mt76: add mt7603 possible workaround for MT7603EN / MT7628AN stability

Add debugfs entry for disabling frames buffering that may be a reason
for mt7603 instability. This patch was sent upstream for review and at
least wasn't rejected yet. Let's add it to let OpenWrt users test if it
really helps.

Example usage:
echo N > /sys/kernel/debug/ieee80211/phy0/mt76/frames_buffering

Signed-off-by: Rafał Miłecki <[email protected]>

package/kernel/mt76/Makefile

@@ -1,7 +1,7 @@
 include $(TOPDIR)/rules.mk
 
 PKG_NAME:=mt76
-PKG_RELEASE=1
+PKG_RELEASE=2
 
 PKG_LICENSE:=GPLv2
 PKG_LICENSE_FILES:=

package/kernel/mt76/patches/101-wifi-mt76-mt7603-add-debugfs-attr-for-disabling-fram.patch

@@ -0,0 +1,81 @@
+From: =?UTF-8?q?Rafa=C5=82=20Mi=C5=82ecki?= <[email protected]>
+Date: Mon, 25 Mar 2024 18:54:02 +0100
+Subject: [PATCH] wifi: mt76: mt7603: add debugfs attr for disabling frames
+ buffering
+MIME-Version: 1.0
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: 8bit
+
+MT7603EN and MT7628AN were reported by multiple users to be unstable
+under high traffic. Transmission of packets could stop for seconds often
+leading to disconnections.
+
+Long research & debugging revelaed a close relation between MCU
+interrupts of type PKT_TYPE_TXS and slowdowns / stalls. That led to
+questioning frames buffering feature.
+
+It turns out that disabling SKBs loopback code makes mt7603 devices much
+more stable under load. There are still some traffic hiccups but those
+happen like once every an hour and end up in recovery in most cases.
+
+Add a debugfs option for disabling frames buffering so users can give it
+a try. If this solution yields a success we can make this feature
+disabled by default.
+
+This change was successfully tested using 2 GHz AP interface on:
+1. Netgear R6220 - MT7621ST (SoC) + MT7603EN (WiFi) + MT7612EN (WiFi)
+2. Xiaomi Mi Router 4C - MT7628AN (Wi-Fi SoC)
+
+Link: https://lore.kernel.org/linux-wireless/[email protected]/
+Closes: https://github.com/openwrt/mt76/issues/865
+Fixes: c8846e101502 ("mt76: add driver for MT7603E and MT7628/7688")
+Signed-off-by: Rafał Miłecki <[email protected]>
+---
+ mt7603/debugfs.c | 2 ++
+ mt7603/dma.c     | 3 +++
+ mt7603/init.c    | 1 +
+ mt7603/mt7603.h  | 2 ++
+ 4 files changed, 8 insertions(+)
+
+--- a/mt7603/debugfs.c
++++ b/mt7603/debugfs.c
+@@ -115,4 +115,6 @@ void mt7603_init_debugfs(struct mt7603_d
+ 			    &dev->sensitivity_limit);
+ 	debugfs_create_bool("dynamic_sensitivity", 0600, dir,
+ 			    &dev->dynamic_sensitivity);
++	debugfs_create_bool("frames_buffering", 0600, dir,
++			    &dev->frames_buffering);
+ }
+--- a/mt7603/dma.c
++++ b/mt7603/dma.c
+@@ -27,6 +27,9 @@ mt7603_rx_loopback_skb(struct mt7603_dev
+ 	u32 val;
+ 	u8 tid = 0;
+ 
++	if (!dev->frames_buffering)
++		goto free;
++
+ 	if (skb->len < MT_TXD_SIZE + sizeof(struct ieee80211_hdr))
+ 		goto free;
+ 
+--- a/mt7603/init.c
++++ b/mt7603/init.c
+@@ -517,6 +517,7 @@ int mt7603_register_device(struct mt7603
+ 	dev->slottime = 9;
+ 	dev->sensitivity_limit = 28;
+ 	dev->dynamic_sensitivity = true;
++	dev->frames_buffering = true;
+ 
+ 	ret = mt7603_init_hardware(dev);
+ 	if (ret)
+--- a/mt7603/mt7603.h
++++ b/mt7603/mt7603.h
+@@ -155,6 +155,8 @@ struct mt7603_dev {
+ 	u32 reset_test;
+ 
+ 	unsigned int reset_cause[__RESET_CAUSE_MAX];
++
++	bool frames_buffering;
+ };
+ 
+ extern const struct mt76_driver_ops mt7603_drv_ops;