linux-kernel - Re: [PATCH V7 04/15] PCI: dwc: Move config space capability search API

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Date:   Fri, 24 May 2019 20:16:04 +0530
From:   Vidya Sagar <vidyas@...dia.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
CC:     <lorenzo.pieralisi@....com>, <robh+dt@...nel.org>,
        <mark.rutland@....com>, <thierry.reding@...il.com>,
        <jonathanh@...dia.com>, <kishon@...com>, <catalin.marinas@....com>,
        <will.deacon@....com>, <jingoohan1@...il.com>,
        <gustavo.pimentel@...opsys.com>, <mperttunen@...dia.com>,
        <linux-pci@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <linux-tegra@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <kthota@...dia.com>,
        <mmaddireddy@...dia.com>, <sagar.tv@...il.com>
Subject: Re: [PATCH V7 04/15] PCI: dwc: Move config space capability search
 API

On 5/22/2019 7:32 PM, Bjorn Helgaas wrote:
> On Wed, May 22, 2019 at 02:26:08PM +0530, Vidya Sagar wrote:
>> On 5/22/2019 2:47 AM, Bjorn Helgaas wrote:
>>> On Fri, May 17, 2019 at 06:08:35PM +0530, Vidya Sagar wrote:
>>>> Move PCIe config space capability search API to common DesignWare file
>>>> as this can be used by both host and ep mode codes.
> 
>>>>    .../pci/controller/dwc/pcie-designware-ep.c   | 37 +----------------
>>>>    drivers/pci/controller/dwc/pcie-designware.c  | 40 +++++++++++++++++++
>>>>    drivers/pci/controller/dwc/pcie-designware.h  |  2 +
>>>>    3 files changed, 44 insertions(+), 35 deletions(-)
> 
>>>> --- a/drivers/pci/controller/dwc/pcie-designware.c
>>>> +++ b/drivers/pci/controller/dwc/pcie-designware.c
>>>> @@ -14,6 +14,46 @@
>>>>    #include "pcie-designware.h"
>>>> +/*
>>>> + * These APIs are different from standard pci_find_*capability() APIs in the
>>>> + * sense that former can only be used post device enumeration as they require
>>>> + * 'struct pci_dev *' pointer whereas these APIs require 'struct dw_pcie *'
>>>> + * pointer and can be used before link up also.
>>>
>>> I think this comment is slightly misleading because it suggests the
>>> reason we need these DW interfaces is because we're doing something
>>> before a pci_dev pointer is available.
>>>
>>> But these DW interfaces are used on devices that will *never* have a
>>> pci_dev pointer because they are not PCI devices.  They're used on
>>> host controller devices, which have a PCIe link on the downstream
>>> side, but the host controller driver operates them using their
>>> upstream, non-PCI interfaces.  Logically, I think they would be
>>> considered parts of Root Complexes, not Root Ports.
>>>
>>> There's actually no reason why that upstream interface should look
>>> anything like PCI; it doesn't need to organize registers into
>>> capability lists at all.  It might be convenient for the hardware to
>>> do that and share things with a Root Port device, which *is* a PCI
>>> device, but it's not required.
>>>
>>> It also really has nothing to do with whether the link is up.  This
>>> code operates on hardware that is upstream from the link, so we can
>>> reach it regardless of the link.
>>
>> I added this comment after receiving a review comment to justify why
>> standard pci_find_*capability() APIs can't be used here. Hence added
>> this.  I understand your comment that DW interface need not have to
>> be a PCI device, but what is present in the hardware is effectively
>> a root port implementation and post enumeration, we get a 'struct
>> pci_dev' created for it, hence I thought it is fine to bring 'struct
>> pci_dev' into picture.
> 
> This code operates on the host controller.  It configures the bridge
> that leads *to* PCI devices.  Since that bridge is not a PCI device,
> the PCI specs don't say anything about how to program it.
> 
> The fact that the host controller programming interface happens to
> resemble the PCI programming interface is purely coincidental.
> 
>> Also, I agree that mention of 'link up' is unwarranted and could be
>> reworded in a better way.
>>
>> Do you suggest to remove this comment altogether or reword it s/and
>> can be used before link up also/and can be used before 'struct
>> pci_dev' is available/ ?
> 
> Maybe something like this?
> 
>      These interfaces resemble the pci_find_*capability() interfaces,
>      but these are for configuring host controllers, which are bridges
>      *to* PCI devices but are not PCI devices themselves.
Ok. Done.

>