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From: David Newall <davidn@davidnewall.com>
To: Greg KH <greg@kroah.com>
Cc: linux-usb@vger.kernel.org,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: Handshaking on USB serial devices
Date: Fri, 15 Feb 2008 15:49:34 +1030	[thread overview]
Message-ID: <47B520E6.5030809@davidnewall.com> (raw)
In-Reply-To: <20080214204744.GB17277@kroah.com>

Greg KH wrote:
> On Thu, Feb 14, 2008 at 07:55:44PM +1030, David Newall wrote:
>   
>> The current 2.6 driver maintains it's own buffer.  I think that's a bad
>> thing: usbserial already buffers writes, and the extra buffer copy seems
>> unnecessary, however it does solve the putchar problem.  Buffered (i.e.
>> by the 2.6 series pl2303 driver) data is written as soon as practicable,
>> regardless of CTS/DTR.  The same general workaround, but placed in
>> pl2303_send seems correct to me; that is, stop submitting write urbs
>> when the remote end lowers CTS/DTR, and trigger the resume from the
>> interrupt callback (specifically in update_line_status.)
>>     
>
> Where does the usbserial core buffer writes on 2.6?  The serial_write()
> function just passes the data straight down to the usb-serial child
> driver directly, no copying or buffering happens that I can see.
>   

You're right.  I haven't examined the 2.6 stack as closely as the 2.4. 
I noticed the buffer in 2.6 pl2303, but didn't check 2.6 usb-serial.  I
think I prefer the buffer in usb-serial, because its centralised rather
than in each driver, but I'm not going to step up to the plate and
propose changing that!

>> To make it clear: Even aside from the buffer in 2.6's pl2303.c, there's
>> a race: An in-flight write URB can fill all hardware buffers, making
>> unsafe what previously appeared to be a safe write.  I think it's
>> essential to delay submission of the URB on a stop-transmit condition.
>>     
>
> It's up to the individual driver to know when their buffers are filled
> up.  The big problem is, a lot of these cheap usb-serial devices (like
> the pl2303) don't have a way to report the uart queue filled-state back
> to the host, so things can easily get over-run as you have found out.
>   

My understanding of the problem has developed over the last couple of
days; going from wrist-deep to elbow-deep into the guts of things does
that.  There is a problem, and the solution I've been developing
addresses it, but maybe there's a simpler answer.  Hope to have a patch
together soon.

  reply	other threads:[~2008-02-15  5:19 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-02-13 14:45 David Newall
2008-02-14  5:02 ` Greg KH
2008-02-14  9:25   ` David Newall
2008-02-14 12:10     ` Alan Cox
2008-02-14 16:16       ` Gene Heskett
2008-02-14 16:31         ` Alan Cox
2008-02-14 17:55           ` Gene Heskett
2008-02-14 19:37             ` Alan Cox
2008-02-14 20:04               ` Gene Heskett
2008-02-14 20:52                 ` Greg KH
2008-02-14 21:32                   ` Gene Heskett
2008-02-15  5:08               ` David Newall
2008-02-14 22:39         ` Krzysztof Halasa
2008-02-14 23:09           ` Gene Heskett
2008-02-14 18:04       ` David Newall
2008-02-14 18:53         ` David Brownell
2008-02-14 19:36         ` Alan Cox
2008-02-21 15:22           ` David Newall
2008-02-21 15:15             ` Alan Cox
2008-02-21 19:35               ` David Newall
2008-02-21 20:58                 ` Greg KH
2008-02-14 20:47     ` Greg KH
2008-02-15  5:19       ` David Newall [this message]
2008-02-14 11:55   ` Alan Cox
     [not found] <9Wr5Z-7cw-1@gated-at.bofh.it>
     [not found] ` <9WSJ4-222-21@gated-at.bofh.it>
     [not found]   ` <9WTlN-2T0-7@gated-at.bofh.it>
     [not found]     ` <9WTYn-3Zb-29@gated-at.bofh.it>
2008-02-15 12:00       ` Bodo Eggert

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