1 =========================================================
2 Converting old watchdog drivers to the watchdog framework
3 =========================================================
4
5 by Wolfram Sang <wsa@kernel.org>
6
7 Before the watchdog framework came into the kernel, every driver had to
8 implement the API on its own. Now, as the framework factored out the common
9 components, those drivers can be lightened making it a user of the framework.
10 This document shall guide you for this task. The necessary steps are described
11 as well as things to look out for.
12
13
14 Remove the file_operations struct
15 ---------------------------------
16
17 Old drivers define their own file_operations for actions like open(), write(),
18 etc... These are now handled by the framework and just call the driver when
19 needed. So, in general, the 'file_operations' struct and assorted functions can
20 go. Only very few driver-specific details have to be moved to other functions.
21 Here is a overview of the functions and probably needed actions:
22
23 - open: Everything dealing with resource management (file-open checks, magic
24 close preparations) can simply go. Device specific stuff needs to go to the
25 driver specific start-function. Note that for some drivers, the start-function
26 also serves as the ping-function. If that is the case and you need start/stop
27 to be balanced (clocks!), you are better off refactoring a separate start-function.
28
29 - close: Same hints as for open apply.
30
31 - write: Can simply go, all defined behaviour is taken care of by the framework,
32 i.e. ping on write and magic char ('V') handling.
33
34 - ioctl: While the driver is allowed to have extensions to the IOCTL interface,
35 the most common ones are handled by the framework, supported by some assistance
36 from the driver:
37
38 WDIOC_GETSUPPORT:
39 Returns the mandatory watchdog_info struct from the driver
40
41 WDIOC_GETSTATUS:
42 Needs the status-callback defined, otherwise returns 0
43
44 WDIOC_GETBOOTSTATUS:
45 Needs the bootstatus member properly set. Make sure it is 0 if you
46 don't have further support!
47
48 WDIOC_SETOPTIONS:
49 No preparations needed
50
51 WDIOC_KEEPALIVE:
52 If wanted, options in watchdog_info need to have WDIOF_KEEPALIVEPING
53 set
54
55 WDIOC_SETTIMEOUT:
56 Options in watchdog_info need to have WDIOF_SETTIMEOUT set
57 and a set_timeout-callback has to be defined. The core will also
58 do limit-checking, if min_timeout and max_timeout in the watchdog
59 device are set. All is optional.
60
61 WDIOC_GETTIMEOUT:
62 No preparations needed
63
64 WDIOC_GETTIMELEFT:
65 It needs get_timeleft() callback to be defined. Otherwise it
66 will return EOPNOTSUPP
67
68 Other IOCTLs can be served using the ioctl-callback. Note that this is mainly
69 intended for porting old drivers; new drivers should not invent private IOCTLs.
70 Private IOCTLs are processed first. When the callback returns with
71 -ENOIOCTLCMD, the IOCTLs of the framework will be tried, too. Any other error
72 is directly given to the user.
73
74 Example conversion::
75
76 -static const struct file_operations s3c2410wdt_fops = {
77 - .owner = THIS_MODULE,
78 - .llseek = no_llseek,
79 - .write = s3c2410wdt_write,
80 - .unlocked_ioctl = s3c2410wdt_ioctl,
81 - .open = s3c2410wdt_open,
82 - .release = s3c2410wdt_release,
83 -};
84
85 Check the functions for device-specific stuff and keep it for later
86 refactoring. The rest can go.
