linux-kernel - Re: [PATCH] pci: Do not read INTx PIN and LINE registers for virtual functions

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Date:   Wed, 17 Jan 2018 12:12:13 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     KarimAllah Ahmed <karahmed@...zon.com>
Cc:     KarimAllah Ahmed <karahmed@...zon.de>, <linux-pci@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        <linux-kernel@...r.kernel.org>,
        "Jan H . Schönherr" <jschoenh@...zon.de>
Subject: Re: [PATCH] pci: Do not read INTx PIN and LINE registers for
 virtual functions

On Wed, 17 Jan 2018 20:02:44 +0100
KarimAllah Ahmed <karahmed@...zon.com> wrote:

> On 01/17/2018 07:49 PM, Alex Williamson wrote:
> > On Wed, 17 Jan 2018 19:30:29 +0100
> > KarimAllah Ahmed <karahmed@...zon.de> wrote:
> >  
> >> ... since INTx is not supported by-spec for virtual functions.  
> > But the spec also states that VFs must implement the interrupt pin
> > register as read-only zero, so either this is redundant or it's a
> > workaround for VFs that aren't quite compliant?  Thanks,  
> 
> The end goal for me is just to NOT do the read across the PCI bus for no 
> good reason. We have devices with thousands of virtual functions and 
> this read is simply not useful in this case and can be optimized as I 
> did. So from a functionality point of view probably the patch does not 
> add any value as you mentioned, but it is really useful as a 
> micro-optimization.

Thanks for the clarification, it's helpful to understand the motivation
for a patch that otherwise seems unnecessary.

Alex

> >> Cc: Bjorn Helgaas <bhelgaas@...gle.com>
> >> Cc: linux-pci@...r.kernel.org
> >> Cc: linux-kernel@...r.kernel.org
> >> Signed-off-by: KarimAllah Ahmed <karahmed@...zon.de>
> >> Signed-off-by: Jan H. Schönherr <jschoenh@...zon.de>
> >> ---
> >>   drivers/pci/probe.c | 7 +++++++
> >>   1 file changed, 7 insertions(+)
> >>
> >> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> >> index 65099d0..61002fb 100644
> >> --- a/drivers/pci/probe.c
> >> +++ b/drivers/pci/probe.c
> >> @@ -1232,6 +1232,13 @@ static void pci_read_irq(struct pci_dev *dev)
> >>   {
> >>   	unsigned char irq;
> >>   
> >> +	/* Virtual functions do not have INTx support */
> >> +	if (dev->is_virtfn) {
> >> +		dev->pin = 0;
> >> +		dev->irq = 0;
> >> +		return;
> >> +	}
> >> +
> >>   	pci_read_config_byte(dev, PCI_INTERRUPT_PIN, &irq);
> >>   	dev->pin = irq;
> >>   	if (irq)  
> >  
> 
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