linux-kernel - Re: [PATCH 2/3] PCI: iproc: Stop using generic config read/write functions

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Date:   Thu, 30 Jul 2020 22:58:03 +0000
From:   Mark Tomlinson <Mark.Tomlinson@...iedtelesis.co.nz>
To:     "helgaas@...nel.org" <helgaas@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "lorenzo.pieralisi@....com" <lorenzo.pieralisi@....com>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "rjui@...adcom.com" <rjui@...adcom.com>,
        "robh@...nel.org" <robh@...nel.org>,
        "sbranden@...adcom.com" <sbranden@...adcom.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>
Subject: Re: [PATCH 2/3] PCI: iproc: Stop using generic config read/write
 functions

On Thu, 2020-07-30 at 11:09 -0500, Bjorn Helgaas wrote:
> I think it would be better to have a warning once per device, so if
> XYZ device has a problem and we look at the dmesg log, we would find a
> single message for device XYZ as a hint.  Would that reduce the
> nuisance level enough?

We would be OK with that.

> So I think I did it wrong in fb2659230120 ("PCI: Warn on possible RW1C
> corruption for sub-32 bit config writes").  Ratelimiting is the wrong
> concept because what we want is a single warning per device, not a
> limit on the similar messages for *all* devices, maybe something like
> this:
> 
> diff --git a/drivers/pci/access.c b/drivers/pci/access.c
> index 79c4a2ef269a..e5f956b7e3b7 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",
> +	if (!(bus->unsafe_warn & (1 << devfn))) {
> +		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 << devfn;
> +	}

As I understand it, devfn is an 8-bit value with five bits of device
and three bits of function. So (1 << devfn) is not going to fit in an
8-bit mask. Am I missing something here? (I do admit that my PCI
knowledge is not great).