linux-kernel - Re: [PATCH v4 04/10] s390/pci: Add architecture specific resource/bus address translation

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Message-ID: <a5ab698977f724e9121f81b9cfec9503d9decc72.camel@linux.ibm.com>
Date: Thu, 02 Oct 2025 14:58:45 +0200
From: Niklas Schnelle <schnelle@...ux.ibm.com>
To: Bjorn Helgaas <helgaas@...nel.org>,
        Ilpo Järvinen
	 <ilpo.jarvinen@...ux.intel.com>
Cc: alex.williamson@...hat.com, clg@...hat.com, mjrosato@...ux.ibm.com,
        Farhan Ali <alifm@...ux.ibm.com>, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pci
 <linux-pci@...r.kernel.org>
Subject: Re: [PATCH v4 04/10] s390/pci: Add architecture specific
 resource/bus address translation

On Wed, 2025-09-24 at 10:16 -0700, Farhan Ali wrote:
> On s390 today we overwrite the PCI BAR resource address to either an
> artificial cookie address or MIO address. However this address is different
> from the bus address of the BARs programmed by firmware. The artificial
> cookie address was created to index into an array of function handles
> (zpci_iomap_start). The MIO (mapped I/O) addresses are provided by firmware
> but maybe different from the bus address. This creates an issue when trying
> to convert the BAR resource address to bus address using the generic
> pcibios_resource_to_bus().
> 
> Implement an architecture specific pcibios_resource_to_bus() function to
> correctly translate PCI BAR resource addresses to bus addresses for s390.
> Similarly add architecture specific pcibios_bus_to_resource function to do
> the reverse translation.
> 
> Signed-off-by: Farhan Ali <alifm@...ux.ibm.com>
> ---
>  arch/s390/pci/pci.c       | 74 +++++++++++++++++++++++++++++++++++++++
>  drivers/pci/host-bridge.c |  4 +--
>  2 files changed, 76 insertions(+), 2 deletions(-)
> 

@Bjorn, interesting new development. This actually fixes a current
linux-next breakage for us. In linux-next commit 06b77d5647a4 ("PCI:
Mark resources IORESOURCE_UNSET when outside bridge windows") from Ilpo
(added) breaks PCI on s390 because the check he added in
__pci_read_base() doesn't find the resource because the BAR address
does not match our MIO / address cookie addresses. With this patch
added however the pcibios_bus_to_resource() in __pci_read_base()
converts  the region correctly and then Ilpo's check works. I was
looking at this code quite intensely today wondering about Benjamin's
comment if we do need to check for containment rather than exact match.
I concluded that I think it is fine as is and was about to give my R-b
before Gerd had tracked down the linux-next issue and I found that this
fixes it.

So now I wonder if we might want to pick this one already to fix the
linux-next regression? Either way I'd like to add my:

Reviewed-by: Niklas Schnelle <schnelle@...ux.ibm.com>

Thanks,
Niklas