Re: [PATCH] hso.c against 2.6.27-rc5 throttle/unthrottle to prevent loss of serial data

Hi Alan,
You are very close to understanding me.
If my understanding of the n_tty.c code is correct it will
only start throttling when there is TTY_THRESHOLD_THROTTLE
i.e. 128 bytes left in the n_tty ring buffer this is a very low limit
for my driver. This means
that if I try to do a tty_flip_buffer_push of
of a tty buffer larger than 128 bytes when the threshold
is reached I will lose data. My urb buffers are 4k in
size 2 buffers of this size up to the tty layer in a row 
quickly I believe I'll lose data in the n_tty layer.

Alan Cox wrote:

O> Alan you'll notice I'm calling tty_flip_buffer_push(tty)

quoted

i.e. flush_to_ldisc every 32 bytes as
TTY_THRESHOLD_THROTTLE in /drivers/char/n_tty.c is
only 128 bytes & I need to get unthrottled before
I lose characters it would be nice if the tty layer
could set the size of the throttle/unthrottle parameters
& the size of the n_tty ring buffer.

I don't really follow what you are trying to do here. You have two layers
of buffering to consider

   Driver -> | tty buffer queue (64k) | n_tty - internal queue | user


And it is when the internal queue gets full that we call throttle not
when the 64K of tty buffering is full.

So you really shouldn't see a problem unless you have many Kbytes of USB
requests floating around to consume all the buffering even after the tty
is throttled.

As to the robustness - the USB tty code is generally pretty bad in this
area so hso won't be any worse. Probably it is sufficient to keep an eye
on what is in flight and when an URB completes don't just reissue it but
reissue any others that didn't get posted due to errors.

The only other case you then have to trap is the nothing could be queued
case which should never happen and I guess if you are passionate about
robustness would need a timer.

Alan

-- 
best regards,
D.J. Barrow
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