linux-kernel - Re: [PATCH] x86: provide a DMI based port 0x80 I/O delay override

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Message-ID: <20071230164828.039916c8@the-village.bc.nu>
Date:	Sun, 30 Dec 2007 16:48:28 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rene Herman <rene.herman@...access.nl>, dpreed@...d.com,
	Islam Amer <pharon@...il.com>, hpa@...or.com,
	Pavel Machek <pavel@....cz>, Ingo Molnar <mingo@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: provide a DMI based port 0x80 I/O delay override

> ok. Is it more of a "gets flushed due to timing out", or a 
> specified-for-sure POST flushing property of all out 0x80 cycles going 
> to the PCI bridge? I thought PCI posting policy is up to the CPU, it can 
> delay PCI space writes arbitrarily (within reasonable timeouts) as long 
> as no read is done from the _same_ IO space address. Note that the port 
> 0x80 cycle is neither a read, nor for the same address.

Its what appears to happen reliably on real computers.

> i'm wondering, how safe would it be to just dumbly replace outb_p() 
> with:
> 
> 	out(port);
> 	in(port);

Catastrophic I imagine. If the delay is for timing access then you've just
broken the timing, if the port has side effects you've just broken the
driver.

> in these drivers. Side-effects of inb() would not be unheard of for the 
> ancient IO ports, but for even relatively old SCSI hardware, would that 
> really be a problem?

The specific drivers need reviewing. There are very few uses in PCI space
so it's a minor job.

> ah, i understand. So i guess a stupid udelay_serialized() which takes a 
> global spinlock would solve these sort of races? But i guess making them 
> more likely to trigger would lead to a better kernel in the end ...

Better to just fix the drivers. I don't think that will take too many
days after everyone is back working.

> doing it - but we'll do the plunge in v2.6.25 and make io_delay=udelay 
> the default, hm? Thomas has a real 386DX system, if that doesnt break 

For processors with TSC I think we should aim for 2.6.25 to do this and
to have the major other _p fixups done. I pity whoever does stuff like
the scc drivers but most of the rest isn't too bad.

Alan
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