pstore: Allow the user to explicitly choose a backend

pstore only allows one backend to be registered at present, but the
system may provide several. Add a parameter to allow the user to choose
which backend will be used rather than just relying on load order.

Signed-off-by: Matthew Garrett <[e-mail]>
Signed-off-by: Tony Luck <[e-mail]>

Documentation/ABI/testing/pstore

···
 		multiple) files based on the record size of the underlying
 		persistent storage until at least this amount is reached.
 		Default is 10 Kbytes.
+
+		Pstore only supports one backend at a time. If multiple
+		backends are available, the preferred backend may be
+		set by passing the pstore.backend= argument to the kernel at
+		boot time.
+

Documentation/kernel-parameters.txt

···
 			[HW,MOUSE] Controls Logitech smartscroll autorepeat.
 			0 = disabled, 1 = enabled (default).
 
+	pstore.backend=	Specify the name of the pstore backend to use
+
 	pt.		[PARIDE]
 			See Documentation/blockdev/paride.txt.
 

fs/pstore/platform.c

···
 static DEFINE_SPINLOCK(pstore_lock);
 static struct pstore_info *psinfo;
 
+static char *backend;
+
 /* How much of the console log to snapshot */
 static unsigned long kmsg_bytes = 10240;
 
···
 		spin_unlock(&pstore_lock);
 		return -EBUSY;
 	}
+
+	if (backend && strcmp(backend, psi->name)) {
+		spin_unlock(&pstore_lock);
+		return -EINVAL;
+	}
+
 	psinfo = psi;
 	spin_unlock(&pstore_lock);
 
···
 	return 0;
 }
 EXPORT_SYMBOL_GPL(pstore_write);
+
+module_param(backend, charp, 0444);
+MODULE_PARM_DESC(backend, "Pstore backend to use");