[PATCH][next] wifi: mt76: Avoid multiple -Wflex-array-member-not-at-end warnings

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

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

So, in order to avoid ending up with a flexible-array member in the
middle of multiple other structs, we use the `struct_group_tagged()`
helper to create a new tagged `struct mt76_connac2_mcu_rxd_hdr`.
This structure groups together all the members of the flexible
`struct mt76_connac2_mcu_rxd` except the flexible array.

As a result, the array is effectively separated from the rest of the
members without modifying the memory layout of the flexible structure.
We then change the type of the middle struct members currently causing
trouble from `struct mt76_connac2_mcu_rxd` to `struct
mt76_connac2_mcu_rxd_hdr`.

We also want to ensure that when new members need to be added to the
flexible structure, they are always included within the newly created
tagged struct. For this, we use `static_assert()`. This ensures that the
memory layout for both the flexible structure and the new tagged struct
is the same after any changes.

This approach avoids having to implement `struct mt76_connac2_mcu_rxd_hdr`
as a completely separate structure, thus preventing having to maintain
two independent but basically identical structures, closing the door
to potential bugs in the future.

So, with these changes, fix the following warnings:

drivers/net/wireless/mediatek/mt76/mt7915/mcu.h:32:37: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/net/wireless/mediatek/mt76/mt7915/mcu.h:40:37: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/net/wireless/mediatek/mt76/mt7915/mcu.h:49:37: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/net/wireless/mediatek/mt76/mt7915/mcu.h:58:37: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 .../wireless/mediatek/mt76/mt76_connac_mcu.h  | 25 +++++++++++--------
 .../net/wireless/mediatek/mt76/mt7915/mcu.h   |  8 +++---
 2 files changed, 19 insertions(+), 14 deletions(-)

diff --git a/drivers/net/wireless/mediatek/mt76/mt76_connac_mcu.h b/drivers/net/wireless/mediatek/mt76/mt76_connac_mcu.h
index 4242d436de26..16689aefa227 100644
--- a/drivers/net/wireless/mediatek/mt76/mt76_connac_mcu.h
+++ b/drivers/net/wireless/mediatek/mt76/mt76_connac_mcu.h
@@ -115,21 +115,26 @@ struct mt76_connac2_mcu_uni_txd {
 } __packed __aligned(4);
 
 struct mt76_connac2_mcu_rxd {
-	__le32 rxd[6];
+	/* New members MUST be added within the struct_group() macro below. */
+	struct_group_tagged(mt76_connac2_mcu_rxd_hdr, hdr,
+		__le32 rxd[6];
 
-	__le16 len;
-	__le16 pkt_type_id;
+		__le16 len;
+		__le16 pkt_type_id;
 
-	u8 eid;
-	u8 seq;
-	u8 option;
-	u8 rsv;
-	u8 ext_eid;
-	u8 rsv1[2];
-	u8 s2d_index;
+		u8 eid;
+		u8 seq;
+		u8 option;
+		u8 rsv;
+		u8 ext_eid;
+		u8 rsv1[2];
+		u8 s2d_index;
+	);
 
 	u8 tlv[];
 };
+static_assert(offsetof(struct mt76_connac2_mcu_rxd, tlv) == sizeof(struct mt76_connac2_mcu_rxd_hdr),
+	      "struct member likely outside of struct_group_tagged()");
 
 struct mt76_connac2_patch_hdr {
 	char build_date[16];
diff --git a/drivers/net/wireless/mediatek/mt76/mt7915/mcu.h b/drivers/net/wireless/mediatek/mt76/mt7915/mcu.h
index b41ac4aaced7..49476a4182fd 100644
--- a/drivers/net/wireless/mediatek/mt76/mt7915/mcu.h
+++ b/drivers/net/wireless/mediatek/mt76/mt7915/mcu.h
@@ -29,7 +29,7 @@ struct mt7915_mcu_thermal_ctrl {
 } __packed;
 
 struct mt7915_mcu_thermal_notify {
-	struct mt76_connac2_mcu_rxd rxd;
+	struct mt76_connac2_mcu_rxd_hdr rxd;
 
 	struct mt7915_mcu_thermal_ctrl ctrl;
 	__le32 temperature;
@@ -37,7 +37,7 @@ struct mt7915_mcu_thermal_notify {
 } __packed;
 
 struct mt7915_mcu_csa_notify {
-	struct mt76_connac2_mcu_rxd rxd;
+	struct mt76_connac2_mcu_rxd_hdr rxd;
 
 	u8 omac_idx;
 	u8 csa_count;
@@ -46,7 +46,7 @@ struct mt7915_mcu_csa_notify {
 } __packed;
 
 struct mt7915_mcu_bcc_notify {
-	struct mt76_connac2_mcu_rxd rxd;
+	struct mt76_connac2_mcu_rxd_hdr rxd;
 
 	u8 band_idx;
 	u8 omac_idx;
@@ -55,7 +55,7 @@ struct mt7915_mcu_bcc_notify {
 } __packed;
 
 struct mt7915_mcu_rdd_report {
-	struct mt76_connac2_mcu_rxd rxd;
+	struct mt76_connac2_mcu_rxd_hdr rxd;
 
 	u8 band_idx;
 	u8 long_detected;
-- 
2.34.1