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doc: ti: Explain how the various gadget devices can be used

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Describe the current situation wrt the handling of USB devices on AM33xx
based boards, taking the example of a common board (the Beagle Bone
Black) and explaining how the different USB gadgets can be used.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
This commit is contained in:
Miquel Raynal 2023-08-07 08:42:41 +02:00 committed by Heinrich Schuchardt
parent 636acb4022
commit c631cf84db
1 changed files with 62 additions and 0 deletions
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doc/board/ti/am335x_evm.rst
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@ -201,3 +201,65 @@ booting and mtdparts have been configured correctly for the board:
U-Boot # spl export fdt ${loadaddr} - ${fdtaddr}
U-Boot # nand erase.part u-boot-spl-os
U-Boot # nand write ${fdtaddr} u-boot-spl-os
USB device
----------
The platform code for am33xx based designs is legacy in the sense that
it is not fully compliant with the driver model in its management of the
various resources. This is particularly true for the USB Ethernet gadget
which will automatically be bound to the first USB Device Controller
(UDC). This make the USB Ethernet gadget work out of the box on common
boards like the Beagle Bone Blacks and by default will prevents other
gadgets to be used.
The output of the 'dm tree' command shows which driver is bound to which
device, so the user can easily configure their platform differently from
the command line:
.. code-block:: text
=> dm tree
Class Index Probed Driver Name
-----------------------------------------------------------
[...]
misc 0 [ + ] ti-musb-wrapper | |-- usb@47400000
usb 0 [ + ] ti-musb-peripheral | | |-- usb@47401000
ethernet 1 [ + ] usb_ether | | | `-- usb_ether
bootdev 3 [ ] eth_bootdev | | | `-- usb_ether.bootdev
usb 0 [ ] ti-musb-host | | `-- usb@47401800
Typically here any network command performed using the usb_ether
interface would work, while using other gadgets would fail:
.. code-block:: text
=> fastboot usb 0
All UDC in use (1 available), use the unbind command
g_dnl_register: failed!, error: -19
exit not allowed from main input shell.
As hinted by the primary error message, the only controller available
(usb@47401000) is currently bound to the usb_ether driver, which makes
it impossible for the fastboot command to bind with this device (at
least from a bootloader point of view). The solution here would be to
use the unbind command specifying the class and index parameters (as
shown above in the 'dm tree' output) to target the driver to unbind:
.. code-block:: text
=> unbind ethernet 1
The output of the 'dm tree' command now shows the availability of the
first USB device controller, the fastboot gadget will now be able to
bind with it:
.. code-block:: text
=> dm tree
Class Index Probed Driver Name
-----------------------------------------------------------
[...]
misc 0 [ + ] ti-musb-wrapper | |-- usb@47400000
usb 0 [ ] ti-musb-peripheral | | |-- usb@47401000
usb 0 [ ] ti-musb-host | | `-- usb@47401800