Subject: Re: [PATCH] detour TTY driver - now ttyprintk
From: Samo Pogacnik <samo_pogacnik@t-2.net>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: linux-embedded <linux-embedded@vger.kernel.org>,
       linux kernel <linux-kernel@vger.kernel.org>
In-Reply-To: <20100621153858.444695c0@lxorguk.ukuu.org.uk>
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Date: Wed, 23 Jun 2010 00:06:50 +0200
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On 21.06.2010 (Mon) at 15:38 +0100 Alan Cox wrote:
> > I'm thinking to leave the ratelimit support in for the time being. I had
> > in mind cases, when someone does
> >  "cat /proc/kmsg > dev/ttyprintk" or
> > suppose the console is redirected to ttyprintk (which i would like to be
> > able to do from user program)
> 
> Console as in the printk sense would then loop.
> 
> If you are going to do the flow control you should probably do something
> like
> 
> 
> write_room()
> {
> 	if (!flow_controlled)
> 		space = 8192;
> 	return space;
> }
> 
> write()
> {
> 	space -= len;
> }
> 
> then your flow control will behave properly and slow down users rather
> than losing data (except stuff sent without blocking)
> 
For correct flow control, i suppose current empty space of the real
(final) printk buffer is needed. On the other hand my intermediate
pre-formatting buffer is only "one line" long, but serialized on access
in a way that it is always completely available (except for the time of
tpk_printk() function running). As such intermediate buffer only defines
maximum write_room space.

Now there are two ideas. The first one is to dig out current real printk
buffer space (smells like hacking?) and adapt write_room to that space
in some logical manner. And the other would be to use ratelimit support
to switch between max and zero in write_room answer and remove other
retelimit response?

What do you suggest, do i miss something?

regards, Samo

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