[PATCH V2 1/3] PCI/MSI: preference to returning -ENOSPC from pci_alloc_irq_vectors_affinity

[helgaas@kernel.org (Bjorn Helgaas)]

On Sat, Dec 29, 2018@11:26:48AM +0800, Ming Lei wrote:
> The API of pci_alloc_irq_vectors_affinity() requires to return -ENOSPC
> if leass than @min_vecs interrupt vectors are available for @dev.

s/leass/fewer/

> However, this way may be changed by falling back to
> __pci_enable_msi_range(), for example, if the device isn't capable of
> MSI, __pci_enable_msi_range() will return -EINVAL, and finally it is
> returned to users of pci_alloc_irq_vectors_affinity() even though
> there are quite MSIX vectors available. This way violates the interface.

I *think* the above means:

  If a device supports MSI-X but not MSI and a caller requests
  @min_vecs that can't be satisfied by MSI-X, we previously returned
  -EINVAL (from the failed attempt to enable MSI), not -ENOSPC.

and I agree that this doesn't match the documented API.

> Users of pci_alloc_irq_vectors_affinity() may try to reduce irq
> vectors and allocate vectors again in case that -ENOSPC is returned, such
> as NVMe, so we need to respect the current interface and give preference to
> -ENOSPC.

I thought the whole point of the (min_vecs, max_vecs) tuple was to
avoid this sort of "reduce and try again" iteration in the callers.

> Cc: Jens Axboe <axboe at fb.com>,
> Cc: Keith Busch <keith.busch at intel.com>,
> Cc: linux-pci at vger.kernel.org,
> Cc: Bjorn Helgaas <bhelgaas at google.com>,
> Signed-off-by: Ming Lei <ming.lei at redhat.com>
> ---
>  drivers/pci/msi.c | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
> index 7a1c8a09efa5..91b4f03fee91 100644
> --- a/drivers/pci/msi.c
> +++ b/drivers/pci/msi.c
> @@ -1168,7 +1168,8 @@ int pci_alloc_irq_vectors_affinity(struct pci_dev *dev, unsigned int min_vecs,
>  				   const struct irq_affinity *affd)
>  {
>  	static const struct irq_affinity msi_default_affd;
> -	int vecs = -ENOSPC;
> +	int msix_vecs = -ENOSPC;
> +	int msi_vecs = -ENOSPC;
>  
>  	if (flags & PCI_IRQ_AFFINITY) {
>  		if (!affd)
> @@ -1179,16 +1180,17 @@ int pci_alloc_irq_vectors_affinity(struct pci_dev *dev, unsigned int min_vecs,
>  	}
>  
>  	if (flags & PCI_IRQ_MSIX) {
> -		vecs = __pci_enable_msix_range(dev, NULL, min_vecs, max_vecs,
> -				affd);
> -		if (vecs > 0)
> -			return vecs;
> +		msix_vecs = __pci_enable_msix_range(dev, NULL, min_vecs,
> +						    max_vecs, affd);
> +		if (msix_vecs > 0)
> +			return msix_vecs;
>  	}
>  
>  	if (flags & PCI_IRQ_MSI) {
> -		vecs = __pci_enable_msi_range(dev, min_vecs, max_vecs, affd);
> -		if (vecs > 0)
> -			return vecs;
> +		msi_vecs = __pci_enable_msi_range(dev, min_vecs, max_vecs,
> +						  affd);
> +		if (msi_vecs > 0)
> +			return msi_vecs;
>  	}
>  
>  	/* use legacy irq if allowed */
> @@ -1199,7 +1201,7 @@ int pci_alloc_irq_vectors_affinity(struct pci_dev *dev, unsigned int min_vecs,
>  		}
>  	}
>  
> -	return vecs;
> +	return msix_vecs == -ENOSPC ? msix_vecs : msi_vecs;

If you know you want to return -ENOSPC, just return that, not a
variable that happens to contain it, i.e.,

  if (msix_vecs == -ENOSPC)
    return -ENOSPC;
  return msi_vecs;

>  }
>  EXPORT_SYMBOL(pci_alloc_irq_vectors_affinity);
>  
> -- 
> 2.9.5
> 
> 
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