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 20:02:44 +0100
From:   KarimAllah Ahmed <karahmed@...zon.com>
To:     Alex Williamson <alex.williamson@...hat.com>,
        KarimAllah Ahmed <karahmed@...zon.de>
CC:     <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 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.

>
> 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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