linux-kernel - Re: [PATCH v2 01/15] PCI/MSI: Forward MSI-X vector enable error code in pci_alloc_irq_vectors

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Date:   Wed, 3 Jun 2020 09:48:26 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Piotr Stankiewicz <piotr.stankiewicz@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org
Cc:     Andy Shevchenko <andriy.shevchenko@...el.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Jian-Hong Pan <jian-hong@...lessm.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 01/15] PCI/MSI: Forward MSI-X vector enable error code
 in pci_alloc_irq_vectors_affinity()



On 2020-06-03 5:44 a.m., Piotr Stankiewicz wrote:
> When debugging an issue where I was asking the PCI machinery to enable a
> set of MSI-X vectors, without falling back on MSI, I ran across a
> behaviour which seems odd. The pci_alloc_irq_vectors_affinity() will
> always return -ENOSPC on failure, when allocating MSI-X vectors only,
> whereas with MSI fallback it will forward any error returned by
> __pci_enable_msi_range(). This is a confusing behaviour, so have the
> pci_alloc_irq_vectors_affinity() forward the error code from
> __pci_enable_msix_range() when appropriate.
> 
> Signed-off-by: Piotr Stankiewicz <piotr.stankiewicz@...el.com>
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@...el.com>
> ---
>  drivers/pci/msi.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
> index 6b43a5455c7a..443cc324b196 100644
> --- a/drivers/pci/msi.c
> +++ b/drivers/pci/msi.c
> @@ -1231,8 +1231,9 @@ int pci_alloc_irq_vectors_affinity(struct pci_dev *dev, unsigned int min_vecs,
>  		}
>  	}
>  
> -	if (msix_vecs == -ENOSPC)
> -		return -ENOSPC;
> +	if (msix_vecs == -ENOSPC ||
> +	    (flags & (PCI_IRQ_MSI | PCI_IRQ_MSIX)) == PCI_IRQ_MSIX)
> +		return msix_vecs;
>  	return msi_vecs;
>  }
>  EXPORT_SYMBOL(pci_alloc_irq_vectors_affinity);
> 

It occurs to me that we could clean this function up a bit more... I
don't see any need to have two variables for msi_vecs and msix_vecs and
then have a complicated bit of logic at the end to decide which to return.

Why not instead just have one variable which is set by
__pci_enable_msix_range(), then __pci_enable_msi_range(), then returned
if they both fail?

Logan