Commit 3a92c319 authored Jun 08, 2016 by Bjorn Helgaas

PCI: Ignore write combining when mapping I/O port space



PCI exposes files like /proc/bus/pci/00/00.0 in procfs.  These files
support operations like this:

  ioctl(fd, PCIIOC_MMAP_IS_IO);           # request I/O port space
  ioctl(fd, PCIIOC_WRITE_COMBINE, 1);     # request write-combining
  mmap(fd, ...)

Write combining is useful on PCI memory space, but I don't think it makes
sense on PCI I/O port space.

We *could* change proc_bus_pci_ioctl() to make it impossible to set
mmap_state == pci_mmap_io and write_combine at the same time, but that
would break the following sequence, which is currently legal:

  mmap(fd, ...)                           # default is I/O, non-combining
  ioctl(fd, PCIIOC_WRITE_COMBINE, 1);     # request write-combining
  ioctl(fd, PCIIOC_MMAP_IS_MEM);          # request memory space
  mmap(fd, ...)                           # get write-combining mapping

Ignore the write-combining flag when mapping I/O port space.

This patch should have no functional effect, based on this analysis of all
implementations of pci_mmap_page_range():

  - ia64 mips parisc sh unicore32 x86 do not support mapping of I/O port
    space at all.

  - arm cris microblaze mn10300 sparc xtensa support mapping of I/O port
    space, but ignore the write_combine argument to pci_mmap_page_range().

  - powerpc supports mapping of I/O port space and uses write_combine, and
    it disables write combining for I/O port space in
    __pci_mmap_set_pgprot().

This patch makes it possible to remove __pci_mmap_set_pgprot() from
powerpc, which simplifies that path.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

parent af8c34ce

drivers/pci/proc.c

+6 −3

Original line number	Diff line number	Diff line
		@@ -231,7 +231,7 @@ static int proc_bus_pci_mmap(struct file file, struct vm_area_struct vma)
		{
		struct pci_dev *dev = PDE_DATA(file_inode(file));
		struct pci_filp_private *fpriv = file->private_data;
		int i, ret;
		int i, ret, write_combine;

		if (!capable(CAP_SYS_RAWIO))
		return -EPERM;
		@@ -245,9 +245,12 @@ static int proc_bus_pci_mmap(struct file file, struct vm_area_struct vma)
		if (i >= PCI_ROM_RESOURCE)
		return -ENODEV;

		if (fpriv->mmap_state == pci_mmap_mem)
		write_combine = fpriv->write_combine;
		else
		write_combine = 0;
		ret = pci_mmap_page_range(dev, vma,
		fpriv->mmap_state,
		fpriv->write_combine);
		fpriv->mmap_state, write_combine);
		if (ret < 0)
		return ret;