linux-kernel - Re: Need help on Linux PCIe

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Message-ID: <CAD6G_RSvrHGa=OEBgoD_iM68eOzb_2Dq9FneuNjSX_+iTj0_zA@mail.gmail.com>
Date:	Thu, 5 Dec 2013 12:00:48 +0530
From:	Jagan Teki <jagannadh.teki@...il.com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Need help on Linux PCIe

On Wed, Dec 4, 2013 at 11:35 PM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
> On Wed, Dec 4, 2013 at 10:00 AM, Jagan Teki <jagannadh.teki@...il.com> wrote:
>> On Wed, Dec 4, 2013 at 8:41 PM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
>>> On Tue, Dec 3, 2013 at 11:20 PM, Jagan Teki <jagannadh.teki@...il.com> wrote:
>>>> Thanks for your quick response.
>>>> Please find my comments below.
>>>>
>>>> On Tue, Dec 3, 2013 at 11:09 PM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
>>>>> On Tue, Dec 3, 2013 at 4:24 AM, Jagan Teki <jagannadh.teki@...il.com> wrote:
>>>>>> Hi,
>>>>>>
>>>>>> I have few question on Linux PCIe subsystem, I am trying to understand
>>>>>> the PCIe on ARM platform.
>>>>>> 1. Compared to PCI, PCIe have an extra port functionalists/services
>>>>>> which is implemented drivers/pci/pcie/* is it true?
>>>>>
>>>>> Yes.
>>>>>
>>>>>> 2. PCIe root complex is same as Host controller drivers in linux drivers/host/*
>>>>>
>>>>> Yes.
>>>>>
>>>>>> 3. As individual endpoint drivers are registered to pci_core as
>>>>>> pci_driver_register, then what is the common call for registering
>>>>>> individual HC driver to pci-core?
>>>>>
>>>>> The host controller-PCI core interface is not as mature as the
>>>>> pci_register_driver() interface.  The basic interface is
>>>>> pci_scan_root_bus().  If you skim through the drivers in
>>>>> drivers/pci/host/* and drivers/acpi/pci_root.c, the interface to the
>>>>> PCI core will be fairly obvious.  And you'll learn what the existing
>>>>> practices are in case you need to add or modify something.
>>>>
>>>> OK.
>>>>
>>>> I understand the flow as below - please correct if am wrong.
>>>>
>>>> From low level (hw) - HC driver has a platform registration using
>>>> platform_driver_register() to lower layer
>>>> and then pci_scan_root_bus() --> pci_common_init_dev() registration to
>>>> upper layer as PCI - BIOS and then ends.
>>>
>>> Yes.  Sometime HC drivers use platform_driver_register(); other use
>>> something else depending on how the HC device is enumerated.  For
>>> example, drivers/acpi/pci_root.c uses something else to deal with host
>>> bridges in the ACPI namespace.
>>>
>>>> From upper level (app) - each endpoint driver has
>>>> pci_driver_register() call to PCI Core for lower level
>>>
>>> Yes.
>>>
>>>> and then the upper level registration is based on endpoint().
>>>
>>> I don't know what you mean here (I don't know of a function named
>>> "endpoint()").  But the driver model matches drivers to PCI functions
>>> based on vendor and device IDs.  A Linux "pci_dev" is what the PCI
>>> specs refer to as a "function."
>> Sorry it's typo - added ()
>>
>>>
>>>> What is the connection here for PCI-BIOS and PCI-Core here, does these
>>>> are two different entities means there is no common call for these?
>>>> I see for ARM - "arch/arm/kernel/bios32.c" is PCI-BIOS is it correct?
>>>> does we have separate BIOS codes for architectures?
>>>
>>> The "pcibios_*" functions are architecture-specific things called by
>>> the generic PCI core.  Generally, things specified by the PCI specs
>>> are architecture-independent and should be in the PCI core
>>> (drivers/pci/*).
>>
>> I have some good information to discuss from this thread.
>> Can you please verify this Linux PCIe subsystem stack - comment
>> whether my understanding is correct/not.
>> (I just draw this based on driver calls flow - to accommodate with in
>> the Linux cores)
>> http://jagannadhteki.blog.com/2013/12/04/linux-pcie-subsystem/
>
> Yes, that makes sense.  I wouldn't label the PCIBIOS - PCI core link
> as "pci_bus_add_device()"; pci_bus_add_device() is part of the PCI
> core's generic enumeration code and shouldn't be called by
> arch-specific code.  The link going from PCI core to PCIBIOS is the
> set of "pcibios_*()" functions.  Going from PCIBIOS to the PCI core,
> it's mostly just pci_scan_root_bus().
Yes - understand your point.
I made few changes accordingly.
http://jagannadhteki.blog.com/files/2013/12/Linux_PCIe_zynq.png

I am planning to document this subsystem into Documentation/PCI/*
with brief description of important blocks, any comments?

> I also probably wouldn't put in links between VFS and AER, HP, PME,
> and VC.  It's true that there are some sysfs files that influence the
> operation of those PCIe features, but mostly for debugging and
> administration.  They aren't something useful to ordinary user
> programs.

-- 
Thanks,
Jagan.
--------
Jagannadha Sutradharudu Teki,
E: jagannadh.teki@...il.com, P: +91-9676773388
Engineer - System Software Hacker
U-boot - SPI Custodian and Zynq APSOC
Ln: http://www.linkedin.com/in/jaganteki
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