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SubjectRe: Handshaking on USB serial devices
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.

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