linux-kernel - RE: Purpose of pci_remap

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Date:	Tue, 12 Jul 2016 08:40:52 +0000
From:	Bharat Kumar Gogada <bharat.kumar.gogada@...inx.com>
To:	Arnd Bergmann <arnd@...db.de>
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

> 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.
>
Thanks Bergmann


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