linux-kernel - Re: [PATCH v9 1/4] pci:host: APM X-Gene PCIe host controller driver

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Date:	Mon, 22 Sep 2014 15:40:03 -0700
From:	Tanmay Inamdar <tinamdar@....com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Rob Landley <rob@...dley.net>,
	Liviu Dudau <liviu.dudau@....com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	linux-arm <linux-arm-kernel@...ts.infradead.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	patches <patches@....com>, Jon Masters <jcm@...hat.com>
Subject: Re: [PATCH v9 1/4] pci:host: APM X-Gene PCIe host controller driver

On Mon, Sep 22, 2014 at 3:09 PM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
> On Mon, Sep 22, 2014 at 3:33 PM, Tanmay Inamdar <tinamdar@....com> wrote:
>
>>>> +static void xgene_pcie_fixup_bridge(struct pci_dev *dev)
>>>> +{
>>>> +     int i;
>>>> +
>>>> +     /* Hide the PCI host BARs from the kernel as their content doesn't
>>>> +      * fit well in the resource management
>>>> +      */
>>>
>>> This needs a better explanation than "doesn't fit well."
>>>
>>> I *think* you're probably talking about something similar to the MVEBU
>>> devices mentioned here:
>>> http://lkml.kernel.org/r/CAErSpo56jB1Bf2JtYCGKXZBZqRF1jXFxGmeewPX_e6vSXueGyA@mail.gmail.com
>>>
>>> where the device can be configured as either an endpoint or a root port,
>>> and the endpoint BARs are still visible when configured as a root port.
>>>
>>
>> It is true that X-Gene PCIe port can be configured as EP, however the
>> the FIXUP is required not because of the BARs are still visible when
>> configured as EP in past. X-Gene PCIe port, when configured as RC,
>> uses BAR0-BAR1 of RC's configuration space as inbound BARs. Entire DDR
>> region is mapped in these BARs so that it is accessible for EP devices
>> for DMA. So if the fixup is not done during enumeration, whole
>> outbound memory resource space gets assigned to RC and nothing is left
>> EP devices.
>
> I'm not familiar with the concept of an "inbound BAR"; at least I'm
> not aware of anything like this in the PCI specs.  I think it might
> reduce confusion if we avoided calling them "BARs".  They happen to be
> at the same addresses where real BARs would be, but they certainly
> don't behave like real BARs.
>
> It sounds like this is basically a trivial IOMMU that determines which
> DMA accesses can reach main memory.
>

Yes. That is correct.

>>> In any event, I'd like a description of exactly what these BARs are and wha
>>> the problem is.
>>
>> Is it ok to put above description in comment to explain the fixup?
>>
>>> Presumably the BARs exist and were sized by the PCI core
>>> in __pci_read_base().  That will generate some log messages and possibly
>>> some warnings, depending on how the host bridge windows are set up.
>
> Instead of doing this in a fixup, can you change the PCI config
> accessors so that when accessing these inbound DMA mapping registers,
> they drop writes and return zeros for reads?  Then the PCI core won't
> think there are BARs there, and it won't mistakenly corrupt them when
> it tries to size them.
>

I suppose this can be done. I will fix this in next revision.

> Bjorn
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