linux-kernel - Re: [PATCH 0/4] Fastboot revisited: Asynchronous function calls

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Message-ID: <20090111131405.GA2217@ucw.cz>
Date:	Sun, 11 Jan 2009 14:14:06 +0100
From:	Pavel Machek <pavel@...e.cz>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, fweisbec@...il.com,
	linux-scsi@...r.kernel.org, linux-ide@...r.kernel.org,
	linux-acpi@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH 0/4] Fastboot revisited: Asynchronous function calls

On Sun 2009-01-04 10:16:57, Linus Torvalds wrote:
> 
> 
> On Sun, 4 Jan 2009, Arjan van de Ven wrote:
> > 
> > To see this working, I uploaded a "before" and "after" kernel bootchart
> > for one of my testboxes at
> > 
> > http://www.fenrus.org/linux/before.svg
> > http://www.fenrus.org/linux/after.svg
> 
> Ok, so why does the serial port init take so long? That's a quarter of a 
> second for you, which is ridiculous.
> 
> I _think_ it's the irq auto-probing, but that's just a guess. The 8250 
> driver does some historical crud, like
> 
> 	probe_irq_off(probe_irq_on());
> 
> to get rid of any pending irq's, but that should be entirely pointless 
> these days. I bet that line basically goes back to pre-history, before we 
> made the auto-probing much stabler.
> 
> The irq auto-probing itself also has a few excessive delays, like waiting 
> for 0.1 s just to wait for spurious interrupts to trigger. Doing the extra 
> unnecessary probe makes that doubly expensive.
> 
> Does this patch make any difference to you? I'm not at all sure that it's 
> the irq probing, but if it is, then this should make the serial probe go 
> much faster.

Do serial ports even matter for the netbooks this is targeted at?
Serial ports are gone even on normal notebooks and getting rare at
desktop...


>  	}
>  
>  	/* forget possible initially masked and pending IRQ */
> -	probe_irq_off(probe_irq_on());

Delete the comment, too? :-).

-- 
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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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