bcm53xx: revert bgmac back to the old limited max frame size

Bumping max frame size has significantly affected network performance.
It was done by upstream commit that first appeared in the 5.7 release.

This change bumps NAT masquarade speed from 196 Mb/s to 383 Mb/s for the
BCM4708 SoC.

Ref: f55f1dbaad33 ("bcm53xx: switch to the kernel 5.10")
Signed-off-by: Rafał Miłecki <[email protected]>

target/linux/bcm53xx/patches-5.10/700-bgmac-reduce-max-frame-size-to-support-just-MTU-1500.patch

@@ -0,0 +1,33 @@
+From: =?UTF-8?q?Rafa=C5=82=20Mi=C5=82ecki?= <[email protected]>
+Date: Fri, 10 Jun 2022 13:10:47 +0200
+Subject: [PATCH] bgmac: reduce max frame size to support just MTU 1500
+MIME-Version: 1.0
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: 8bit
+
+bgmac allocates new replacement buffer before handling each received
+frame. Allocating & DMA-preparing 9724 B each time consumes a lot of CPU
+time. Ideally bgmac should just respect currently set MTU but it isn't
+the case right now. For now just revert back to the old limited frame
+size.
+
+This change bumps NAT masquarade speed by ~95%.
+
+Ref: 8c7da63978f1 ("bgmac: configure MTU and add support for frames beyond 8192 byte size")
+Signed-off-by: Rafał Miłecki <[email protected]>
+---
+ drivers/net/ethernet/broadcom/bgmac.h | 3 +--
+ 1 file changed, 1 insertion(+), 2 deletions(-)
+
+--- a/drivers/net/ethernet/broadcom/bgmac.h
++++ b/drivers/net/ethernet/broadcom/bgmac.h
+@@ -366,8 +366,7 @@
+ #define BGMAC_RX_FRAME_OFFSET			30		/* There are 2 unused bytes between header and real data */
+ #define BGMAC_RX_BUF_OFFSET			(NET_SKB_PAD + NET_IP_ALIGN - \
+ 						 BGMAC_RX_FRAME_OFFSET)
+-/* Jumbo frame size with FCS */
+-#define BGMAC_RX_MAX_FRAME_SIZE			9724
++#define BGMAC_RX_MAX_FRAME_SIZE			1536
+ #define BGMAC_RX_BUF_SIZE			(BGMAC_RX_FRAME_OFFSET + BGMAC_RX_MAX_FRAME_SIZE)
+ #define BGMAC_RX_ALLOC_SIZE			(SKB_DATA_ALIGN(BGMAC_RX_BUF_SIZE + BGMAC_RX_BUF_OFFSET) + \
+ 						 SKB_DATA_ALIGN(sizeof(struct skb_shared_info)))