net: ethernet: ti: netcp: Standardize knav_dma_open_channel to return NULL on error

Make knav_dma_open_channel consistently return NULL on error instead
of ERR_PTR. Currently the header include/linux/soc/ti/knav_dma.h
returns NULL when the driver is disabled, but the driver
implementation does not even return NULL or ERR_PTR on failure,
causing inconsistency in the users. This results in a crash in
netcp_free_navigator_resources as followed (trimmed):

Unhandled fault: alignment exception (0x221) at 0xfffffff2
[fffffff2] *pgd=80000800207003, *pmd=82ffda003, *pte=00000000
Internal error: : 221 [#1] SMP ARM
Modules linked in:
CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.17.0-rc7 #1 NONE
Hardware name: Keystone
PC is at knav_dma_close_channel+0x30/0x19c
LR is at netcp_free_navigator_resources+0x2c/0x28c

[... TRIM...]

Call trace:
knav_dma_close_channel from netcp_free_navigator_resources+0x2c/0x28c
netcp_free_navigator_resources from netcp_ndo_open+0x430/0x46c
netcp_ndo_open from __dev_open+0x114/0x29c
__dev_open from __dev_change_flags+0x190/0x208
__dev_change_flags from netif_change_flags+0x1c/0x58
netif_change_flags from dev_change_flags+0x38/0xa0
dev_change_flags from ip_auto_config+0x2c4/0x11f0
ip_auto_config from do_one_initcall+0x58/0x200
do_one_initcall from kernel_init_freeable+0x1cc/0x238
kernel_init_freeable from kernel_init+0x1c/0x12c
kernel_init from ret_from_fork+0x14/0x38
[... TRIM...]

Standardize the error handling by making the function return NULL on
all error conditions. The API is used in just the netcp_core.c so the
impact is limited.

Note, this change, in effect reverts commit 5b6cb43b4d62 ("net:
ethernet: ti: netcp_core: return error while dma channel open issue"),
but provides a less error prone implementation.

Suggested-by: Simon Horman <[e-mail]>
Suggested-by: Jacob Keller <[e-mail]>
Signed-off-by: Nishanth Menon <[e-mail]>
Reviewed-by: Jacob Keller <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ethernet/ti/netcp_core.c

···
 
 	tx_pipe->dma_channel = knav_dma_open_channel(dev,
 				tx_pipe->dma_chan_name, &config);
-	if (IS_ERR(tx_pipe->dma_channel)) {
+	if (!tx_pipe->dma_channel) {
 		dev_err(dev, "failed opening tx chan(%s)\n",
 			tx_pipe->dma_chan_name);
-		ret = PTR_ERR(tx_pipe->dma_channel);
+		ret = -EINVAL;
 		goto err;
 	}
 
···
 	return 0;
 
 err:
-	if (!IS_ERR_OR_NULL(tx_pipe->dma_channel))
+	if (tx_pipe->dma_channel)
 		knav_dma_close_channel(tx_pipe->dma_channel);
 	tx_pipe->dma_channel = NULL;
 	return ret;
···
 
 	netcp->rx_channel = knav_dma_open_channel(netcp->netcp_device->device,
 					netcp->dma_chan_name, &config);
-	if (IS_ERR(netcp->rx_channel)) {
+	if (!netcp->rx_channel) {
 		dev_err(netcp->ndev_dev, "failed opening rx chan(%s\n",
 			netcp->dma_chan_name);
-		ret = PTR_ERR(netcp->rx_channel);
+		ret = -EINVAL;
 		goto fail;
 	}
 

drivers/soc/ti/knav_dma.c

···
  * @name:	slave channel name
  * @config:	dma configuration parameters
  *
- * Returns pointer to appropriate DMA channel on success or error.
+ * Return: Pointer to appropriate DMA channel on success or NULL on error.
  */
 void *knav_dma_open_channel(struct device *dev, const char *name,
 					struct knav_dma_cfg *config)
···
 
 	if (!kdev) {
 		pr_err("keystone-navigator-dma driver not registered\n");
-		return (void *)-EINVAL;
+		return NULL;
 	}
 
 	chan_num = of_channel_match_helper(dev->of_node, name, &instance);
 	if (chan_num < 0) {
 		dev_err(kdev->dev, "No DMA instance with name %s\n", name);
-		return (void *)-EINVAL;
+		return NULL;
 	}
 
 	dev_dbg(kdev->dev, "initializing %s channel %d from DMA %s\n",
···
 	if (config->direction != DMA_MEM_TO_DEV &&
 	    config->direction != DMA_DEV_TO_MEM) {
 		dev_err(kdev->dev, "bad direction\n");
-		return (void *)-EINVAL;
+		return NULL;
 	}
 
 	/* Look for correct dma instance */
···
 	}
 	if (!dma) {
 		dev_err(kdev->dev, "No DMA instance with name %s\n", instance);
-		return (void *)-EINVAL;
+		return NULL;
 	}
 
 	/* Look for correct dma channel from dma instance */
···
 	if (!chan) {
 		dev_err(kdev->dev, "channel %d is not in DMA %s\n",
 				chan_num, instance);
-		return (void *)-EINVAL;
+		return NULL;
 	}
 
 	if (atomic_read(&chan->ref_count) >= 1) {
 		if (!check_config(chan, config)) {
 			dev_err(kdev->dev, "channel %d config miss-match\n",
 				chan_num);
-			return (void *)-EINVAL;
+			return NULL;
 		}
 	}