PCI: Create revision file in sysfs

Currently the revision isn't available via sysfs/libudev thus if one wants
to know the value one needs to read through the config file, which can be
quite time-consuming because it wakes/powers up the device.

There are at least two userspace components which could make use the new
file: libpciaccess and libdrm.  The former wakes up _every_ PCI device,
which can be observed via glxinfo when using Mesa 10.0+ drivers.  The
latter, in association with Mesa 13.0, can lead to 2-3 second delays while
starting firefox, thunderbird or chromium.

Link: https://bugs.freedesktop.org/show_bug.cgi?id=98502
Tested-by: Mauro Santos <registo.mailling@gmail.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch
CC: Greg KH <gregkh@linuxfoundation.org>

Documentation/ABI/testing/sysfs-bus-pci

@@ -294,3 +294,10 @@ Description:
 		a firmware bug to the system vendor.  Writing to this file
 		taints the kernel with TAINT_FIRMWARE_WORKAROUND, which
 		reduces the supportability of your system.
+
+What:		/sys/bus/pci/devices/.../revision
+Date:		November 2016
+Contact:	Emil Velikov <emil.l.velikov@gmail.com>
+Description:
+		This file contains the revision field of the the PCI device.
+		The value comes from device config space. The file is read only.

Documentation/filesystems/sysfs-pci.txt

@@ -17,6 +17,7 @@ that support it.  For example, a given bus might look like this:
      |   |-- resource0
      |   |-- resource1
      |   |-- resource2
+     |   |-- revision
      |   |-- rom
      |   |-- subsystem_device
      |   |-- subsystem_vendor
@@ -41,6 +42,7 @@ files, each with their own function.
        resource		   PCI resource host addresses (ascii, ro)
        resource0..N	   PCI resource N, if present (binary, mmap, rw[1])
        resource0_wc..N_wc  PCI WC map resource N, if prefetchable (binary, mmap)
+       revision		   PCI revision (ascii, ro)
        rom		   PCI ROM resource, if present (binary, ro)
        subsystem_device	   PCI subsystem device (ascii, ro)
        subsystem_vendor	   PCI subsystem vendor (ascii, ro)

drivers/pci/pci-sysfs.c

@@ -50,6 +50,7 @@ pci_config_attr(vendor, "0x%04x\n");
 pci_config_attr(device, "0x%04x\n");
 pci_config_attr(subsystem_vendor, "0x%04x\n");
 pci_config_attr(subsystem_device, "0x%04x\n");
+pci_config_attr(revision, "0x%02x\n");
 pci_config_attr(class, "0x%06x\n");
 pci_config_attr(irq, "%u\n");
 
@@ -568,6 +569,7 @@ static struct attribute *pci_dev_attrs[] = {
 	&dev_attr_device.attr,
 	&dev_attr_subsystem_vendor.attr,
 	&dev_attr_subsystem_device.attr,
+	&dev_attr_revision.attr,
 	&dev_attr_class.attr,
 	&dev_attr_irq.attr,
 	&dev_attr_local_cpus.attr,