[PATCH 3.16 057/134] PCI: Ignore write combining when mapping I/O port space

Ben Hutchings
2017-08-18T13:13:20Z

From: Bjorn Helgaas <bhelgaas@google.com>

commit 3a92c319c44a7bcee9f48dff9d97d001943b54c6 upstream.

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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/pci/proc.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

--- a/drivers/pci/proc.c
+++ b/drivers/pci/proc.c
@@ -231,7 +231,7 @@ static int proc_bus_pci_mmap(struct file
{
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
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;