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

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Date:	Wed, 5 Jun 2013 15:09:34 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Sebastian Andrzej Siewior <sebastian@...akpoint.cc>
Cc:	Alexander Gordeev <agordeev@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-pci@...r.kernel.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>
Subject: Re: [PATCH v3 -tip x86/apic 1/2] PCI/MSI: Allocate as many
 multiple-MSIs as requested

On Wed, Jun 5, 2013 at 2:56 PM, Sebastian Andrzej Siewior
<sebastian@...akpoint.cc> wrote:
> On Mon, May 13, 2013 at 11:05:48AM +0200, Alexander Gordeev wrote:
>> 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.
>
> The last paragraph makes me curious. The only place where 'multiple' is set is
> in do_setup_msi_irqs() and this uses the next power of two for it. And since a
> device is not enabled twice, it is not overridden.
> So it should be possible to compute 'multiple' out of 'nvec' but it saves
> cycles not do to so. I agree to keep 'multiple' but your argument does not
> seem to make sense.

Alexander had an example device that advertised 16 vectors, but the
driver knew that it could only generate 6.  That's a case where we
can't compute 'multiple' from 'nvec' (assuming the driver supplies
'nvec == 6').  If we just rounded up to compute 'multiple', I think
we'd compute 8 instead of 16.

> While nitpicking, 'nvec' might deserve a better comment than 'number of
> messages' since it holds the number of allocated interrupts. :)

I did change the name 'nvec' to 'nvec_used', which should help a bit.
But I agree that it's still somewhat confusing.

BTW, the patches actually in my tree are at
http://git.kernel.org/cgit/linux/kernel/git/helgaas/pci.git/log/?h=pci/alexander-msi
(I tweaked this name and some comments slightly).

Bjorn
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