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dm: Add docs to explain how to enable DM_SERIAL for a board
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> Reviewed-by: Fabio Estevam <festevam@denx.de> Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
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Simon Glass
committed by
Heinrich Schuchardt
Heinrich Schuchardt
parent
d46e86d25c
commit
67ae5d3d46
@@ -34,7 +34,7 @@ In terms of patches a conversion series typically has these patches:
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- convert at least one existing board to use driver model serial
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- (if no boards remain that don't use driver model) remove the old code
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This may be a good time to move your board to use device tree also. Mostly
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This may be a good time to move your board to use the device tree too. Mostly
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this involves these steps:
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- define CONFIG_OF_CONTROL and CONFIG_OF_SEPARATE
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@@ -44,3 +44,154 @@ this involves these steps:
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- build and get u-boot-dtb.bin so you can test it
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- Your drivers can now use device tree
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- For device tree in SPL, define CONFIG_SPL_OF_CONTROL
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Converting boards to CONFIG_DM_SERIAL
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-------------------------------------
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If your SoC has a serial driver that uses driver model (has U_BOOT_DRIVER() in
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it), then you may still find that your board has not been converted. To convert
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your board, enable the option and see if you can get it working.
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Firstly you will have a lot more success if you have a method of debugging your
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board, such as a JTAG connection. Failing that the debug UART is useful,
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although since you are trying to get the UART driver running, it will interfere
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with your efforts eventually.
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Secondly, while the UART is a relatively simple peripheral, it may need quite a
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few pieces to be up and running before it will work, such as the correct pin
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muxing, clocks, power domains and possibly even GPIOs, if an external
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transceiver is used. Look at other boards that use the same SoC, for clues as to
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what is needed.
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Thirdly, when added tags, put them in a xxx-u-boot.dtsi file, where xxx is your
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board name, or SoC name. There may already be a file for your SoC which contains
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what you need. U-Boot automatically includes these files: see :ref:`dttweaks`.
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Here are some things you might need to consider:
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1. The serial driver itself needs to be present before relocation, so that the
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U-Boot banner appears. Make sure it has a u-boot,pre-reloc tag in the device
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tree, so that the serial driver is bound when U-Boot starts.
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For example, on iMX8::
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lpuart3: serial@5a090000 {
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compatible = "fsl,imx8qm-lpuart";
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...
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};
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put this in your xxx-u-boot.dtsi file::
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&lpuart3 {
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u-boot,dm-pre-proper;
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};
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2. If your serial port requires a particular pinmux configuration, you may need
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a pinctrl driver. This needs to have a u-boot,pre-reloc tag also. Take care
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that any subnodes have the same tag, if they are needed to make the correct
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pinctrl available.
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For example, on RK3288, the UART2 uses uart2_xfer::
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uart2: serial@ff690000 {
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...
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pinctrl-0 = <&uart2_xfer>;
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};
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which is defined as follows::
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pinctrl: pinctrl {
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compatible = "rockchip,rk3228-pinctrl";
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uart2: uart2 {
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uart2_xfer: uart2-xfer {
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rockchip,pins = <1 RK_PC2 RK_FUNC_2 &pcfg_pull_up>,
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<1 RK_PC3 RK_FUNC_2 &pcfg_pull_none>;
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};
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...
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};
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This means you must make the uart2-xfer node available as well as all its
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parents, so put this in your xxx-u-boot.dtsi file::
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&pinctrl {
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u-boot,dm-pre-reloc;
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};
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&uart2 {
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u-boot,dm-pre-reloc;
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};
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&uart2_xfer {
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u-boot,dm-pre-reloc;
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};
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3. The same applies to power domains. For example, if a particular power domain
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must be enabled for the serial port to work, you need to ensure it is
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available before relocation:
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For example, on iMX8, put this in your xxx-u-boot.dtsi file::
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&pd_dma {
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u-boot,dm-pre-proper;
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};
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&pd_dma_lpuart3 {
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u-boot,dm-pre-proper;
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};
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4. The same applies to clocks, in the same way. Make sure that when your driver
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requests a clock, typically with clk_get_by_index(), it is available.
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Generally a failure to find a required device will cause an error which you can
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catch, if you have the debug UART working. U-Boot outputs serial data to the
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debug UART until the point where the real serial driver takes over. This point
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is marked by gd->flags having the GD_FLG_SERIAL_READY flag set. This change
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happens in serial_init() in serial-uclass.c so until that point the debug UART
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is used. You can see the relevant code in putc()
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, for example::
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/* if we don't have a console yet, use the debug UART */
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if (IS_ENABLED(CONFIG_DEBUG_UART) && !(gd->flags & GD_FLG_SERIAL_READY)) {
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printch(c);
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return;
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}
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... carries on to use the console / serial driver
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Note that in device_probe() the call to pinctrl_select_state() silently fails
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if the pinctrl driver fails. You can add a temporary check there if needed.
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Why do we have all these tags? The problem is that before relocation we don't
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want to bind all the drivers since memory is limited and the CPU may be running
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at a slow speed. So many boards will fail to boot without this optimisation, or
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may take a long time to start up (e.g. hundreds of milliseconds). The tags tell
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U-Boot which drivers to bind.
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The good news is that this problem is normally solved by the SoC, so that any
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boards that use it will work as normal. But in some cases there are multiple
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UARTs or multiple pinmux options, which means that each board may need to do
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some customisation.
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Serial in SPL
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-------------
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A similar process is needed in SPL, but in this case the u-boot,dm-spl or
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u-boot,dm-tpl tags are used. Add these in the same way as above, to ensure that
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the SPL device tree contains the required nodes (see spl/u-boot-spl.dtb for
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what it actually contains).
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Removing old code
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-----------------
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In some cases there may be initialisation code that is no-longer needed when
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driver model is used, such as setting up the pin muxing, or enabling a clock.
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Be sure to remove this.
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Example patch
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-------------
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See this serial_patch_ for iMX7.
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.. _serial_patch: https://patchwork.ozlabs.org/project/uboot/patch/20220314232406.1945308-1-festevam@gmail.com/
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