linux-kernel - Re: [PATCH v3 -tip x86/apic 1/2] PCI/MSI: Allocate as many multiple-MSIs as requested

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Date:	Tue, 28 May 2013 11:50:52 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Alexander Gordeev <agordeev@...hat.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	linux-pci@...r.kernel.org,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Yinghai Lu <yinghai@...nel.org>,
	Joerg Roedel <joro@...tes.org>,
	Jan Beulich <JBeulich@...e.com>,
	Ingo Molnar <mingo@...hat.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v3 -tip x86/apic 1/2] PCI/MSI: Allocate as many
 multiple-MSIs as requested


* Alexander Gordeev <agordeev@...hat.com> wrote:

> When multiple MSIs are enabled with pci_enable_msi_block(), the
> requested number of interrupts 'nvec' is rounded up to the nearest
> power-of-two value. The result is then used for setting up the
> number of MSI messages in the PCI device and allocation of
> interrupt resources in the operating system (i.e. vector numbers).
> Thus, in cases when a device driver requests some number of MSIs
> and this number is not a power-of-two value, the extra operating
> system resources (allocated as the result of rounding) are wasted.
> 
> This fix introduces 'msi_desc::nvec' field to address the above
> issue. When non-zero, it will report the actual number of MSIs the
> device will send, as requested by the device driver. This value
> should be used by architectures to properly set up and tear down
> associated interrupt resources.
> 
> Note, although the existing 'msi_desc::multiple' field might seem
> redundant, in fact in does not. In general case the number of MSIs a
> PCI device is initialized with is not necessarily the closest power-
> of-two value of the number of MSIs the device will send. Thus, in
> theory it would not be always possible to derive the former from the
> latter and we need to keep them both, to stress this corner case.
> Besides, since 'msi_desc::multiple' is a bitfield, throwing it out
> would not save us any space.
> 
> Signed-off-by: Alexander Gordeev <agordeev@...hat.com>

Would be nice to have an Acked-by from Bjorn for this patch.

Thanks,

	Ingo
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