[PATCH 1/2][next] firewire: Avoid -Wflex-array-member-not-at-end warning

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

-Wflex-array-member-not-at-end is coming in GCC-14, and we are getting
ready to enable it globally.

There is currently a local structure `u` that is using a flexible
`struct fw_iso_packet` as header for an on-stack array `u8 header[256]`.

struct {
	struct fw_iso_packet packet;
	u8 header[256];
} u;

However, we are deprecating flexible arrays in the middle of another
struct. So, in order to avoid this, we use the `struct_group_tagged()`
helper to separate the flexible array from the rest of the members in
the flexible structure:

struct fw_iso_packet {
        struct_group_tagged(fw_iso_packet_hdr, hdr,
		... the rest of the members
        );
        u32 header[];           /* tx: Top of 1394 isoch. data_block    */
};

With the change described above, we can now declare an object of the
type of the tagged struct, without embedding the flexible array in the
middle of another struct:

struct {
        struct fw_iso_packet_hdr packet;
        u8 header[256];
} u;

We also use `container_of()` whenever we need to retrieve a pointer to
the flexible structure, through which the flexible-array member can be
accessed, as in this case.

So, with these changes, fix the following warning:

drivers/firewire/core-cdev.c: In function ‘ioctl_queue_iso’:
drivers/firewire/core-cdev.c:1129:38: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
 1129 |                 struct fw_iso_packet packet;
      |                                      ^~~~~~

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 drivers/firewire/core-cdev.c |  9 +++++----
 include/linux/firewire.h     | 16 +++++++++-------
 2 files changed, 14 insertions(+), 11 deletions(-)

diff --git a/drivers/firewire/core-cdev.c b/drivers/firewire/core-cdev.c
index 6274b86eb943..e1f1daa2e667 100644
--- a/drivers/firewire/core-cdev.c
+++ b/drivers/firewire/core-cdev.c
@@ -1126,9 +1126,11 @@ static int ioctl_queue_iso(struct client *client, union ioctl_arg *arg)
 	u32 control;
 	int count;
 	struct {
-		struct fw_iso_packet packet;
+		struct fw_iso_packet_hdr packet;
 		u8 header[256];
 	} u;
+	struct fw_iso_packet *packet =
+			container_of(&u.packet, struct fw_iso_packet, hdr);
 
 	if (ctx == NULL || a->handle != 0)
 		return -EINVAL;
@@ -1192,7 +1194,7 @@ static int ioctl_queue_iso(struct client *client, union ioctl_arg *arg)
 		if (next > end)
 			return -EINVAL;
 		if (copy_from_user
-		    (u.packet.header, p->header, transmit_header_bytes))
+		    (packet->header, p->header, transmit_header_bytes))
 			return -EFAULT;
 		if (u.packet.skip && ctx->type == FW_ISO_CONTEXT_TRANSMIT &&
 		    u.packet.header_length + u.packet.payload_length > 0)
@@ -1200,8 +1202,7 @@ static int ioctl_queue_iso(struct client *client, union ioctl_arg *arg)
 		if (payload + u.packet.payload_length > buffer_end)
 			return -EINVAL;
 
-		if (fw_iso_context_queue(ctx, &u.packet,
-					 &client->buffer, payload))
+		if (fw_iso_context_queue(ctx, packet, &client->buffer, payload))
 			break;
 
 		p = next;
diff --git a/include/linux/firewire.h b/include/linux/firewire.h
index dd9f2d765e68..becd3a60d0fb 100644
--- a/include/linux/firewire.h
+++ b/include/linux/firewire.h
@@ -456,13 +456,15 @@ void fw_core_remove_descriptor(struct fw_descriptor *desc);
  * scatter-gather streaming (e.g. assembling video frame automatically).
  */
 struct fw_iso_packet {
-	u16 payload_length;	/* Length of indirect payload		*/
-	u32 interrupt:1;	/* Generate interrupt on this packet	*/
-	u32 skip:1;		/* tx: Set to not send packet at all	*/
-				/* rx: Sync bit, wait for matching sy	*/
-	u32 tag:2;		/* tx: Tag in packet header		*/
-	u32 sy:4;		/* tx: Sy in packet header		*/
-	u32 header_length:8;	/* Length of immediate header		*/
+	struct_group_tagged(fw_iso_packet_hdr, hdr,
+		u16 payload_length;	/* Length of indirect payload		*/
+		u32 interrupt:1;	/* Generate interrupt on this packet	*/
+		u32 skip:1;		/* tx: Set to not send packet at all	*/
+		/* rx: Sync bit, wait for matching sy	*/
+		u32 tag:2;		/* tx: Tag in packet header		*/
+		u32 sy:4;		/* tx: Sy in packet header		*/
+		u32 header_length:8;	/* Length of immediate header		*/
+	);
 	u32 header[];		/* tx: Top of 1394 isoch. data_block	*/
 };
 
-- 
2.34.1