linux-kernel - Re: serial flow control appears broken

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Message-ID: <46AA2748.80703@howardsilvan.com>
Date:	Fri, 27 Jul 2007 10:11:36 -0700
From:	Lee Howard <faxguy@...ardsilvan.com>
To:	"Maciej W. Rozycki" <macro@...ux-mips.org>
CC:	Robert Hancock <hancockr@...w.ca>, linux-serial@...r.kernel.org,
	tytso@....edu, rmk@....linux.org.uk, linux-kernel@...r.kernel.org
Subject: Re: serial flow control appears broken

Maciej W. Rozycki wrote:

>On Thu, 26 Jul 2007, Lee Howard wrote:
>
>  
>
>>If the application were to use TIOCM_RTS how would it know when to apply it or
>>not?  Is there some approach that the application could take to manage flow
>>control on the serial port?  What about software flow control?  Does the
>>    
>>
>
> Well, an application could negate RTS when it receives a character and 
>is running out of resources for further processing of incoming data.
>
> Smarter UARTs may be able to negate RTS themselves based on the amount of 
>data in their receive FIFO.  The threshold may be configurable.
>

Okay, so let's say we've got a loop around a blocking read on the modem 
file descriptor...

  for (;;) {
      read some data from modem
      process data from modem
      if (end-of-data detected) break;
  }

Are you suggesting that the application should be using deasserting RTS 
after the read and asserting it before?

I had previously thought that the control of RTS was something that the 
serial/tty driver was supposed to do independently based on the buffer 
fill.  Was I wrong?

Thanks,

Lee.
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