iomem and ioports

[anish singh]

On Wed, Feb 16, 2011 at 5:13 PM, Rajat Jain <rajatjain at juniper.net> 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:
>
> >
> > if (port > PIO_MASK) return NULL;
> >
> Checks that the ioport number is within the range of ioports.
>
> >
> > 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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