linux-kernel - Re: [PATCHv2] PCI: QDF2432 32 bit config space accessors

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Date:   Thu, 3 Nov 2016 19:49:12 -0400
From:   Sinan Kaya <okaya@...eaurora.org>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     cov@...eaurora.org, Tomasz Nowicki <tn@...ihalf.com>,
        will.deacon@....com, catalin.marinas@....com, rafael@...nel.org,
        Lorenzo.Pieralisi@....com, arnd@...db.de, hanjun.guo@...aro.org,
        jchandra@...adcom.com, dhdang@....com, ard.biesheuvel@...aro.org,
        robert.richter@...iumnetworks.com, mw@...ihalf.com,
        Liviu.Dudau@....com, ddaney@...iumnetworks.com,
        wangyijing@...wei.com, msalter@...hat.com,
        linux-pci@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linaro-acpi@...ts.linaro.org, jcm@...hat.com,
        andrea.gallo@...aro.org, jeremy.linton@....com,
        liudongdong3@...wei.com, gabriele.paoloni@...wei.com,
        jhugo@...eaurora.org, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2] PCI: QDF2432 32 bit config space accessors

On 11/3/2016 4:43 PM, Bjorn Helgaas wrote:
> On Thu, Nov 03, 2016 at 12:58:10PM -0400, Sinan Kaya wrote:
>>
>> On 11/3/2016 10:00 AM, Bjorn Helgaas wrote:
>>> On Wed, Nov 02, 2016 at 12:36:16PM -0400, Sinan Kaya wrote:
>>>> Hi Bjorn,
>>>>
>>>> On 11/2/2016 12:08 PM, Bjorn Helgaas wrote:
>>>>> On Tue, Nov 01, 2016 at 07:06:31AM -0600, cov@...eaurora.org wrote:
>>>>>> Hi Bjorn,
>>>>>>
>>>>>> On 2016-10-31 15:48, Bjorn Helgaas wrote:
>>>>>>> On Wed, Sep 21, 2016 at 06:38:05PM -0400, Christopher Covington wrote:
>>>>>>>> The Qualcomm Technologies QDF2432 SoC does not support accesses
>>>>>>>> smaller
>>>>>>>> than 32 bits to the PCI configuration space. Register the appropriate
>>>>>>>> quirk.
>>>>>>>>
>>>>>>>> Signed-off-by: Christopher Covington <cov@...eaurora.org>
>>>>>>>
>>>>>>> Hi Christopher,
>>>>>>>
>>>>>>> Can you rebase this against v4.9-rc1?  It no longer applies to my tree.
>>>>>>
>>>>>> I apologize for not being clearer. This patch depends on:
>>>>>>
>>>>>> PCI/ACPI: Extend pci_mcfg_lookup() responsibilities
>>>>>> PCI/ACPI: Check platform-specific ECAM quirks
>>>>>>
>>>>>> These patches from Tomasz Nowicki were previously in your pci/ecam-v6
>>>>>> branch, but that seems to have come and gone. How would you like to
>>>>>> proceed?
>>>>>
>>>>> Oh yes, that's right, I forgot that connection.  I'm afraid I kind of
>>>>> dropped the ball on that thread, so I went back and read through it
>>>>> again.
>>>>>
>>>>> I *think* the current state is:
>>>>>
>>>>>   - I'm OK with the first two patches that add the quirk
>>>>>     infrastructure.
>>>>>
>>>>>   - My issue with the last three patches that add ThunderX quirks is
>>>>>     that there's no generic description of the ECAM address space.
>>>>>
>>>>> So if I understand correctly, your Qualcomm patch depends only on the
>>>>> first two patches.
>>>>>
>>>>> Then the question is how the Qualcomm ECAM address space is described.
>>>>> Your quirk overrides the default pci_generic_ecam_ops with the
>>>>> &pci_32b_ops, but it doesn't touch the address space part, so I assume
>>>>> the bus ranges and corresponding address space in your MCFG is
>>>>> correct.  So far, so good.
>>>>
>>>> Qualcomm ECAM space includes both the root port and the endpoint address
>>>> space with a single contiguous 256 MB address space described in MCFG table.
>>>> There is no need to describe additional resources like PNP0C02.
>>>
>>> This is the crucial point I have failed to communicate clearly: the
>>> PNP0C02 resource is *always* required, even if the MCFG is correct.
>>>
>>
>> Interesting...
>>
>> It looks like there is a lot of lessons learnt here from history.
>>
>> I think this requirement is only true if your system DDR space and PCIe
>> space overlaps in the memory map. I understand that Intel systems allow
>> sharing of these two memory ranges. An OS could potentially reclaim this
>> address range.
>>
>> If there is no overlap and PCI is not enabled, there can't be any SW entity
>> to reclaim this space. 
> 
> No, this isn't really anything to do with DDR/PCIe overlaps.  This is
> just a fundamental part of the ACPI model: the firmware should
> communicate all address space usage to the OS either via ACPI or via
> standard self-describing mechanisms like PCI BARs.
> 
> You can argue that this isn't "necessary", but that's an assumption
> based on your knowledge of this particular system, and we don't want
> the OS to have to make that assumption.  For example, ACPI allows the
> hot-addition of new ACPI devices, and we may have to assign address
> space for them, and we don't want to collide with existing devices.

Thanks for the description. 

> 
> Bjorn
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> 


-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.