[PATCH v4] PCI: Reduce warnings on possible RW1C corruption

[Mark Tomlinson <mark.tomlinson@alliedtelesis.co.nz>]

For hardware that only supports 32-bit writes to PCI there is the
possibility of clearing RW1C (write-one-to-clear) bits. A rate-limited
messages was introduced by fb2659230120, but rate-limiting is not the
best choice here. Some devices may not show the warnings they should if
another device has just produced a bunch of warnings. Also, the number
of messages can be a nuisance on devices which are otherwise working
fine.

This patch changes the ratelimit to a single warning per bus. This
ensures no bus is 'starved' of emitting a warning and also that there
isn't a continuous stream of warnings. It would be preferable to have a
warning per device, but the pci_dev structure is not available here, and
a lookup from devfn would be far too slow.

Suggested-by: Bjorn Helgaas <helgaas@kernel.org>
Fixes: fb2659230120 ("PCI: Warn on possible RW1C corruption for sub-32 bit config writes")
Signed-off-by: Mark Tomlinson <mark.tomlinson@alliedtelesis.co.nz>
---
changes in v4:
 - Use bitfield rather than bool to save memory (was meant to be in v3).

 drivers/pci/access.c | 9 ++++++---
 include/linux/pci.h  | 1 +
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/pci/access.c b/drivers/pci/access.c
index 79c4a2ef269a..b452467fd133 100644
--- a/drivers/pci/access.c
+++ b/drivers/pci/access.c
@@ -160,9 +160,12 @@ int pci_generic_config_write32(struct pci_bus *bus, unsigned int devfn,
 	 * write happen to have any RW1C (write-one-to-clear) bits set, we
 	 * just inadvertently cleared something we shouldn't have.
 	 */
-	dev_warn_ratelimited(&bus->dev, "%d-byte config write to %04x:%02x:%02x.%d offset %#x may corrupt adjacent RW1C bits\n",
-			     size, pci_domain_nr(bus), bus->number,
-			     PCI_SLOT(devfn), PCI_FUNC(devfn), where);
+	if (!bus->unsafe_warn) {
+		dev_warn(&bus->dev, "%d-byte config write to %04x:%02x:%02x.%d offset %#x may corrupt adjacent RW1C bits\n",
+			 size, pci_domain_nr(bus), bus->number,
+			 PCI_SLOT(devfn), PCI_FUNC(devfn), where);
+		bus->unsafe_warn = 1;
+	}
 
 	mask = ~(((1 << (size * 8)) - 1) << ((where & 0x3) * 8));
 	tmp = readl(addr) & mask;
diff --git a/include/linux/pci.h b/include/linux/pci.h
index 34c1c4f45288..85211a787f8b 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -626,6 +626,7 @@ struct pci_bus {
 	struct bin_attribute	*legacy_io;	/* Legacy I/O for this bus */
 	struct bin_attribute	*legacy_mem;	/* Legacy mem */
 	unsigned int		is_added:1;
+	unsigned int		unsafe_warn:1;	/* warned about RW1C config write */
 };
 
 #define to_pci_bus(n)	container_of(n, struct pci_bus, dev)
-- 
2.28.0