linux-kernel - Re: Disabling DMA with ICH10?

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Date:	Wed, 30 Sep 2009 14:21:53 +0200
From:	Krzysztof Halasa <khc@...waw.pl>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	"Lennart Baruschka" <FunFlyer@....net>,
	linux-kernel@...r.kernel.org
Subject: Re: Disabling DMA with ICH10?

Alan Cox <alan@...rguk.ukuu.org.uk> writes:

> INB foo
>
> 	CPU -> PCI device		read this register
> 	PCI device -> Disk
> 			trundle whirr whirrr
> 			clunk clunk thud
> 	Disk -> PCI device
> 	PCI device -> CPU... "5"
>
> INB completes
>
> The INB is not interruptible mid instruction and stalls the CPU for the
> full period of the message passing back and forth across the bus. So PIO
> 0 on PCI stalls the bus for the equivalent of an ISA access, and PIO4
> while a good deal faster is still a very long stall in hard real time
> terms.

Sure :-)

That's why I wrote it's a bad idea. I just wanted to correct the
statement about ISA speed - it's not (necessarily) ISA speed, though
it's slow and uses 100% CPU (core) time. (And PIO on PCI* in general
is not much faster, though usually faster than ISA).
-- 
Krzysztof Halasa
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