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>

1 files changed, 152 insertions, 1 deletions

diff --git a/doc/develop/driver-model/serial-howto.rst b/doc/develop/driver-model/serial-howto.rst
index 1469131124..8af79a90f4 100644
--- a/doc/develop/driver-model/serial-howto.rst
+++ b/doc/develop/driver-model/serial-howto.rst
@@ -34,7 +34,7 @@ In terms of patches a conversion series typically has these patches:
 - convert at least one existing board to use driver model serial
 - (if no boards remain that don't use driver model) remove the old code
 
-This may be a good time to move your board to use device tree also. Mostly
+This may be a good time to move your board to use the device tree too. Mostly
 this involves these steps:
 
 - define CONFIG_OF_CONTROL and CONFIG_OF_SEPARATE
@@ -44,3 +44,154 @@ 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-festevam@gmail.com/