linux-kernel - Re: Need help on Linux PCIe

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Message-ID: <CAErSpo57gPPY73nCHbfGVynZRde0jj9_kjpR=FKwY-uTXzY2cw@mail.gmail.com>
Date:	Wed, 4 Dec 2013 08:11:04 -0700
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Jagan Teki <jagannadh.teki@...il.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 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."

> 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/*).

Bjorn
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