linux-kernel - Re: [PATCH] Fix data loss in cdc-acm

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Date:	Wed, 21 Oct 2015 12:12:32 +0200
From:	Oliver Neukum <oneukum@...e.com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Johan Hovold <johan@...nel.org>, Sven Brauch <mail@...nbrauch.de>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Toby Gray <toby.gray@...lvnc.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-serial@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [PATCH] Fix data loss in cdc-acm

On Tue, 2015-10-20 at 14:16 -0400, Peter Hurley wrote:
> ECHO is on by default and the cdc-acm driver does not implement the
> put_char() and flush_chars() tty driver methods, which made the
> problem
> _way worse_, since every echoed char is sent as it's own URB.

That can be fixed. How can I test this? Will the tty layer use write()
and put_char()?

	Regards
		Oliver

>From ac75a2c9ea67ec22c704a6bcaa30e6fc415d64d3 Mon Sep 17 00:00:00 2001
From: Oliver Neukum <oneukum@...e.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@...e.com>
---
 drivers/usb/class/cdc-acm.c | 58 +++++++++++++++++++++++++++++++++++++++++++++
 drivers/usb/class/cdc-acm.h |  1 +
 2 files changed, 59 insertions(+)

diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index b30e742..262f179 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -725,6 +725,62 @@ 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);
+		goto out;
+	}
+
+	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 +1944,8 @@ 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



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