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Date: Tue, 16 Dec 2003 19:12:23 -0500 (EST)
From: "Richard B. Johnson" <root@chaos.analogic.com>
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To: Vladimir Kondratiev <vladimir.kondratiev@intel.com>
cc: arjanv@redhat.com, Jeff Garzik <jgarzik@pobox.com>,
       Linus Torvalds <torvalds@osdl.org>, greg@kroah.com,
       linux-kernel@vger.kernel.org, Alan Cox <alan@redhat.com>,
       Marcelo Tosatti <marcelo@conectiva.com.br>, Martin Mares <mj@ucw.cz>
Subject: Re: PCI Express support for 2.4 kernel
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On Wed, 17 Dec 2003, Vladimir Kondratiev wrote:

> Arjan van de Ven wrote:
>
> >>>+	/* dummy read to flush PCI write */
> >>>+	readb(addr);
> >>>
> >>>
> >>This is going to choke some hardware, I guarantee.
> >>
> >>You always want to make sure your flush is of the same size at the
> >>write.  Reading a byte from an address that the hardware defines as
> >>"32-bit writes only" can get ugly real quick ;-)
> >>
> >>
> >
> >also reading back addr might not be the best choice in case some
> >registers have side effects on reading, it's probably better to read
> >back an address that is known to be ok to read (like the vendor ID
> >field)
> >
> >
> >
> Good idea!


You should not abitrarily flush all the writes with a read
after each write. The PCI/Bus is a FIFO, stuff will get to
the hardware in the order written. If you "flush" every write,
you will seriously hurt the performance. I don't like that
MACRO/Function, whatever it is with the switch shown previously.
All that code just for a "@(*^&$(*^(" write will screw up
performance. I've looked at the result of some the the MACRO
expansions, in particular the get_user, put_user, and copy_to/from
macros where there is a switch. It is not pre-processed out.


Cheers,
Dick Johnson
Penguin : Linux version 2.4.22 on an i686 machine (797.90 BogoMips).
            Note 96.31% of all statistics are fiction.