Re: Ordering between PCI config space writes and MMIO reads?

Matthew Wilcox
2006-10-24T21:47:27Z

On Tue, Oct 24, 2006 at 03:22:10PM -0400, Jeff Garzik wrote:
> The PCI config APIs have traditionally enforced very strong ordering.
> Heck, the PCI config APIs often take a spinlock on each read or write;
> so they are definitely not intended to be as fast as MMIO.

s/often/always/. It's implemented in drivers/pci/access.c.

I think the right way to fix this is to ensure mmio write ordering in
the pci_write_config_*() implementations. Like this.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>

diff --git a/drivers/pci/access.c b/drivers/pci/access.c
index ea16805..c80f1ba 100644
--- a/drivers/pci/access.c
+++ b/drivers/pci/access.c
@@ -1,6 +1,6 @@
#include <linux/pci.h>
#include <linux/module.h>
-#include <linux/ioport.h>
+#include <linux/io.h>

#include "pci.h"

@@ -45,6 +45,7 @@ int pci_bus_write_config_##size \
if (PCI_##size##_BAD) return PCIBIOS_BAD_REGISTER_NUMBER; \
spin_lock_irqsave(&pci_lock, flags); \
res = bus->ops->write(bus, devfn, pos, len, value); \
+ mmiowb(); \
spin_unlock_irqrestore(&pci_lock, flags); \
return res; \
}
@@ -102,6 +103,7 @@ int pci_user_write_config_##size \
if (likely(!dev->block_ucfg_access)) \
ret = dev->bus->ops->write(dev->bus, dev->devfn, \
pos, sizeof(type), val); \
+ mmiowb(); \
spin_unlock_irqrestore(&pci_lock, flags); \
return ret; \
}