Re: [PATCH v2 19/27] pci: PCIe driver for Marvell Armada 370/XP systems

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Thu, Jan 31, 2013 at 04:22:37PM +0100, Thomas Petazzoni wrote:
> Dear Russell King - ARM Linux,
> 
> On Thu, 31 Jan 2013 15:08:01 +0000, Russell King - ARM Linux wrote:
> > If that's what your bridge requires, then we need to detect it via
> > its vendor and device IDs and only apply this fixup to those bridges
> > which require a 64K alignment.
> > 
> > So, the IDs are vendor:device = 0x11ab:0x1092 ?  And let me get this
> > straight, it _is_ a specific requirement for this particular bridge
> > P2P bridge?
> 
> The vendor/device ID will change. This P2P bridge is emulated. However,
> I'm not sure why you need to know the vendor:device ID to make the
> fixup specific.
> 
> The fixup is already specific to those bridges, since I am just setting
> pci_sys_data->win_align_io to 64K for the particular buses that are
> downstream the problematic bridges. So it doesn't affect any other bus
> on the system, and therefore I don't think this fixup needs to be made
> specific to a given vendor:device, no?

The pci_sys_data is not specific to one bus.  It's specific from the
root bus downwards, and is shared by all child busses.

The problem is if you have some card or a conventional P2P bridge which
has 4K windows.  If you merely set the alignment to 64K for all bridges,
then all bridges get this treatment whether or not they need it.  That's
what I'm trying to avoid.

Take, for instance, a cardbus bridge (remember, there are PCI cards which
can be plugged in to give you a cardbus slot.)  I have a device here which
can be plugged into a cardbus slot which has not just one P2P bridge but
two, and a bunch of downsteam devices, including VGA, ethernet, USB, PS/2
etc.  (Okay, Linux doesn't support this hardware because of crappy X86
stuff, despite the fact Windows cope with it just fine.)

There have been cards in the past which have had P2P bridges on them as
well.

So, simply believing that the only P2P bridges in the system will be
those on the physical board is a mistake.