[PATCH] PCI:MSI Return -ENOSPC when requested vectors is not enough

[Bjorn Helgaas]

On Thu, Dec 01, 2016 at 10:15:04AM +0800, Dennis Chen wrote:
> The __pci_enable_msi_range() should return -ENOSPC instead of -EINVAL
> when the device doesn't have enough vectors as required, just as the 
> MSI-X vector allocator does in __pci_enable_msix_range(). Otherwise, 
> some drivers depending on that return value will probably fallback to
> the legacy interrupt directly, for example, in commit 17a51f12cfbd2814
> ("ahci: only try to use multi-MSI mode if there is more than 1 port"), the
> ahci driver will fallback to single MSI mode only when the return value
> is -ENOSPC in case of required vectors is not enough, else the driver will
> use legacy interrupt which has been observed on a x86 box with 6-port SATA
> controller.

Unless Christoph objects, I'll apply this, but I don't understand the
situation with 17a51f12cfbd.  That commit doesn't check for EINVAL or
ENOSPC so I don't know what the connection with this patch is.

I know Christoph said he changed something in ahci to treat all errors
the same, but I don't know where that is, either.

If there's a revision of Linus' tree that is broken, please give the
details so I can at least describe which versions are broken, when it
got fixed by Christoph, and figure out whether we need stable
backports or anything.

Bjorn

> With this patch, when a MSI-capable device doesn't have enough MSI
> vectors as requested, it will fallback to single MSI mode while not
> legacy interrupt.
> 
> Signed-off-by: Dennis Chen <dennis.chen at arm.com>
> Cc: Tejun Heo <tj at kernel.org>
> Cc: Christoph Hellwig <hch at lst.de>
> Cc: Tom Long Nguyen <tom.l.nguyen at intel.com>
> Cc: Bjorn Helgaas <bhelgaas at google.com>
> Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
> Cc: Marc Zyngier <marc.zyngier at arm.com>
> Cc: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>
> Cc: Steve Capper <steve.capper at arm.com>
> Cc: linux-ide at vger.kernel.org
> Cc: linux-arm-kernel at lists.infradead.org
> ---
>  drivers/pci/msi.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
> index ad70507..da37113 100644
> --- a/drivers/pci/msi.c
> +++ b/drivers/pci/msi.c
> @@ -1084,7 +1084,7 @@ static int __pci_enable_msi_range(struct pci_dev *dev, int minvec, int maxvec,
>  	if (nvec < 0)
>  		return nvec;
>  	if (nvec < minvec)
> -		return -EINVAL;
> +		return -ENOSPC;
>  
>  	if (nvec > maxvec)
>  		nvec = maxvec;
> -- 
> 2.7.4
> 
> 
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