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

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Date:   Thu, 30 Jul 2020 18:06:15 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Mark Tomlinson <Mark.Tomlinson@...iedtelesis.co.nz>
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, Jul 30, 2020 at 10:58:03PM +0000, Mark Tomlinson wrote:
> 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).

You're not missing anything, I just screwed up.  What I was really
*hoping* to do was just put a bit in the pci_dev, but of course, these
functions don't have a pci_dev.  256 bits in the bus seems like a
little overkill though.  Maybe we just give up on the exact device and
warn only once per *bus* instead of once per device.

Bjorn