Re: 2.4.18 serial drops characters with 16654

From: Alan Cox (
Date: Thu Sep 19 2002 - 12:38:52 EST

On Thu, 2002-09-19 at 18:27, Dan Christian wrote:
> The problem seems to be related to the RTS/CTS flow control handling.
> The 16654 handles flow control in hardware, but the 16550 does it in
> software (I've verified this with a digital oscilloscope). I don't
> currently have the equipment to compare when the lines drop and which
> characters are lost.

Actually you can do it in hardware on the 16550 depending how its wired.
Take a look at the usenet-2 serial port design some day. The software
mode we do does in theory mean heavy delay to the bh handling might
delay the assertion excessively. That I think may be the real
explanation here.

         buffer full
         bh handler delayed by bh load (tasklet nowdays I guess I mean)
         Oh look I should do carrier

Russell - does that sound reasonable.

If so the answer yet again (as with the gige performance and some
others) might be to make it much much harder for stuff tofall back to

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