linux-kernel - Re: [PATCH v2 2/2] PCI: Reduce warnings on possible RW1C corruption

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Date:   Fri, 31 Jul 2020 09:32:24 -0600
From:   Rob Herring <robh@...nel.org>
To:     Mark Tomlinson <mark.tomlinson@...iedtelesis.co.nz>
Cc:     Ray Jui <ray.jui@...adcom.com>, Bjorn Helgaas <helgaas@...nel.org>,
        Scott Branden <sbranden@...adcom.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        PCI <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] PCI: Reduce warnings on possible RW1C corruption

On Thu, Jul 30, 2020 at 9:40 PM Mark Tomlinson
<mark.tomlinson@...iedtelesis.co.nz> 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.

If we don't want to just warn when a 8 or 16 bit access occurs (I'm
not sure if 32-bit only accesses is possible or common. Seems like
PCI_COMMAND would always get written?), then a simple way to do this
is just move this out of line and do something like this where the bus
or device is created/registered:

if (bus->ops->write == pci_generic_config_write32)
    warn()

>
> Suggested-by: Bjorn Helgaas <helgaas@...nel.org>
> Fixes: fb2659230120 ("PCI: Warn on possible RW1C corruption for sub-32 bit config writes")
> Signed-off-by: Mark Tomlinson <mark.tomlinson@...iedtelesis.co.nz>
> ---
>  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..ab85cb7df9b6 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 = true;
> +       }
>
>         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..5b6ab593ae09 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -613,6 +613,7 @@ struct pci_bus {
>         unsigned char   primary;        /* Number of primary bridge */
>         unsigned char   max_bus_speed;  /* enum pci_bus_speed */
>         unsigned char   cur_bus_speed;  /* enum pci_bus_speed */
> +       bool            unsafe_warn;    /* warned about RW1C config write */

Make this a bitfield next to 'is_added'.

>  #ifdef CONFIG_PCI_DOMAINS_GENERIC
>         int             domain_nr;
>  #endif
> --
> 2.28.0
>