linux-kernel - Re: [RFC] disable PCIE 'Enable No Snoop' bit by default

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Message-Id: <1189492022.25021.0.camel@sli10-conroe.sh.intel.com>
Date:	Tue, 11 Sep 2007 14:27:02 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Greg KH <greg@...ah.com>
Cc:	David Miller <davem@...emloft.net>, matthew@....cx,
	linux-kernel@...r.kernel.org, linux-pci@...ey.karlin.mff.cuni.cz,
	akpm@...ux-foundation.org, gregkh@...e.de
Subject: Re: [RFC] disable PCIE 'Enable No Snoop' bit by default

On Mon, 2007-09-10 at 23:15 -0700, Greg KH wrote:
> On Tue, Sep 11, 2007 at 09:29:43AM +0800, Shaohua Li wrote:
> > On Sun, 2007-09-09 at 09:43 -0700, Greg KH wrote:
> > > On Thu, Sep 06, 2007 at 05:40:38AM -0700, David Miller wrote:
> > > > From: Matthew Wilcox <matthew@....cx>
> > > > Date: Thu, 6 Sep 2007 05:57:31 -0600
> > > > 
> > > > > I'm not sure your analysis is correct.  Here's what my draft copy of
> > > > > the pcie 2.0 spec says:
> > > > > 
> > > > >   Enble No Snoop ? If this bit is Set, the Function is permitted to
> > > > >   Set the No Snoop bit in the Requester Attributes of transactions it
> > > > >   initiates that do not require hardware enforced cache coherency (see
> > > > >   Section 2.2.6.5). Note that setting this bit to 1b should not cause
> > > > >   a Function to Set the No Snoop attribute on all transactions that it
> > > > >   initiates. Even when this bit is Set, a Function is only permitted
> > > > >   to Set the No Snoop attribute on a transaction when it can guarantee
> > > > >   that the address of the transaction is not stored in any cache in
> > > > >   the system.  This bit permitted to be hardwired to 0b if a Function
> > > > >   would never Set the No Snoop attribute in transactions it initiates.
> > > > >   Default value of this bit is 1b.
> > > > > 
> > > > > That implies that devices are only allowed to set it when it's safe to
> > > > > do so ... and we don't need to turn it off.
> > > > 
> > > > This is my understanding of this area of PCI-E as well, and I
> > > > also agree that therefore we should not turn this bit off.
> > > 
> > > I agree.  But Shaohua, do you see any problems that this patch fixes?
> > No, I didn't see any breakage, just worry about it's a potential issue.
> 
> Hm, well, if you don't mind, I'd like to leave it as is for now, as no
> one is reporting any problems with this, and there seems to be some
> disagreement as to if it is really needed or not.
> 
> Is that ok?
Ok.

Regards,
Shaohua
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