Re: [RFC][PATCH] misc: Introduce reboot_reason driver

On Tue, Dec 8, 2015 at 1:52 PM, Arnd Bergmann [off-list ref] wrote:

On Tuesday 08 December 2015 13:29:22 John Stultz wrote:

quoted

diff --git a/arch/arm/boot/dts/qcom-apq8064-nexus7-flo.dts b/arch/arm/boot/dts/qcom-apq8064-nexus7-flo.dts
index 5183d18..ee5dcb7 100644
--- a/arch/arm/boot/dts/qcom-apq8064-nexus7-flo.dts
+++ b/arch/arm/boot/dts/qcom-apq8064-nexus7-flo.dts

@@ -282,6 +282,15 @@
                      };
              };

+             reboot_reason: reboot_reason@2a03f65c {
+                     compatible              = "reboot_reason";

This is not a good compatible string. There should generally be a vendor
name associated with it (use "linux," if nothing else, and you should have
'-' instead of '_'.

Ack.

quoted

+                     reg                     = <0x2A03F65C 0x4>;

This may easily conflict with the device it is part of. We should have
non-overlapping register areas in general. For the example you are
looking at, which register block is this?

So Bjorn says its IMEM, but I was assuming it was just a reserved
magic phys addr from the bootloader.

Ideally I'm hoping to use this same driver for another device, which
plans to reserve a page from memory that the bootloader won't clear.

quoted

+/* Types of reasons */
+static enum {
+     NONE,
+     BOOTLOADER,
+     RECOVERY,
+     OEM,
+     MAX_REASONS
+} __maybe_unused reason_types;

The variable seems to always be unused, not just "__maybe_unused". Maybe remove it?

Yea. I initially just had the empty enum, but the compiler was giving
me "useless class storage specifier in empty declaration" warnings. So
I added a variable to it, but then I got unused variable warnings. So
I ended up with this. :P

Is there a better way? Are enums for array indexes out of fashion?
Just a list of #defines ?

quoted

+static int reboot_reason(struct notifier_block *nb, unsigned long action,
+                                                             void *data)
+{
+     char *cmd = (char *)data;
+     long reason = reasons[NONE];
+
+     if (!reboot_reason_addr)
+             return NOTIFY_DONE;
+
+     if (cmd != NULL) {
+             if (!strncmp(cmd, "bootloader", 10))
+                     reason = reasons[BOOTLOADER];
+             else if (!strncmp(cmd, "recovery", 8))
+                     reason = reasons[RECOVERY];
+             else if (!strncmp(cmd, "oem-", 4)) {
+                     unsigned long code;
+
+                     if (!kstrtoul(cmd+4, 0, &code))
+                             reason = reasons[OEM] | (code & 0xff);
+             }
+     }
+
+     if (reason != -1)
+             writel(reason, reboot_reason_addr);
+     return NOTIFY_DONE;
+}

Will this reboot the machine?

No. Just sets the data for the bootloader before the reboot occurs.

quoted

+     /* Install the notifier */
+     restart_nb.notifier_call = reboot_reason;
+     restart_nb.priority = 256;
+     if (register_restart_handler(&restart_nb)) {

If not, you should use register_reboot_notifier() rather than
register_restart_handler(). The former gets called to do something
just before rebooting, while the latter attempts to actually reboot
the machine.

Ah. Thanks.

quoted

+static int __init reboot_reason_init(void)
+{
+     return platform_driver_register(&reboot_reason_driver);
+}
+arch_initcall(reboot_reason_init);

Why this early? If it can be a normal device_initcall, you can use
module_platform_driver().

Yea. Mostly its just from the driver I stole the init path from, but I
thought it might be useful to have early. But I'm not passionate about
it one way or the other, so module_platform_driver is fine.


Thanks so much for the review and feedback!

thanks
-john
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