On Wed, Feb 16, 2011 at 5:13 PM, Rajat Jain [off-list ref] wrote:

Hi,

This code says that there are 0x10000 possible ioports i.e. in the range (0
- 0xFFFF). To map these into iomemory, simply an address = (0x10000 + ioport
num) is used. Thus this code:

quoted

if (port > PIO_MASK) return NULL;

Checks that the ioport number is within the range of ioports.

quoted

return (void __iomem *) (unsigned long) (port + PIO_OFFSET);

And if yes, offsets it by 0x10000 and returns the resulting address that
shall be used as iomemory.

Some places i have seen that directly IO port is typecasted to  (__iomem * )
and used as it is i.e. returned value is given to ioread and iowrite
functions.Kindly explain why in some cases we dont add offset and in some
cases we add??
Is it because in some board we need to memory map the port number and then
read/write and in some boards we don't need to do that as we can directly
access the port using arch specific functions??

Thanks,

Rajat Jain

________________________________________
From: kernelnewbies-bounces at kernelnewbies.org [mailto:
kernelnewbies-bounces at kernelnewbies.org] On Behalf Of prabhu
Sent: Wednesday, February 16, 2011 1:15 PM
To: kernelnewbies
Subject: iomem and ioports

HI All,

I started to understand output of /proc/oiports and /proc/iomem. I confused
to relate these two's output.

Below is the kernel source for Mapping of io-port to io-mem. Could anyone
please explain below code.

/* We encode the physical PIO addresses (0-0xffff) into the
                                           (0-
 * pointer by offsetting them with a constant (0x10000) and
 * assuming that all the low addresses are always PIO. That means
                                                            means
 * we can do some sanity checks on the low bits, and don't
 * need to just take things for granted.
 */
#define PIO_OFFSET              0x10000UL
#define PIO_MASK                0x0ffffUL
#define PIO_RESERVED            0x40000UL
void __iomem *ioport_map(unsigned long port, unsigned int nr) {
     __iomem
           if (port > PIO_MASK) return NULL;
           return (void __iomem *) (unsigned long) (port + PIO_OFFSET);
                        __iomem
}

Thanks,
Prabhu

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