Re: [PATCH v7 1/6] pci: Introduce pci_register_io_range() helper function.

[Liviu Dudau <Liviu.Dudau@arm.com>]

On Sun, Apr 06, 2014 at 10:49:53AM +0100, Benjamin Herrenschmidt wrote:
> On Fri, 2014-04-04 at 18:19 -0600, Bjorn Helgaas wrote:
> > > Introduce a pci_register_io_range() helper function that can be used
> > > by the architecture code to keep track of the I/O ranges described by the
> > > PCI bindings. If the PCI_IOBASE macro is not defined that signals
> > > lack of support for PCI and we return an error.
> > 
> > I don't quite see how you intend to use this, because this series doesn't
> > include any non-stub implementation of pci_register_io_range().
> > 
> > Is this anything like the ia64 strategy I mentioned above?  If so, it would
> > be really nice to unify some of this stuff.
> 
> We also use two different strategies on ppc32 and ppc64
> 
>  - On ppc32, inb/outb turn into an MMIO access to _IO_BASE + port
> 
> That _IO_BASE is a variable which is initialized to the ioremapped address
> of the IO space MMIO aperture of the first bridge we discover. Then port
> numbers are "fixed up" on all other bridges so that the addition _IO_BASE + port
> fits the ioremapped address of the IO space on that bridge. A bit messy... and breaks
> whenever drivers copy port numbers into variables of the wrong type such as shorts.
> 
>  - On ppc64, we have more virtual space, so instead we reserve a range
> of address space (fixed) for IO space, it's always the same. Bridges IO spaces
> are then mapped into that range, so we always have a positive offset from _IO_BASE
> which makes things a bit more robust and less "surprising" than ppc32. Additionally,
> the first 64k are reserved. They are only mapped if we see an ISA bridge (which some
> older machines have). Otherwise it's left unmapped, so crappy drivers trying to
> hard code x86 IO ports will blow up immediately which I deem better than silently
> whacking the wrong hardware. In addition, we have a mechanism we use on powernv to
> re-route accesses to that first 64k to the power8 built-in LPC bus which can
> have some legacy IOs on it such as a UART or a RTC.
> 
> Cheers,
> Ben.
> 

Hi Benjamin,

Thanks for the summary, is really useful as I was recently looking into code in that
area. One thing I was trying to understand is why ppc needs _IO_BASE at all rather
than using the generic PCI_IOBASE? 

Best regards,
Liviu
 

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