linux-kernel - Re: [PATCH] pci: Export pci device msi table via sysfs

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Date:	Thu, 21 Apr 2011 13:10:41 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	Neil Horman <nhorman@...driver.com>
Cc:	linux-kernel@...r.kernel.org,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	linux-pci@...r.kernel.org
Subject: Re: [PATCH] pci: Export pci device msi table via sysfs

On Thu, Apr 21, 2011 at 02:57:47PM -0400, Neil Horman wrote:
> b) msi_table - populated only if msi(x) is enabled, it lists the irqs allocated
> to the pci device

You're not the first one to try this ... the problem is, you can easily
overflow a single 4k page.  A device can have up to 2k MSI-X entries,
and we might take up to 5 bytes for each one, so we'd need a 10k buffer.

> +#ifdef CONFIG_PCI_MSI
> +static ssize_t msi_list_show(struct device *dev,
> +			struct device_attribute *attr, char *buf)
> +{
> +	struct pci_dev *pdev = to_pci_dev(dev);
> +	struct msi_desc *entry;
> +	int first, last;
> +	ssize_t count = 0;
> +
> +	if (!(pdev->msi_enabled || pdev->msix_enabled))
> +		return 0;
> +
> +	list_for_each_entry(entry, &pdev->msi_list, list)
> +		count += sprintf(&buf[count], "%d ", entry->irq);
> +
> +	return count;
> +}
> +#endif

The fundamental problem is that the way Linux uses MSI-X is completely
bollocks.  I've got a few hours to myself on a plane coming up in six
weeks, and I hope to rewrite it then (I've already written my talk, so
what else am I going to do?  :-)

-- 
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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