mac80211: debugfs option to force TX status frames

At Technical University of Munich we use MAC 802.11 TX status frames to
perform several measurements in MAC 802.11 setups.

With ath based drivers this was possible until commit d94a461d
("ath9k: use ieee80211_tx_status_noskb where possible") as the driver
ignored the IEEE80211_TX_CTL_REQ_TX_STATUS flag and always delivered
tx_status frames. Since that commit, this behavior was changed and the
driver now adheres to IEEE80211_TX_CTL_REQ_TX_STATUS.

Due to performance reasons, IEEE80211_TX_CTL_REQ_TX_STATUS is not set for
data frames from interfaces in managed mode. Hence, frames that are sent
from a managed mode interface do never deliver tx_status frames. This
remains true even if a monitor mode interface (the measurement interface)
is added to the same ieee80211 physical device. Thus, there is no
possibility for receiving tx_status frames for frames sent on an interface
in managed mode, if the driver adheres to IEEE80211_TX_CTL_REQ_TX_STATUS.

In order to force delivery of tx_status frames for research and debugging
purposes, implement a debugfs option force_tx_status for ieee80211 physical
devices. When this option is set for a physical device,
IEEE80211_TX_CTL_REQ_TX_STATUS is enabled in all packets sent from that
device. This option can be set via
/sys/kernel/debug/ieee80211/<dev>/force_tx_status. The default is disabled.
Co-developed-by: Charlie Groh <ga58taw@mytum.de>
Signed-off-by: Charlie Groh <ga58taw@mytum.de>
Signed-off-by: Julius Niedworok <julius.n@gmx.net>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/debugfs.c

@@ -150,6 +150,58 @@ static const struct file_operations aqm_ops = {
 	.llseek = default_llseek,
 };
 
+static ssize_t force_tx_status_read(struct file *file,
+				    char __user *user_buf,
+				    size_t count,
+				    loff_t *ppos)
+{
+	struct ieee80211_local *local = file->private_data;
+	char buf[3];
+	int len = 0;
+
+	len = scnprintf(buf, sizeof(buf), "%d\n", (int)local->force_tx_status);
+
+	return simple_read_from_buffer(user_buf, count, ppos,
+				       buf, len);
+}
+
+static ssize_t force_tx_status_write(struct file *file,
+				     const char __user *user_buf,
+				     size_t count,
+				     loff_t *ppos)
+{
+	struct ieee80211_local *local = file->private_data;
+	char buf[3];
+	size_t len;
+
+	if (count > sizeof(buf))
+		return -EINVAL;
+
+	if (copy_from_user(buf, user_buf, count))
+		return -EFAULT;
+
+	buf[sizeof(buf) - 1] = '\0';
+	len = strlen(buf);
+	if (len > 0 && buf[len - 1] == '\n')
+		buf[len - 1] = 0;
+
+	if (buf[0] == '0' && buf[1] == '\0')
+		local->force_tx_status = 0;
+	else if (buf[0] == '1' && buf[1] == '\0')
+		local->force_tx_status = 1;
+	else
+		return -EINVAL;
+
+	return count;
+}
+
+static const struct file_operations force_tx_status_ops = {
+	.write = force_tx_status_write,
+	.read = force_tx_status_read,
+	.open = simple_open,
+	.llseek = default_llseek,
+};
+
 #ifdef CONFIG_PM
 static ssize_t reset_write(struct file *file, const char __user *user_buf,
 			   size_t count, loff_t *ppos)
@@ -383,6 +435,7 @@ void debugfs_hw_add(struct ieee80211_local *local)
 	DEBUGFS_ADD(hwflags);
 	DEBUGFS_ADD(user_power);
 	DEBUGFS_ADD(power);
+	DEBUGFS_ADD_MODE(force_tx_status, 0600);
 
 	if (local->ops->wake_tx_queue)
 		DEBUGFS_ADD_MODE(aqm, 0600);

net/mac80211/ieee80211_i.h

@@ -1384,6 +1384,7 @@ struct ieee80211_local {
 		struct dentry *rcdir;
 		struct dentry *keys;
 	} debugfs;
+	bool force_tx_status;
 #endif
 
 	/*

net/mac80211/tx.c

@@ -2471,6 +2471,11 @@ static struct sk_buff *ieee80211_build_hdr(struct ieee80211_sub_if_data *sdata,
 	if (IS_ERR(sta))
 		sta = NULL;
 
+#ifdef CONFIG_MAC80211_DEBUGFS
+	if (local->force_tx_status)
+		info_flags |= IEEE80211_TX_CTL_REQ_TX_STATUS;
+#endif
+
 	/* convert Ethernet header to proper 802.11 header (based on
 	 * operation mode) */
 	ethertype = (skb->data[12] << 8) | skb->data[13];
@@ -3473,6 +3478,11 @@ static bool ieee80211_xmit_fast(struct ieee80211_sub_if_data *sdata,
 		      (tid_tx ? IEEE80211_TX_CTL_AMPDU : 0);
 	info->control.flags = IEEE80211_TX_CTRL_FAST_XMIT;
 
+#ifdef CONFIG_MAC80211_DEBUGFS
+	if (local->force_tx_status)
+		info->flags |= IEEE80211_TX_CTL_REQ_TX_STATUS;
+#endif
+
 	if (hdr->frame_control & cpu_to_le16(IEEE80211_STYPE_QOS_DATA)) {
 		tid = skb->priority & IEEE80211_QOS_CTL_TAG1D_MASK;
 		*ieee80211_get_qos_ctl(hdr) = tid;