soc: aspeed: lpc-snoop: Don't disable channels that aren't enabled

Mitigate e.g. the following:

    # echo 1e789080.lpc-snoop > /sys/bus/platform/drivers/aspeed-lpc-snoop/unbind
    ...
    [  120.363594] Unable to handle kernel NULL pointer dereference at virtual address 00000004 when write
    [  120.373866] [00000004] *pgd=00000000
    [  120.377910] Internal error: Oops: 805 [kernel-patches#1] SMP ARM
    [  120.383306] CPU: 1 UID: 0 PID: 315 Comm: sh Not tainted 6.15.0-rc1-00009-g926217bc7d7d-dirty kernel-patches#20 NONE
    ...
    [  120.679543] Call trace:
    [  120.679559]  misc_deregister from aspeed_lpc_snoop_remove+0x84/0xac
    [  120.692462]  aspeed_lpc_snoop_remove from platform_remove+0x28/0x38
    [  120.700996]  platform_remove from device_release_driver_internal+0x188/0x200
    ...

Fixes: 9f4f9ae ("drivers/misc: add Aspeed LPC snoop driver")
Cc: stable@vger.kernel.org
Cc: Jean Delvare <jdelvare@suse.de>
Acked-by: Jean Delvare <jdelvare@suse.de>
Link: https://patch.msgid.link/20250616-aspeed-lpc-snoop-fixes-v2-2-3cdd59c934d3@codeconstruct.com.au
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>

Author: amboar

drivers/soc/aspeed/aspeed-lpc-snoop.c

@@ -58,6 +58,7 @@ struct aspeed_lpc_snoop_model_data {
 };
 
 struct aspeed_lpc_snoop_channel {
+	bool enabled;
 	struct kfifo		fifo;
 	wait_queue_head_t	wq;
 	struct miscdevice	miscdev;
@@ -190,6 +191,9 @@ static int aspeed_lpc_enable_snoop(struct aspeed_lpc_snoop *lpc_snoop,
 	const struct aspeed_lpc_snoop_model_data *model_data =
 		of_device_get_match_data(dev);
 
+	if (WARN_ON(lpc_snoop->chan[channel].enabled))
+		return -EBUSY;
+
 	init_waitqueue_head(&lpc_snoop->chan[channel].wq);
 	/* Create FIFO datastructure */
 	rc = kfifo_alloc(&lpc_snoop->chan[channel].fifo,
@@ -236,6 +240,8 @@ static int aspeed_lpc_enable_snoop(struct aspeed_lpc_snoop *lpc_snoop,
 		regmap_update_bits(lpc_snoop->regmap, HICRB,
 				hicrb_en, hicrb_en);
 
+	lpc_snoop->chan[channel].enabled = true;
+
 	return 0;
 
 err_misc_deregister:
@@ -248,6 +254,9 @@ static int aspeed_lpc_enable_snoop(struct aspeed_lpc_snoop *lpc_snoop,
 static void aspeed_lpc_disable_snoop(struct aspeed_lpc_snoop *lpc_snoop,
 				     int channel)
 {
+	if (!lpc_snoop->chan[channel].enabled)
+		return;
+
 	switch (channel) {
 	case 0:
 		regmap_update_bits(lpc_snoop->regmap, HICR5,
@@ -263,6 +272,8 @@ static void aspeed_lpc_disable_snoop(struct aspeed_lpc_snoop *lpc_snoop,
 		return;
 	}
 
+	lpc_snoop->chan[channel].enabled = false;
+	/* Consider improving safety wrt concurrent reader(s) */
 	misc_deregister(&lpc_snoop->chan[channel].miscdev);
 	kfifo_free(&lpc_snoop->chan[channel].fifo);
 }