linux-kernel - Re: [PATCH 3/4] AHCI: Conserve interrupts with pci_enable_msi_block

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Message-ID: <20130903161906.GC14221@dhcp-26-207.brq.redhat.com>
Date:	Tue, 3 Sep 2013 18:19:06 +0200
From:	Alexander Gordeev <agordeev@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	linux-pci@...r.kernel.org, linux-ide@...r.kernel.org,
	Ingo Molnar <mingo@...nel.org>, Joerg Roedel <joro@...tes.org>,
	Jan Beulich <JBeulich@...e.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH 3/4] AHCI: Conserve interrupts with
 pci_enable_msi_block_part() interface

On Tue, Sep 03, 2013 at 10:18:24AM -0400, Tejun Heo wrote:
> On Mon, Sep 02, 2013 at 11:00:28AM +0200, Alexander Gordeev wrote:
> > +	if (hpriv->flags & AHCI_HFLAG_NO_MSI)
> > +		goto intx;
> > +
> > +	rc = pci_enable_msi_block_part(pdev, n_ports, AHCI_MAX_PORTS);

We start with maximum possible number of ports AHCI_MAX_PORTS

> > +	if (!rc)
> > +		return AHCI_MAX_PORTS;

If we succeeded the device is indeed supports all AHCI_MAX_PORTS
and we report it.

> > +	if (rc < 0)
> > +		goto intx;

If pci_enable_msi_block_part() failed we should not make further
attempts and fallback to simple IRQ.

> > +	maxvec = rc;

The device supports a lesser of AHCI_MAX_PORTS, because the previous
pci_enable_msi_block_part() has not succeeded nor failed. Thus, rc
contains number of supported MSIs. In case of ICH this will be 16
rather than 32.

Actually, while I was writing this I realized this could be a number
of MSIs that could have been enabled this device, not the maximum
number of supported MSIs - these two may differ. I think MRSM should
be checked. But I will continue as if it always the same.

> > +	rc = pci_enable_msi_block_part(pdev, n_ports, maxvec);

Try pci_enable_msi_block_part() with the maximum number of supported MSIs.

> > +	if (!rc)
> > +		return maxvec;

If we succeeded report the number of enabled MSIs.

> > +	if (rc < 0)
> > +		goto intx;

If pci_enable_msi_block_part() failed we should not make further
attempts and fallback to simple IRQ.

> Why is the above fallback necessary?  The only other number which
> makes sense is roundup_pow_of_two(n_ports), right?  What does the
> above fallback logic buy us?

We must enable maximum possible number of MSIs - the one reported in
Multiple Message Capable register. Otherwise ICH device will fallback
to MRSM. IOW, if the result of roundup_pow_of_two(n_ports) is not what
in Multiple Message Capable register (i.e. as roundup_pow_of_two(6) vs 16)
ICH will enforce MRSM mode.

> Thanks.
> 
> -- 
> tejun

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