[PATCH] firmware: coreboot: Register bus in module init

[Brian Norris <briannorris@chromium.org>]

The coreboot_table driver registers a coreboot bus while probing a
"coreboot_table" device representing the coreboot table memory region.
Probing this device (i.e., registering the bus) is a dependency for the
module_init() functions of any driver for this bus (e.g.,
memconsole-coreboot.c / memconsole_driver_init()).

With synchronous probe, this dependency works OK, as the link order in
the Makefile ensures coreboot_table_driver_init() (and thus,
coreboot_table_probe()) completes before a coreboot device driver tries
to add itself to the bus.

With asynchronous probe, however, coreboot_table_probe() may race with
memconsole_driver_init(), and so we're liable to hit one of these two:

1. coreboot_driver_register() eventually hits "[...] the bus was not
   initialized.", and the memconsole driver fails to register; or
2. coreboot_driver_register() gets past #1, but still races with
   bus_register() and hits some other undefined/crashing behavior (e.g.,
   in driver_find() [1])

We can resolve this by registering the bus in our initcall, and only
deferring "device" work (scanning the coreboot memory region and
creating sub-devices) to probe().

[1] Example failure, using 'driver_async_probe=*' kernel command line:

[    0.114217] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000010
...
[    0.114307] CPU: 1 PID: 1 Comm: swapper/0 Not tainted 6.1.0-rc1 #63
[    0.114316] Hardware name: Google Scarlet (DT)
...
[    0.114488] Call trace:
[    0.114494]  _raw_spin_lock+0x34/0x60
[    0.114502]  kset_find_obj+0x28/0x84
[    0.114511]  driver_find+0x30/0x50
[    0.114520]  driver_register+0x64/0x10c
[    0.114528]  coreboot_driver_register+0x30/0x3c
[    0.114540]  memconsole_driver_init+0x24/0x30
[    0.114550]  do_one_initcall+0x154/0x2e0
[    0.114560]  do_initcall_level+0x134/0x160
[    0.114571]  do_initcalls+0x60/0xa0
[    0.114579]  do_basic_setup+0x28/0x34
[    0.114588]  kernel_init_freeable+0xf8/0x150
[    0.114596]  kernel_init+0x2c/0x12c
[    0.114607]  ret_from_fork+0x10/0x20
[    0.114624] Code: 5280002b 1100054a b900092a f9800011 (885ffc01)
[    0.114631] ---[ end trace 0000000000000000 ]---

Fixes: b81e3140e412 ("firmware: coreboot: Make bus registration symmetric")
Cc: <stable@vger.kernel.org>
Signed-off-by: Brian Norris <briannorris@chromium.org>
---
Currently, get_maintainers.pl tells me Greg should pick this up. But I
CC the chrome-platform list too, since it seems reasonable for Google
folks (probably ChromeOS folks are most active here?) to maintain
Google/Chrome drivers.

Let me know if y'all would like this official, and I'll push out a
MAINTAINERS patch.

 drivers/firmware/google/coreboot_table.c | 37 +++++++++++++++++++-----
 1 file changed, 29 insertions(+), 8 deletions(-)

diff --git a/drivers/firmware/google/coreboot_table.c b/drivers/firmware/google/coreboot_table.c
index c52bcaa9def6..9ca21feb9d45 100644
--- a/drivers/firmware/google/coreboot_table.c
+++ b/drivers/firmware/google/coreboot_table.c
@@ -149,12 +149,8 @@ static int coreboot_table_probe(struct platform_device *pdev)
 	if (!ptr)
 		return -ENOMEM;
 
-	ret = bus_register(&coreboot_bus_type);
-	if (!ret) {
-		ret = coreboot_table_populate(dev, ptr);
-		if (ret)
-			bus_unregister(&coreboot_bus_type);
-	}
+	ret = coreboot_table_populate(dev, ptr);
+
 	memunmap(ptr);
 
 	return ret;
@@ -169,7 +165,6 @@ static int __cb_dev_unregister(struct device *dev, void *dummy)
 static int coreboot_table_remove(struct platform_device *pdev)
 {
 	bus_for_each_dev(&coreboot_bus_type, NULL, NULL, __cb_dev_unregister);
-	bus_unregister(&coreboot_bus_type);
 	return 0;
 }
 
@@ -199,6 +194,32 @@ static struct platform_driver coreboot_table_driver = {
 		.of_match_table = of_match_ptr(coreboot_of_match),
 	},
 };
-module_platform_driver(coreboot_table_driver);
+
+static int __init coreboot_table_driver_init(void)
+{
+	int ret;
+
+	ret = bus_register(&coreboot_bus_type);
+	if (ret)
+		return ret;
+
+	ret = platform_driver_register(&coreboot_table_driver);
+	if (ret) {
+		bus_unregister(&coreboot_bus_type);
+		return ret;
+	}
+
+	return 0;
+}
+
+static void __exit coreboot_table_driver_exit(void)
+{
+	platform_driver_unregister(&coreboot_table_driver);
+	bus_unregister(&coreboot_bus_type);
+}
+
+module_init(coreboot_table_driver_init);
+module_exit(coreboot_table_driver_exit);
+
 MODULE_AUTHOR("Google, Inc.");
 MODULE_LICENSE("GPL");
-- 
2.38.0.413.g74048e4d9e-goog