This is an attempt to cover the common cases found when enabling driver model for serial on a new board.
Signed-off-by: Simon Glass sjg@chromium.org ---
Changes in v2: - Add an example patch - Mention removing old code that is not used
doc/develop/driver-model/serial-howto.rst | 152 ++++++++++++++++++++++ 1 file changed, 152 insertions(+)
diff --git a/doc/develop/driver-model/serial-howto.rst b/doc/develop/driver-model/serial-howto.rst index 1469131124b..e595049c0a3 100644 --- a/doc/develop/driver-model/serial-howto.rst +++ b/doc/develop/driver-model/serial-howto.rst @@ -44,3 +44,155 @@ this involves these steps: - build and get u-boot-dtb.bin so you can test it - Your drivers can now use device tree - For device tree in SPL, define CONFIG_SPL_OF_CONTROL + + +Converting boards to CONFIG_DM_SERIAL +------------------------------------- + +If your SoC has a serial driver that uses driver model (has U_BOOT_DRIVER() in +it) then you may still find that your board has not been converted. To convert +your board, enable the option and see if you can get it working. + +Firstly you will have a lot more success if you have a method of debugging your +board, such as a JTAG connection. Failing that the debug UART is useful, +although since you are trying to get the UART driver running, it will interfere +with your efforts eventually. + +Secondly, while the UART is a relatively simple peripheral, it may need quite a +few pieces to be up and running before it will work, such as the correct pin +muxing, clocks, power domains and possibly even GPIOs, if an external +transceiver is used. Look at other boards that use the same SoC, for clues as to +what is needed. + +Thirdly, when added tags, put them in a xxx-u-boot.dtsi file, where xxx is your +board name, or SoC name. There may already be a file for your SoC which contains +what you need. U-Boot automatically includes these files: see :ref:`dttweaks`. + +Here are some things you might need to consider: + +1. The serial driver itself needs to be present before relocation, so that the +U-Boot banner appears. Make sure it has a u-boot,pre-reloc tag in the device +tree, so that the serial driver is bound when U-Boot starts. + +For example, on iMX8:: + + lpuart3: serial@5a090000 { + compatible = "fsl,imx8qm-lpuart"; + ... + }; + +put this in your xxx-u-boot.dtsi file:: + + &lpuart3 { + u-boot,dm-pre-proper; + }; + +2. If your serial port requires a particular pinmux configuration, you may need +a pinctrl driver. This needs to have a u-boot,pre-reloc tag also. Take care that +any subnodes have the same tag, if they are needed to make the correct pinctrl +available. + +For example, on RK3288, the UART2 uses uart2_xfer:: + + uart2: serial@ff690000 { + ... + pinctrl-0 = <&uart2_xfer>; + }; + +which is defined as follows:: + + pinctrl: pinctrl { + compatible = "rockchip,rk3228-pinctrl"; + + uart2: uart2 { + uart2_xfer: uart2-xfer { + rockchip,pins = <1 RK_PC2 RK_FUNC_2 &pcfg_pull_up>, + <1 RK_PC3 RK_FUNC_2 &pcfg_pull_none>; + }; + + ... + }; + +This means you must make the uart2-xfer node available as well as all its +parents, so put this in your xxx-u-boot.dtsi file:: + + &pinctrl { + u-boot,dm-pre-reloc; + }; + + &uart2 { + u-boot,dm-pre-reloc; + }; + + &uart2_xfer { + u-boot,dm-pre-reloc; + }; + +3. The same applies to power domains. For example, if a particular power domain +must be enabled for the serial port to work, you need to ensure it is available +before relocation: + +For example, on iMX8, put this in your xxx-u-boot.dtsi file:: + + &pd_dma { + u-boot,dm-pre-proper; + }; + + &pd_dma_lpuart3 { + u-boot,dm-pre-proper; + }; + +4. The same applies to clocks, in the same way. Make sure that when your driver +requests a clock, typically with clk_get_by_index(), it is available. + + +Generally a failure to find a required device will cause an error which you can +catch, if you have the debug UART working. U-Boot outputs serial data to the +debug UART until the point where the real serial driver takes over. This point +is marked by gd->flags having the GD_FLG_SERIAL_READY flag set. This change +happens in serial_init() in serial-uclass.c so until that point the debug UART +is used. You can see the relevant code in putc() +, for example:: + + /* if we don't have a console yet, use the debug UART */ + if (IS_ENABLED(CONFIG_DEBUG_UART) && !(gd->flags & GD_FLG_SERIAL_READY)) { + printch(c); + return; + } + ... carries on to use the console / serial driver + +Note that in device_probe() the call to pinctrl_select_state() silently fails +if the pinctrl driver fails. You can add a temporary check there if needed. + +Why do we have all these tags? The problem is that before relocation we don't +want to bind all the drivers since memory is limited and the CPU may be running +at a slow speed. So many boards will fail to boot without this optimisation, or +may take a long time to start up (e.g. hundreds of milliseconds). The tags tell +U-Boot which drivers to bind. + +The good news is that this problem is normally solved by the SoC, so that any +boards that use it will work as normal. But in some cases there are multiple +UARTs or multiple pinmux options, which means that each board may need to do +some customisation. + +Serial in SPL +------------- + +A similar process is needed in SPL, but in this case the u-boot,dm-spl or +u-boot,dm-tpl tags are used. Add these in the same way as above, to ensure that +the SPL device tree contains the required nodes (see spl/u-boot-spl.dtb for +what it actually contains). + +Removing old code +----------------- + +In some cases there may be initialisation code that is no-longer needed when +driver model is used, such as setting up the pin muxing, or enabling a clock. +Be sure to remove this. + +Example patch +------------- + +See this serial_patch_ for iMX7. + +.. _serial_patch: https://patchwork.ozlabs.org/project/uboot/patch/20220314232406.1945308-1-fe...