[PATCH 1/3] powerpc: Make rtas console _much_ faster

[Michael Ellerman]

I'll clean this one up a little before merging it as per Ryan's email of
a week or two ago. New patch today or tomorrow.

Even though this is 1/3 the rest of the series should be fine to merge,
right Arnd?

cheers

On Sat, 2006-04-29 at 02:45 +0200, Arnd Bergmann wrote:
> Currently the hvc_rtas driver is painfully slow to use. Our "benchmark" is
> ls -R /etc, which spits out about 27866 characters. The theoretical maximum
> speed would be about 2.2 seconds, the current code takes ~50 seconds.
> 
> The core of the problem is that sometimes when the tty layer asks us to push
> characters the firmware isn't able to handle some or all of them, and so
> returns an error. The current code sees this and just returns to the tty code
> with the buffer half sent.
> 
> There's the khvcd thread which will eventually wake up and try to push more
> characters, that will usually work because the firmware's had time to push
> the characters out. But the thread only wakes up every 10 milliseconds, which
> isn't fast enough.
> 
> There's already code in the hvc_console driver to make the khvcd thread do
> a "quick" loop, where it just calls yield() instead of sleeping. The only code
> that triggered that behaviour was recently removed though, which I don't
> quite understand.
> 
> Still, if we set HVC_POLL_QUICK whenever the push hvc_push() doesn't push all
> characters (ie. RTAS blocks), we can get good performance out of the hvc_rtas
> backend. With this patch the "benchmark" takes ~2.8 seconds.
> 
> Cc: Ryan Arnold <rsa at us.ibm.com>
> Signed-off-by: Michael Ellerman <michael at ellerman.id.au>
> Signed-off-by: Arnd Bergmann <arnd.bergmann at de.ibm.com>
> 
> ---
> 
>  drivers/char/hvc_console.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Index: linus-2.6/drivers/char/hvc_console.c
> ===================================================================
> --- linus-2.6.orig/drivers/char/hvc_console.c
> +++ linus-2.6/drivers/char/hvc_console.c
> @@ -570,7 +570,7 @@ static int hvc_poll(struct hvc_struct *h
>  		hvc_push(hp);
>  	/* Reschedule us if still some write pending */
>  	if (hp->n_outbuf > 0)
> -		poll_mask |= HVC_POLL_WRITE;
> +		poll_mask |= HVC_POLL_WRITE | HVC_POLL_QUICK;
>  
>  	/* No tty attached, just skip */
>  	tty = hp->tty;
> 
> --
-- 
Michael Ellerman
IBM OzLabs

wwweb: http://michael.ellerman.id.au
phone: +61 2 6212 1183 (tie line 70 21183)

We do not inherit the earth from our ancestors,
we borrow it from our children. - S.M.A.R.T Person
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