linux-kernel - Re: [PATCH v10 3/7] PCI: reserve bus range for SR-IOV device

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Date:	Fri, 6 Mar 2009 13:20:24 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	Yu Zhao <yu.zhao@...el.com>
Cc:	jbarnes@...tuousgeek.org, linux-pci@...r.kernel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10 3/7] PCI: reserve bus range for SR-IOV device

On Fri, Feb 20, 2009 at 02:54:44PM +0800, Yu Zhao wrote:
> +static inline void virtfn_bdf(struct pci_dev *dev, int id, u8 *busnr, u8 *devfn)
> +{
> +	u16 bdf;
> +
> +	bdf = (dev->bus->number << 8) + dev->devfn +
> +	      dev->sriov->offset + dev->sriov->stride * id;
> +	*busnr = bdf >> 8;
> +	*devfn = bdf & 0xff;
> +}

I find the interface here a bit clunky -- a function returning void
while having two OUT parameters.  How about this variation on the theme
(viewers are encouraged to come up with their own preferred
implementations and interfaces):

static inline __pure u16 virtfn_bdf(struct pci_dev *dev, int id)
{
	return (dev->bus->number << 8) + dev->devfn + dev->sriov->offset +
		dev->sriov->stride * id;
}

#define VIRT_BUS(dev, id)	(virtfn_bdf(dev, id) >> 8)
#define VIRT_DEVFN(dev, id)	(virtfn_bdf(dev, id) & 0xff)

We rely on GCC to do CSE and not actually invoke virtfn_bdf more than
once.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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