1 How to Get Your Patch Accepted Into the Hwmon Subsystem
2 =======================================================
3
4 This text is a collection of suggestions for people writing patches or
5 drivers for the hwmon subsystem. Following these suggestions will greatly
6 increase the chances of your change being accepted.
7
8
9 1. General
10 ----------
11
12 * It should be unnecessary to mention, but please read and follow:
13
14 - Documentation/process/submit-checklist.rst
15 - Documentation/process/submitting-patches.rst
16 - Documentation/process/coding-style.rst
17
18 * Please run your patch through 'checkpatch --strict'. There should be no
19 errors, no warnings, and few if any check messages. If there are any
20 messages, please be prepared to explain.
21
22 * Please use the standard multi-line comment style. Do not mix C and C++
23 style comments in a single driver (with the exception of the SPDX license
24 identifier).
25
26 * If your patch generates checkpatch errors, warnings, or check messages,
27 please refrain from explanations such as "I prefer that coding style".
28 Keep in mind that each unnecessary message helps hiding a real problem,
29 and a consistent coding style makes it easier for others to understand
30 and review the code.
31
32 * Please test your patch thoroughly. We are not your test group.
33 Sometimes a patch can not or not completely be tested because of missing
34 hardware. In such cases, you should test-build the code on at least one
35 architecture. If run-time testing was not achieved, it should be written
36 explicitly below the patch header.
37
38 * If your patch (or the driver) is affected by configuration options such as
39 CONFIG_SMP, make sure it compiles for all configuration variants.
40
41
42 2. Adding functionality to existing drivers
43 -------------------------------------------
44
45 * Make sure the documentation in Documentation/hwmon/<driver_name>.rst is up to
46 date.
47
48 * Make sure the information in Kconfig is up to date.
49
50 * If the added functionality requires some cleanup or structural changes, split
51 your patch into a cleanup part and the actual addition. This makes it easier
52 to review your changes, and to bisect any resulting problems.
53
54 * Never mix bug fixes, cleanup, and functional enhancements in a single patch.
55
56
57 3. New drivers
58 --------------
59
60 * Running your patch or driver file(s) through checkpatch does not mean its
61 formatting is clean. If unsure about formatting in your new driver, run it
62 through Lindent. Lindent is not perfect, and you may have to do some minor
63 cleanup, but it is a good start.
64
65 * Consider adding yourself to MAINTAINERS.
66
67 * Document the driver in Documentation/hwmon/<driver_name>.rst.
68
69 * Add the driver to Kconfig and Makefile in alphabetical order.
70
71 * Make sure that all dependencies are listed in Kconfig.
72
73 * Please list include files in alphabetic order.
74
75 * Please align continuation lines with '(' on the previous line.
76
77 * Avoid forward declarations if you can. Rearrange the code if necessary.
78
79 * Avoid macros to generate groups of sensor attributes. It not only confuses
80 checkpatch, but also makes it more difficult to review the code.
81
82 * Avoid calculations in macros and macro-generated functions. While such macros
83 may save a line or so in the source, it obfuscates the code and makes code
84 review more difficult. It may also result in code which is more complicated
85 than necessary. Use inline functions or just regular functions instead.
86
87 * Limit the number of kernel log messages. In general, your driver should not
88 generate an error message just because a runtime operation failed. Report
89 errors to user space instead, using an appropriate error code. Keep in mind
90 that kernel error log messages not only fill up the kernel log, but also are
91 printed synchronously, most likely with interrupt disabled, often to a serial
92 console. Excessive logging can seriously affect system performance.
93
94 * Use devres functions whenever possible to allocate resources. For rationale
95 and supported functions, please see Documentation/driver-api/driver-model/devres.rst.
96 If a function is not supported by devres, consider using devm_add_action().
97
98 * If the driver has a detect function, make sure it is silent. Debug messages
99 and messages printed after a successful detection are acceptable, but it
100 must not print messages such as "Chip XXX not found/supported".
101
102 Keep in mind that the detect function will run for all drivers supporting an
103 address if a chip is detected on that address. Unnecessary messages will just
104 pollute the kernel log and not provide any value.
105
106 * Provide a detect function if and only if a chip can be detected reliably.
107
108 * Only the following I2C addresses shall be probed: 0x18-0x1f, 0x28-0x2f,
109 0x48-0x4f, 0x58, 0x5c, 0x73 and 0x77. Probing other addresses is strongly
110 discouraged as it is known to cause trouble with other (non-hwmon) I2C
111 chips. If your chip lives at an address which can't be probed then the
112 device will have to be instantiated explicitly (which is always better
113 anyway.)
114
115 * Avoid writing to chip registers in the detect function. If you have to write,
116 only do it after you have already gathered enough data to be certain that the
117 detection is going to be successful.
118
119 Keep in mind that the chip might not be what your driver believes it is, and
120 writing to it might cause a bad misconfiguration.
121
122 * Make sure there are no race conditions in the probe function. Specifically,
123 completely initialize your chip and your driver first, then register with
124 the hwmon subsystem.
125
126 * Use devm_hwmon_device_register_with_info() or, if your driver needs a remove
127 function, hwmon_device_register_with_info() to register your driver with the
128 hwmon subsystem. Try using devm_add_action() instead of a remove function if
129 possible. Do not use any of the deprecated registration functions.
130
131 * Your driver should be buildable as module. If not, please be prepared to
132 explain why it has to be built into the kernel.
133
134 * Do not provide support for deprecated sysfs attributes.
135
136 * Do not create non-standard attributes unless really needed. If you have to use
137 non-standard attributes, or you believe you do, discuss it on the mailing list
138 first. Either case, provide a detailed explanation why you need the
139 non-standard attribute(s).
140 Standard attributes are specified in Documentation/hwmon/sysfs-interface.rst.
141
142 * When deciding which sysfs attributes to support, look at the chip's
143 capabilities. While we do not expect your driver to support everything the
144 chip may offer, it should at least support all limits and alarms.
145
146 * Last but not least, please check if a driver for your chip already exists
147 before starting to write a new driver. Especially for temperature sensors,
148 new chips are often variants of previously released chips. In some cases,
149 a presumably new chip may simply have been relabeled.
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