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

From: Bjorn Helgaas <bhelgaas@google.com>
To: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Cc: Alexander Gordeev <agordeev@redhat.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"x86@kernel.org" <x86@kernel.org>,
	"linux-pci@vger.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>
Subject: Re: [PATCH v3 -tip x86/apic 1/2] PCI/MSI: Allocate as many multiple-MSIs as requested
Date: Wed, 5 Jun 2013 15:09:34 -0600	[thread overview]
Message-ID: <CAErSpo5hJYhWpBfN6GpyPv-ViaYNXyyLtw96xcN59CtQjfZi6w@mail.gmail.com> (raw)
In-Reply-To: <20130605205638.GA5816@breakpoint.cc>

On Wed, Jun 5, 2013 at 2:56 PM, Sebastian Andrzej Siewior
<sebastian@breakpoint.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