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SubjectRE: accessing on-card ram/rom


On Wed, 8 Nov 2000 hiren_mehta@agilent.com wrote:

> I looked at the IO-mapping.txt file. It says that
> on x86 architecture it should not make any difference.
> It also says that "on x86 it _is_ the same memory space. So
> on x86 it actually works to just dereference a pointer".

For bus_to_virt() to give a usable virtual address, such a virtual
mapping must exist and additionnally be part of the linear kernel
mapping. A PCI MMIO address is generally _not_ even mapped by default
by the kernel.

On the other hand, bus_to_virt() hasn't the proper semantic for your
problem, since it applies to main memory addresses as seen from the PCI
BUS, and you want an MMIO address usable by the CPU (=virtual).

Hmmm... ioremap() just create a virtual mapping for the CPU to access
the MMIO window of your PCI chip just fine.

Gérard.

> Any inputs on this ?
>
> Thanks and regards,
> -hiren
>
> > -----Original Message-----
> > From: Jeff Garzik [mailto:jgarzik@mandrakesoft.com]
> > Sent: Wednesday, November 08, 2000 2:53 PM
> > To: MEHTA,HIREN (A-SanJose,ex1)
> > Cc: 'linux-kernel@vger.kernel.org'
> > Subject: Re: accessing on-card ram/rom
> >
> >
> > "MEHTA,HIREN (A-SanJose,ex1)" wrote:
> > > I have a PCI card which has on-card ram/rom which gets mapped
> > > into pci address space and there is a separate base register
> > > for this memory. Now the question is : can I access this on-card
> > > memory by converting the pci base address into the virtual address
> > > using bus_to_virt and adding the required offset ? Or do I need
> > > to use ioremap function to map the physical address space starting
> > > from the pci base address into the kernel virtual address space ?
> > > Or is there any other interface to access the on-card memory ?
> > > Is it that bus_to_virt can be used only for the normal RAM ?
> >
> > Use ioremap.
> >
> > For more details, read linux/Documentation/IO-mapping.txt.
> >
> > Jeff
> >
> >
> > --
> > Jeff Garzik | "When I do this, my computer freezes."
> > Building 1024 | -user
> > MandrakeSoft | "Don't do that."
> > | -level 1
> >
> -
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