implement put_char() in cdc-acm

[Oliver Neukum <oneukum-IBi9RG/b67k@public.gmane.org>]

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Hi,

the theory that the lack of support for put_char() is a
major contributor to character loss in cdc-acm can be tested.
Sven, could you test the attached patch? It implements
the support.

	Regards
		Oliver


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From f3871a76d7e2876b0e6ad66aee91085ecf9b2785 Mon Sep 17 00:00:00 2001
From: Oliver Neukum <oneukum@suse.com>
Date: Wed, 21 Oct 2015 12:10:07 +0200
Subject: [PATCH] cdc-acm: implement put_char() and flush_chars()

This should cut down latencies and waste if the tty layer writes single bytes.

Signed-off-by: Oliver Neukum >oneukum@suse.com>
---
 drivers/usb/class/cdc-acm.c | 67 +++++++++++++++++++++++++++++++++++++++++++++
 drivers/usb/class/cdc-acm.h |  1 +
 2 files changed, 68 insertions(+)

diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index b30e742..1cb3124 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -712,9 +712,20 @@ static int acm_tty_write(struct tty_struct *tty,
 	}
 
 	if (acm->susp_count) {
+		if (acm->putbuffer) {
+			/* now to preserve order */
+			usb_anchor_urb(acm->putbuffer->urb, &acm->delayed);
+			acm->putbuffer = NULL;
+		}
 		usb_anchor_urb(wb->urb, &acm->delayed);
 		spin_unlock_irqrestore(&acm->write_lock, flags);
 		return count;
+	} else {
+		if (acm->putbuffer) {
+			/* at this point there is no good way to handle errors */
+			acm_start_wb(acm, acm->putbuffer);
+			acm->putbuffer = NULL;
+		}
 	}
 
 	stat = acm_start_wb(acm, wb);
@@ -725,6 +736,60 @@ static int acm_tty_write(struct tty_struct *tty,
 	return count;
 }
 
+static void acm_tty_flush_chars(struct tty_struct *tty)
+{
+	struct acm *acm = tty->driver_data;
+	struct acm_wb *cur = acm->putbuffer;
+	int err;
+	unsigned long flags;
+
+	acm->putbuffer = NULL;
+	err = usb_autopm_get_interface_async(acm->control);
+	spin_lock_irqsave(&acm->write_lock, flags);
+	if (err < 0) {
+		cur->use = 0;
+		goto out;
+	}
+
+	if (acm->susp_count)
+		usb_anchor_urb(cur->urb, &acm->delayed);
+	else
+		acm_start_wb(acm, cur);
+out:
+	spin_unlock_irqrestore(&acm->write_lock, flags);
+	return;
+}
+
+static int acm_tty_put_char(struct tty_struct *tty, unsigned char ch)
+{
+	struct acm *acm = tty->driver_data;
+	struct acm_wb *cur;
+	int wbn;
+	unsigned long flags;
+
+overflow:
+	cur = acm->putbuffer;
+	if (!cur) {
+		spin_lock_irqsave(&acm->write_lock, flags);
+		wbn = acm_wb_alloc(acm);
+		if (wbn >= 0) {
+			cur = &acm->wb[wbn];
+			acm->putbuffer = cur;
+		}
+		spin_unlock_irqrestore(&acm->write_lock, flags);
+		if (!cur)
+			return 0;
+	}
+
+	if (cur->len == acm->writesize) {
+		acm_tty_flush_chars(tty);
+		goto overflow;
+	}
+
+	cur->buf[cur->len++] = ch;
+	return 1;
+}
+
 static int acm_tty_write_room(struct tty_struct *tty)
 {
 	struct acm *acm = tty->driver_data;
@@ -1888,6 +1953,8 @@ static const struct tty_operations acm_ops = {
 	.cleanup =		acm_tty_cleanup,
 	.hangup =		acm_tty_hangup,
 	.write =		acm_tty_write,
+	.put_char =		acm_tty_put_char,
+	.flush_chars =		acm_tty_flush_chars,
 	.write_room =		acm_tty_write_room,
 	.ioctl =		acm_tty_ioctl,
 	.throttle =		acm_tty_throttle,
diff --git a/drivers/usb/class/cdc-acm.h b/drivers/usb/class/cdc-acm.h
index dd9af38..648a6f7 100644
--- a/drivers/usb/class/cdc-acm.h
+++ b/drivers/usb/class/cdc-acm.h
@@ -94,6 +94,7 @@ struct acm {
 	unsigned long read_urbs_free;
 	struct urb *read_urbs[ACM_NR];
 	struct acm_rb read_buffers[ACM_NR];
+	struct acm_wb *putbuffer;			/* for acm_tty_put_char() */
 	int rx_buflimit;
 	int rx_endpoint;
 	spinlock_t read_lock;
-- 
2.1.4