linux-kernel - Re: [PATCH] drivers/char/mem.c: Add /dev/ioports, supporting 16-bit and 32-bit ports

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Date:	Fri, 30 May 2014 13:32:50 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	josh@...htriplett.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-api@...r.kernel.org
Subject: Re: [PATCH] drivers/char/mem.c: Add /dev/ioports, supporting 16-bit and 32-bit ports

On Thursday 29 May 2014 06:38:35 H. Peter Anvin wrote:
> On 05/29/2014 02:26 AM, Arnd Bergmann wrote:
> > On Wednesday 28 May 2014 14:41:52 H. Peter Anvin wrote:
> >> On 05/19/2014 05:36 AM, Arnd Bergmann wrote:
> >>>
> >>> My feeling is that all devices we can think of fall into at least one
> >>> of these categories:
> >>>
> >>> * legacy PC stuff that needs only byte access
> >>> * PCI devices that can be accessed through sysfs
> >>> * devices on x86 that can be accessed using iopl
> >>>
> >>
> >> I don't believe PCI I/O space devices can be accessed through sysfs, but
> >> perhaps I'm wrong?  (mmapping I/O space is not portable.)
> > 
> > The interface is there, both a read/write and mmap on the resource
> > bin_attribute. But it seems you're right, neither of them is implemented
> > on all architectures.
> > 
> > Only powerpc, microblaze, alpha, sparc and xtensa allow users to mmap
> > I/O space, even though a lot of others could. The read-write interface
> > is only defined for alpha, ia64, microblaze and powerpc.
> > 
> 
> And how is that read/write interface defined?  Does it have the same
> silly handling of data sizes?

In architecture specific code, e.g. for powerpc:

int pci_legacy_read(struct pci_bus *bus, loff_t port, u32 *val, size_t size)
{
        unsigned long offset;
        struct pci_controller *hose = pci_bus_to_host(bus);
        struct resource *rp = &hose->io_resource;
        void __iomem *addr;

        /* Check if port can be supported by that bus. We only check
         * the ranges of the PHB though, not the bus itself as the rules
         * for forwarding legacy cycles down bridges are not our problem
         * here. So if the host bridge supports it, we do it.
         */
        offset = (unsigned long)hose->io_base_virt - _IO_BASE;
        offset += port;

        if (!(rp->flags & IORESOURCE_IO))
                return -ENXIO;
        if (offset < rp->start || (offset + size) > rp->end)
                return -ENXIO;
        addr = hose->io_base_virt + port;

        switch(size) {
        case 1:
                *((u8 *)val) = in_8(addr);
                return 1;
        case 2:
                if (port & 1)
                        return -EINVAL;
                *((u16 *)val) = in_le16(addr);
                return 2;
        case 4:
                if (port & 3)
                        return -EINVAL;
                *((u32 *)val) = in_le32(addr);
                return 4;
        }
        return -EINVAL;
}

The common code already enforces size to be 1, 2 or 4.

	Arnd
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