Re: PCI Express support for 2.4 kernel

From: "Richard B. Johnson" <root@chaos.analogic.com>
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
Date: Tue, 16 Dec 2003 19:12:23 -0500 (EST)	[thread overview]
Message-ID: <Pine.LNX.4.53.0312161906340.23063@chaos> (raw)
In-Reply-To: <3FDF898D.2060902@intel.com>

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.