linux-kernel - Re: Purpose of pci_remap

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Date:	Tue, 12 Jul 2016 10:31:59 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Bharat Kumar Gogada <bharat.kumar.gogada@...inx.com>
Cc:	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	"Liviu.Dudau@....com" <Liviu.Dudau@....com>,
	nofooter <nofooter@...inx.com>,
	"thomas.petazzoni@...e-electrons.com" 
	<thomas.petazzoni@...e-electrons.com>
Subject: Re: Purpose of pci_remap_iospace

On Tuesday, July 12, 2016 6:57:10 AM CEST Bharat Kumar Gogada wrote:
> Hi,
> 
> I have a query.
> 
> Can any once explain the purpose of pci_remap_iospace function in root port driver.
> 
> What is its dependency with architecture ?
> 
> Here is my understanding, the above API takes PCIe IO resource and its to be mapped CPU address from
> ranges property and remaps into virtual address space.
> 
> So my question is who uses this virtual addresses ?

The inb()/outb() functions declared in asm/io.h

> When End Point requests for IO BARs doesn't it get
> from the above resource range (first parameter of API) and
> do ioremap to access this region ?

Device drivers generally do not ioremap() the I/O BARs but they
use inb()/outb() directly. They can also call pci_iomap() and
do ioread8()/iowrite8() on the pointer returned from that function,
but generally the call to pci_iomap() then returns a pointer into
the virtual address that is already mapped.
 
> But why root complex driver is mapping this address region ?

The PCI core does not know that the I/O space is memory mapped.
On x86 and a few others, I/O space is not memory mapped but requires
the use of special CPU instructions.

	Arnd