linux-kernel - Re: [PATCH] cdc-acm: some enhancement on acm delayed write

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Message-ID: <1396952542.25633.13.camel@linux-fkkt.site>
Date:	Tue, 08 Apr 2014 12:22:22 +0200
From:	Oliver Neukum <oneukum@...e.de>
To:	Johan Hovold <jhovold@...il.com>
Cc:	Xiao Jin <jin.xiao@...el.com>, gregkh@...uxfoundation.org,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	david.a.cohen@...ux.intel.com, yanmin.zhang@...el.com
Subject: Re: [PATCH] cdc-acm: some enhancement on acm delayed write

On Tue, 2014-04-08 at 09:33 +0200, Johan Hovold wrote:
> On Tue, Apr 08, 2014 at 11:05:20AM +0800, Xiao Jin wrote:
> > We find two problems on acm tty write delayed mechanism.
> 
> Then you should split this into two patches.
> 
> > (1) When acm resume, the delayed wb will be started. But now
> > only one write can be saved during acm suspend. More acm write
> > may be abandoned.
> 
> Why not simply return 0 in write and use the tty buffering rather than
> implement another buffer in cdc_acm?

Yes. We need a single buffer because the tty layer is not happy
when you accept no data. But we should be able to refuse subsequent
writes. Could you test this patch?

	Regards
		Oliver

>From 1d44c1f2a10b5617824a37c8ec51f5547e482259 Mon Sep 17 00:00:00 2001
From: Oliver Neukum <oneukum@...e.de>
Date: Tue, 8 Apr 2014 12:17:39 +0200
Subject: [PATCH] cdc-acm: fix consecutive writes while device is suspended

CDC-ACM needs to handle one attempt to write to a suspended
device because we told the tty layer that there is room.
A second attempt may and must fail or we drop data.

Signed-off-by: Oliver Neukum <oneukum@...e.de>
CC: stable@...r.kernel.org
---
 drivers/usb/class/cdc-acm.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index 900f7ff..7ad3105 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -646,10 +646,12 @@ static int acm_tty_write(struct tty_struct *tty,
 
 	usb_autopm_get_interface_async(acm->control);
 	if (acm->susp_count) {
-		if (!acm->delayed_wb)
+		if (!acm->delayed_wb) {
 			acm->delayed_wb = wb;
-		else
+		} else {
 			usb_autopm_put_interface_async(acm->control);
+			count = 0;
+		}
 		spin_unlock_irqrestore(&acm->write_lock, flags);
 		return count;	/* A white lie */
 	}
-- 
1.8.4.5



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