From: Alexander Gordeev <agordeev@redhat.com>
To: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
linux-pci@vger.kernel.org,
Suresh Siddha <suresh.b.siddha@intel.com>,
Yinghai Lu <yinghai@kernel.org>, Joerg Roedel <joro@8bytes.org>,
Jan Beulich <JBeulich@suse.com>, Ingo Molnar <mingo@redhat.com>,
Bjorn Helgaas <bhelgaas@google.com>
Subject: Re: [PATCH v3 -tip x86/apic 1/2] PCI/MSI: Allocate as many multiple-MSIs as requested
Date: Thu, 6 Jun 2013 10:30:20 +0200 [thread overview]
Message-ID: <20130606083019.GA28569@dhcp-26-207.brq.redhat.com> (raw)
In-Reply-To: <20130605205638.GA5816@breakpoint.cc>
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@redhat.com
next prev parent reply other threads:[~2013-06-06 8:31 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-05-13 9:05 [PATCH v3 -tip x86/apic 0/2] PCI/MSI: Allocate as many multiple-MSIs as requested Alexander Gordeev
2013-05-13 9:05 ` [PATCH v3 -tip x86/apic 1/2] " Alexander Gordeev
2013-05-28 9:50 ` Ingo Molnar
2013-06-05 20:56 ` Sebastian Andrzej Siewior
2013-06-05 21:09 ` Bjorn Helgaas
2013-06-05 21:28 ` Sebastian Andrzej Siewior
2013-06-06 8:30 ` Alexander Gordeev [this message]
2013-06-06 19:51 ` Sebastian Andrzej Siewior
2013-05-13 9:06 ` [PATCH v3 -tip x86/apic 2/2] x86/MSI: Conserve interrupt resources when using multiple-MSIs Alexander Gordeev
2013-06-05 20:08 ` Sebastian Andrzej Siewior
2013-05-28 21:51 [PATCH v3 -tip x86/apic 1/2] PCI/MSI: Allocate as many multiple-MSIs as requested Bjorn Helgaas
2013-05-29 8:36 ` Alexander Gordeev
2013-05-29 20:58 ` Bjorn Helgaas
2013-06-03 20:46 ` Bjorn Helgaas
2013-06-04 13:14 ` Alexander Gordeev
2013-06-05 17:18 ` Bjorn Helgaas
2013-06-05 18:33 ` Konrad Rzeszutek Wilk
2013-06-05 18:35 ` Bjorn Helgaas
2013-06-20 12:51 ` Joerg Roedel
2013-06-25 17:34 ` Bjorn Helgaas
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