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

[David Miller <davem@davemloft.net>]

From: Matthew Wilcox <matthew@wil.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.