1.. SPDX-License-Identifier: GPL-2.0+
2
3How to port a serial driver to driver model
4===========================================
5
6Here is a suggested approach for converting your serial driver over to driver
7model. Please feel free to update this file with your ideas and suggestions.
8
9- #ifdef out all your own serial driver code (#ifndef CONFIG_DM_SERIAL)
10- Define CONFIG_DM_SERIAL for your board, vendor or architecture
11- If the board does not already use driver model, you need CONFIG_DM also
12- Your board should then build, but will not boot since there will be no serial
13  driver
14- Add the U_BOOT_DRIVER piece at the end (e.g. copy serial_s5p.c for example)
15- Add a private struct for the driver data - avoid using static variables
16- Implement each of the driver methods, perhaps by calling your old methods
17- You may need to adjust the function parameters so that the old and new
18  implementations can share most of the existing code
19- If you convert all existing users of the driver, remove the pre-driver-model
20  code
21
22In terms of patches a conversion series typically has these patches:
23- clean up / prepare the driver for conversion
24- add driver model code
25- convert at least one existing board to use driver model serial
26- (if no boards remain that don't use driver model) remove the old code
27
28This may be a good time to move your board to use the device tree too. Mostly
29this involves these steps:
30
31- define CONFIG_OF_CONTROL and CONFIG_OF_SEPARATE
32- add your device tree files to arch/<arch>/dts
33- update the Makefile there
34- Add stdout-path to your /chosen device tree node if it is not already there
35- build and get u-boot-dtb.bin so you can test it
36- Your drivers can now use device tree
37- For device tree in SPL, define CONFIG_SPL_OF_CONTROL
38
39
40Converting boards to CONFIG_DM_SERIAL
41-------------------------------------
42
43If your SoC has a serial driver that uses driver model (has U_BOOT_DRIVER() in
44it), then you may still find that your board has not been converted. To convert
45your board, enable the option and see if you can get it working.
46
47Firstly you will have a lot more success if you have a method of debugging your
48board, such as a JTAG connection. Failing that the debug UART is useful,
49although since you are trying to get the UART driver running, it will interfere
50with your efforts eventually.
51
52Secondly, while the UART is a relatively simple peripheral, it may need quite a
53few pieces to be up and running before it will work, such as the correct pin
54muxing, clocks, power domains and possibly even GPIOs, if an external
55transceiver is used. Look at other boards that use the same SoC, for clues as to
56what is needed.
57
58Thirdly, when added tags, put them in a xxx-u-boot.dtsi file, where xxx is your
59board name, or SoC name. There may already be a file for your SoC which contains
60what you need. U-Boot automatically includes these files: see :ref:`dttweaks`.
61
62Here are some things you might need to consider:
63
641. The serial driver itself needs to be present before relocation, so that the
65   U-Boot banner appears. Make sure it has a bootph-all tag in the device
66   tree, so that the serial driver is bound when U-Boot starts.
67
68   For example, on iMX8::
69
70       lpuart3: serial@5a090000 {
71           compatible = "fsl,imx8qm-lpuart";
72           ...
73       };
74
75   put this in your xxx-u-boot.dtsi file::
76
77       &lpuart3 {
78           bootph-some-ram;
79       };
80
812. If your serial port requires a particular pinmux configuration, you may need
82   a pinctrl driver. This needs to have a bootph-all tag also. Take care
83   that any subnodes have the same tag, if they are needed to make the correct
84   pinctrl available.
85
86   For example, on RK3288, the UART2 uses uart2_xfer::
87
88       uart2: serial@ff690000 {
89           ...
90           pinctrl-0 = <&uart2_xfer>;
91       };
92
93   which is defined as follows::
94
95       pinctrl: pinctrl {
96           compatible = "rockchip,rk3228-pinctrl";
97
98           uart2: uart2 {
99               uart2_xfer: uart2-xfer {
100                   rockchip,pins = <1 RK_PC2 RK_FUNC_2 &pcfg_pull_up>,
101                         <1 RK_PC3 RK_FUNC_2 &pcfg_pull_none>;
102           };
103           ...
104       };
105
106   This means you must make the uart2-xfer node available as well as all its
107   parents, so put this in your xxx-u-boot.dtsi file::
108
109       &pinctrl {
110           bootph-all;
111       };
112
113       &uart2 {
114           bootph-all;
115       };
116
117       &uart2_xfer {
118           bootph-all;
119       };
120
1213. The same applies to power domains. For example, if a particular power domain
122   must be enabled for the serial port to work, you need to ensure it is
123   available before relocation:
124
125   For example, on iMX8, put this in your xxx-u-boot.dtsi file::
126
127       &pd_dma {
128           bootph-some-ram;
129       };
130
131       &pd_dma_lpuart3 {
132           bootph-some-ram;
133       };
134
1354. The same applies to clocks, in the same way. Make sure that when your driver
136   requests a clock, typically with clk_get_by_index(), it is available.
137
138
139Generally a failure to find a required device will cause an error which you can
140catch, if you have the debug UART working. U-Boot outputs serial data to the
141debug UART until the point where the real serial driver takes over. This point
142is marked by gd->flags having the GD_FLG_SERIAL_READY flag set. This change
143happens in serial_init() in serial-uclass.c so until that point the debug UART
144is used. You can see the relevant code in putc()
145, for example::
146
147   /* if we don't have a console yet, use the debug UART */
148   if (IS_ENABLED(CONFIG_DEBUG_UART) && !(gd->flags & GD_FLG_SERIAL_READY)) {
149      printch(c);
150      return;
151   }
152   ... carries on to use the console / serial driver
153
154Note that in device_probe() the call to pinctrl_select_state() silently fails
155if the pinctrl driver fails. You can add a temporary check there if needed.
156
157Why do we have all these tags? The problem is that before relocation we don't
158want to bind all the drivers since memory is limited and the CPU may be running
159at a slow speed. So many boards will fail to boot without this optimisation, or
160may take a long time to start up (e.g. hundreds of milliseconds). The tags tell
161U-Boot which drivers to bind.
162
163The good news is that this problem is normally solved by the SoC, so that any
164boards that use it will work as normal. But in some cases there are multiple
165UARTs or multiple pinmux options, which means that each board may need to do
166some customisation.
167
168Serial in SPL
169-------------
170
171A similar process is needed in SPL, but in this case the bootph-pre-ram or
172bootph-pre-sram tags are used. Add these in the same way as above, to ensure
173that the SPL device tree contains the required nodes (see spl/u-boot-spl.dtb
174for what it actually contains).
175
176Removing old code
177-----------------
178
179In some cases there may be initialisation code that is no-longer needed when
180driver model is used, such as setting up the pin muxing, or enabling a clock.
181Be sure to remove this.
182
183Example patch
184-------------
185
186See this serial_patch_ for iMX7.
187
188.. _serial_patch: https://patchwork.ozlabs.org/project/uboot/patch/20220314232406.1945308-1-festevam@gmail.com/
189