linux-kernel - Re: UIO: uio_mem does not handle devices above 4 GB address

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Date:	Thu, 02 Apr 2009 09:20:31 -0400
From:	"Steven A. Falco" <sfalco@...ris.com>
To:	linux-kernel@...r.kernel.org, hjk@...utronix.de,
	"Herrera-Bendezu, Luis" <lherrera@...ris.com>
CC:	"linuxppc-dev@...abs.org" <linuxppc-dev@...abs.org>
Subject: Re: UIO: uio_mem does not handle devices above 4 GB address

> On Wed, Apr 01, 2009 at 01:20:49PM -0400, Herrera-Bendezu, Luis wrote:
> >> I am working on a PPC440EPx board with some FPGAs located at
> >> addresses above 4 GB

Cc'ing the PPC list, since they have the most experience with this
CPU, and may have some useful insights.  The topic is "UIO on the
PPC440EPx - 36-bit physical addresses".

The PPC440EPx uses 36-bit addressing but is a 32-bit processor.
*All* peripherals are above the first 4 GB - that cannot be changed
if you use this CPU chip.

In general, the solution is to use "struct resource" to carry physical
addresses and sizes.  The basic type is:

#ifdef CONFIG_PHYS_ADDR_T_64BIT
typedef u64 phys_addr_t;
#else
typedef u32 phys_addr_t;
#endif

and for this processor, we use u64 rather than u32.

So, if we want to make UIO usable on this class of CPU, we might
replace:

struct uio_mem {
	unsigned long		addr;
	unsigned long		size;
	int			memtype;
	void __iomem		*internal_addr;
	struct uio_map		*map;
};

with:

struct uio_mem {
	phys_addr_t		addr;
	phys_addr_t		size;
	int			memtype;
	void __iomem		*internal_addr;
	struct uio_map		*map;
};

A few other changes would be needed.  We'd have to use something
other than

	return sprintf(buf, "0x%lx\n", mem->addr);

because the format would be incorrect for a u64.  Also, I believe
we would get warnings in uio_vma_fault().  However, the phys_addr
is really only applicable to UIO_MEM_PHYS, so perhaps we want a
union of phys_addr_t and unsigned long for addr and size?

Anyway, this is a real problem for the PPC440EPx, that board 
designers cannot work around.

	Steve

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