. If not, then the bus device can
+allocate the space itself before the child device is probed.
+
+Also the bus driver can define the child_pre_probe() and child_post_remove()
+methods to allow it to do some processing before the child is activated or
+after it is deactivated.
+
+Note that the information that controls this behaviour is in the bus's
+driver, not the child's. In fact it is possible that child has no knowledge
+that it is connected to a bus. The same child device may even be used on two
+different bus types. As an example. the 'flash' device shown above may also
+be connected on a SATA bus or standalone with no bus:
+
+   xhci_usb (UCLASS_USB)
+      flash (UCLASS_FLASH_STORAGE)  - parent data/methods defined by USB bus
+
+   sata (UCLASS_SATA)
+      flash (UCLASS_FLASH_STORAGE)  - parent data/methods defined by SATA bus
+
+   flash (UCLASS_FLASH_STORAGE)  - no parent data/methods (not on a bus)
+
+Above you can see that the driver for xhci_usb/sata controls the child's
+bus methods. In the third example the device is not on a bus, and therefore
+will not have these methods at all. Consider the case where the flash
+device defines child methods. These would be used for *its* children, and
+would be quite separate from the methods defined by the driver for the bus
+that the flash device is connetced to. The act of attaching a device to a
+parent device which is a bus, causes the device to start behaving like a
+bus device, regardless of its own views on the matter.
+
+The uclass for the device can also contain data private to that uclass.
+But note that each device on the bus may be a memeber of a different
+uclass, and this data has nothing to do with the child data for each child
+on the bus.
+
+
 Driver Lifecycle
 ----------------