87
88
89 Remove the miscdevice
90 ---------------------
91
92 Since the file_operations are gone now, you can also remove the 'struct
93 miscdevice'. The framework will create it on watchdog_dev_register() called by
94 watchdog_register_device()::
95
96 -static struct miscdevice s3c2410wdt_miscdev = {
97 - .minor = WATCHDOG_MINOR,
98 - .name = "watchdog",
99 - .fops = &s3c2410wdt_fops,
100 -};
101
102
103 Remove obsolete includes and defines
104 ------------------------------------
105
106 Because of the simplifications, a few defines are probably unused now. Remove
107 them. Includes can be removed, too. For example::
108
109 - #include <linux/fs.h>
110 - #include <linux/miscdevice.h> (if MODULE_ALIAS_MISCDEV is not used)
111 - #include <linux/uaccess.h> (if no custom IOCTLs are used)
112
113
114 Add the watchdog operations
115 ---------------------------
116
117 All possible callbacks are defined in 'struct watchdog_ops'. You can find it
118 explained in 'watchdog-kernel-api.txt' in this directory. start() and
119 owner must be set, the rest are optional. You will easily find corresponding
120 functions in the old driver. Note that you will now get a pointer to the
121 watchdog_device as a parameter to these functions, so you probably have to
122 change the function header. Other changes are most likely not needed, because
123 here simply happens the direct hardware access. If you have device-specific
124 code left from the above steps, it should be refactored into these callbacks.
125
126 Here is a simple example::
127
128 +static struct watchdog_ops s3c2410wdt_ops = {
129 + .owner = THIS_MODULE,
130 + .start = s3c2410wdt_start,
131 + .stop = s3c2410wdt_stop,
132 + .ping = s3c2410wdt_keepalive,
133 + .set_timeout = s3c2410wdt_set_heartbeat,
134 +};
135
136 A typical function-header change looks like::
137
138 -static void s3c2410wdt_keepalive(void)
139 +static int s3c2410wdt_keepalive(struct watchdog_device *wdd)
140 {
141 ...
142 +
143 + return 0;
144 }
145
146 ...
147
148 - s3c2410wdt_keepalive();
149 + s3c2410wdt_keepalive(&s3c2410_wdd);
150
151
152 Add the watchdog device
153 -----------------------
154
155 Now we need to create a 'struct watchdog_device' and populate it with the
156 necessary information for the framework. The struct is also explained in detail
157 in 'watchdog-kernel-api.txt' in this directory. We pass it the mandatory
158 watchdog_info struct and the newly created watchdog_ops. Often, old drivers
159 have their own record-keeping for things like bootstatus and timeout using
160 static variables. Those have to be converted to use the members in
161 watchdog_device. Note that the timeout values are unsigned int. Some drivers
162 use signed int, so this has to be converted, too.
163
164 Here is a simple example for a watchdog device::
165
166 +static struct watchdog_device s3c2410_wdd = {
167 + .info = &s3c2410_wdt_ident,
168 + .ops = &s3c2410wdt_ops,
169 +};
170
171
172 Handle the 'nowayout' feature
173 -----------------------------
174
175 A few drivers use nowayout statically, i.e. there is no module parameter for it
176 and only CONFIG_WATCHDOG_NOWAYOUT determines if the feature is going to be
177 used. This needs to be converted by initializing the status variable of the
178 watchdog_device like this::
179
180 .status = WATCHDOG_NOWAYOUT_INIT_STATUS,
181
182 Most drivers, however, also allow runtime configuration of nowayout, usually
183 by adding a module parameter. The conversion for this would be something like::
184
185 watchdog_set_nowayout(&s3c2410_wdd, nowayout);
186
187 The module parameter itself needs to stay, everything else related to nowayout
188 can go, though. This will likely be some code in open(), close() or write().
189
190
191 Register the watchdog device
192 ----------------------------
193
194 Replace misc_register(&miscdev) with watchdog_register_device(&watchdog_dev).
195 Make sure the return value gets checked and the error message, if present,
196 still fits. Also convert the unregister case::
197
198 - ret = misc_register(&s3c2410wdt_miscdev);
199 + ret = watchdog_register_device(&s3c2410_wdd);
200
201 ...
202
203 - misc_deregister(&s3c2410wdt_miscdev);
204 + watchdog_unregister_device(&s3c2410_wdd);
205
206
207 Update the Kconfig-entry
208 ------------------------
209
210 The entry for the driver now needs to select WATCHDOG_CORE:
211
212 + select WATCHDOG_CORE
213
214
215 Create a patch and send it to upstream
216 --------------------------------------
217
218 Make sure you understood Documentation/process/submitting-patches.rst and send your patch to
219 linux-watchdog@vger.kernel.org. We are looking forward to it :)
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