linux-kernel - Re: [RFC 2/4] PCI, MSI: Optionally free legacy PCI IRQ when enabling MSI/MSI-X

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Date:	Tue, 19 May 2015 17:16:54 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Jiang Liu <jiang.liu@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Rafael J . Wysocki" <rjw@...ysocki.net>,
	Lv Zheng <lv.zheng@...el.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-pci@...r.kernel.org,
	linux-acpi@...r.kernel.org, "x86 @ kernel . org" <x86@...nel.org>
Subject: Re: [RFC 2/4] PCI, MSI: Optionally free legacy PCI IRQ when enabling
 MSI/MSI-X

On Tue, May 19, 2015 at 10:08:38AM -0500, Bjorn Helgaas wrote:
> [+cc Michael]
> 
> On Thu, May 07, 2015 at 11:12:52AM +0800, Jiang Liu wrote:
> > Once PCI MSI/MSI-X is enabled by the device driver, PCI device won't
> > make use of legacy PCI IRQ until PCI MSI/MSI-X is disabled again.
> > So optionally free legacy PCI IRQ when enabling MSI/MSI-X and reallocate
> > when disabling MSI/MSI-X.
> > 
> > Signed-off-by: Jiang Liu <jiang.liu@...ux.intel.com>
> > ---
> >  drivers/pci/msi.c |    6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
> > index c3e7dfcf9ff5..47cf72c669f0 100644
> > --- a/drivers/pci/msi.c
> > +++ b/drivers/pci/msi.c
> > @@ -686,6 +686,7 @@ static int msi_capability_init(struct pci_dev *dev, int nvec)
> >  	msi_set_enable(dev, 1);
> >  	dev->msi_enabled = 1;
> >  
> > +	pcibios_free_irq(dev);
> 
> >From the recent discussion about disabling MSI at shutdown, I seem to
> recall that turning off bus mastering also effectively disables MSI and
> causes at least some devices to revert to using INTx.
> 
> So with this patch, do we now have a problem in the following scenario?

Can't say, I didn't follow the discussion. But I'll address the
comment below:

>     pci_enable_msi(dev);	/* now frees INTx IRQ */
>     pci_clear_master(dev);
>     <device interrupts>		/* uses INTx since bus mastering disabled */

Not really, this does not use INTx. Disabling bus master shuts down MSI
but does not enable INTx, you effectively get no interrupts at all: MSI
disabled INTx and bus master disabled MSI.


> >  	dev->irq = entry->irq;
> >  	return 0;
> >  }
> > @@ -813,9 +814,10 @@ static int msix_capability_init(struct pci_dev *dev,
> >  	/* Set MSI-X enabled bits and unmask the function */
> >  	pci_intx_for_msi(dev, 0);
> >  	dev->msix_enabled = 1;
> > -
> >  	msix_clear_and_set_ctrl(dev, PCI_MSIX_FLAGS_MASKALL, 0);
> >  
> > +	pcibios_free_irq(dev);
> > +
> >  	return 0;
> >  
> >  out_avail:
> > @@ -930,6 +932,7 @@ void pci_msi_shutdown(struct pci_dev *dev)
> >  
> >  	/* Restore dev->irq to its default pin-assertion irq */
> >  	dev->irq = desc->msi_attrib.default_irq;
> > +	pcibios_alloc_irq(dev);
> >  }
> >  
> >  void pci_disable_msi(struct pci_dev *dev)
> > @@ -1030,6 +1033,7 @@ void pci_msix_shutdown(struct pci_dev *dev)
> >  	msix_clear_and_set_ctrl(dev, PCI_MSIX_FLAGS_ENABLE, 0);
> >  	pci_intx_for_msi(dev, 1);
> >  	dev->msix_enabled = 0;
> > +	pcibios_alloc_irq(dev);
> >  }
> >  
> >  void pci_disable_msix(struct pci_dev *dev)
> > -- 
> > 1.7.10.4
> > 
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