Date: Sun, 9 Oct 2005 13:33:13 +0200
From: Samuel Thibault <samuel.thibault@ens-lyon.org>
To: akpm@osdl.org, Alan Cox <alan@lxorguk.ukuu.org.uk>,
       linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org
Subject: Re: [patch 3/4] new serial flow control
Message-ID: <20051009113313.GD5104@bouh.residence.ens-lyon.fr>
Mail-Followup-To: Samuel Thibault <samuel.thibault@ens-lyon.org>,
	akpm@osdl.org, Alan Cox <alan@lxorguk.ukuu.org.uk>,
	linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org
References: <200501052341.j05Nfod27823@mail.osdl.org> <20050105235301.B26633@flint.arm.linux.org.uk> <20051008222711.GA5150@bouh.residence.ens-lyon.fr> <20051009000153.GA23083@flint.arm.linux.org.uk> <20051009002129.GJ5150@bouh.residence.ens-lyon.fr> <20051009083724.GA14335@flint.arm.linux.org.uk> <20051009100909.GF5150@bouh.residence.ens-lyon.fr> <20051009111718.GA13144@flint.arm.linux.org.uk>
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Hi,

Russell King, le Sun 09 Oct 2005 12:17:18 +0100, a ?crit :
> On Sun, Oct 09, 2005 at 12:09:09PM +0200, Samuel Thibault wrote:
> > - some other use it as a strobe for acknowledging characters, some other
> > use it as a strobe for acknowledging frames (announced by CTS).
> 
> The last has no business being in the serial driver though -
> the driver knows nothing about frames of characters.  It's more
> a userland (in which case it's TIOCM* ioctls) or ldisc issue
> (tty_driver->tiocmset).

In principle, yes. But implementing it in userland is not responsive
enough doesn't work reliably, while it can actually be implemented in
the serial driver as "acknowledge any CTS upfront" (if the device is not
throttled), and then it works fine. Writing a whole line discipline (and
getting it accepted in the kernel) just for this is a bit overkill.

> > > However, bear in mind that the majority of the more inteligent 8250-
> > > compatible UARTs with large FIFOs only do hardware flow control on
> > > RTS/CTS
> > 
> > Hardward flow control is usually performed in software. Can't their
> > hardware implementation of hardware flow control be disabled when
> > control method is not usual RTS/CTS?
> 
> You missed the point.  Of course the hardware flow control can be
> disabled.  However, if you do have on-chip CTS flow control disabled
> with UARTs with large FIFOs,
> etc..

Yes, of course. But can't this be disabled too? (people won't mind cpu
overhead if they can at least get their device working).

Regards,
Samuel
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