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

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Aug 06, 2020 at 04:14:55PM +1200, Mark Tomlinson wrote:
> 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>

Applied with collected reviews/acks to pci/enumeration for v5.10,
thanks!

> ---
> 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
>