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

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Date:	Thu, 6 Jun 2013 10:30:20 +0200
From:	Alexander Gordeev <agordeev@...hat.com>
To:	Sebastian Andrzej Siewior <sebastian@...akpoint.cc>
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

On Wed, Jun 05, 2013 at 10:56:38PM +0200, Sebastian Andrzej Siewior 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.
> While nitpicking, 'nvec' might deserve a better comment than 'number of
> messages' since it holds the number of allocated interrupts. :)

Sebastian,

I re-read my comment few times and I admit it might be confusing. You are
right - 'multiple' is set by rounding up only. The part '...not necessarily
the closest power-of-two value...' implied an abstract PCI device rather than
the described code, but the wording is less than perfect, indeed. 

In fact, at the moment of writing I kept in mind a follow-up patch that could
help with aforementioned devices. That would be a new interface:

	int pci_enable_msi_block_partial(struct pci_dev *dev,
					 unsigned int nvec_use,
					 unsigned int nvec_init);

In this case 'nvec_use' would go to 'msi_desc::nvec_used' and 'nvec_init'
would translate to 'msi_desc::multiple' in case 'nvec_init' is not zero.
In case 'nvec_init' is zero, 'msi_desc::multiple' would be initialized
with the maximum possible value for the device (the way it is done now for
pci_enable_msi_block_auto() interface). So, for the AHCI device (Bjorn
mentioned) such a call would conserve on 10 of 16 vectors:

	pci_enable_msi_block_partial(pdev, 6, 0);

What I am not sure is whether we need to read out the maximum possible
number of vectors like pci_enable_msi_block_auto() does:

	int pci_enable_msi_block_partial(struct pci_dev *dev,
					 unsigned int nvec_use,
					 unsigned int nvec_init,
					 unsigned int *maxvec);

I can not think of any use of 'maxvec' with this interface, but the second
variant completes the whole picture about a device...

> Sebastian

-- 
Regards,
Alexander Gordeev
agordeev@...hat.com
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