linux-kernel - Re: [Fwd: Re: [PATCH 0/5]PCI: x86 MMCONFIG]

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Date:	Thu, 20 Dec 2007 15:05:52 -0500
From:	Loic Prylli <loic@...i.com>
To:	Matthew Wilcox <matthew@....cx>
CC:	Tony Camuso <tcamuso@...hat.com>, Greg KH <gregkh@...e.de>,
	linux-kernel@...r.kernel.org, linux-pci@...ey.karlin.mff.cuni.cz
Subject: Re: [Fwd: Re: [PATCH 0/5]PCI: x86 MMCONFIG]

On 12/20/2007 2:08 PM, Matthew Wilcox wrote:
> On Thu, Dec 20, 2007 at 02:04:31PM -0500, Tony Camuso wrote:
>   
>> Also, this solution also would allow us to remove the unreachable_devices()
>> routine and bitmap.
>>     
>
> Not really ... we probe reading address 0x100 to see if the device
> supports extended config space or not.  So we need to make that fail
> gracefully for the amd7111 case.
>   

pci_cfg_space_size()  is only done for PCI-express  or PCI-X mode 2
devices, so you still have eliminated the bulk of the problems which are
typically handling the legacy busses on modern machines. I don't know
what is the amd7111.

>   
>> Does anybody see a down side to this?
>>     
>
> It'll be slower than it would be if we used mmconfig directly.  Now yes,
> nobody should be using pci config space in performance critical paths
> ... but see the tg3 driver.
>   

I am not familiar with the tg3 driver, just trying to give a 5 minutes
look, it seems the typical cases where the pci-conf-space is used
intensively are with some rev in combination with the 82801
(TG3_FLG2_ICH_WORKAROUND) which I don't think support mmconfig anyway,
as well as some very specific PCI-X combinations
(TG3_FLAG_PCIX_TARGET_HWBUG) which are also very unlikely to support
mmconfig.

Even if I am wrong for the tg3, I don't really think mmconfig vs type1
could make a noticeable performance on any common systems (obscure
systems or hardware where it could potentially have a performance impact
could  use a non-default configuration).

Loic

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