linux-kernel - Re: [RFC PATCH] pci: add pci_irq_get_affinity

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Date:   Tue, 8 Nov 2016 16:08:51 +0100
From:   Hannes Reinecke <hare@...e.de>
To:     Christoph Hellwig <hch@....de>
Cc:     Thomas Gleixner <tglx@...utronix.de>, Jens Axboe <axboe@...nel.dk>,
        linux-block@...r.kernel.org, linux-pci@...r.kernel.org,
        Linux Kernel Mailinglist <linux-kernel@...r.kernel.org>,
        Hannes Reinecke <hare@...e.com>
Subject: Re: [RFC PATCH] pci: add pci_irq_get_affinity_vector()

On 11/08/2016 03:56 PM, Christoph Hellwig wrote:
> On Tue, Nov 08, 2016 at 08:47:21AM +0100, Hannes Reinecke wrote:
>> Add a reverse-mapping function to return the interrupt vector for
>> any CPU if interrupt affinity is enabled.
>
> What's the use case of it?
>
> Also as-is this won't work due to the non-affinity vectors that
> have the affinity set to all cpus.  It will get even worse if we have
> to support things like virtio_net that have multiple interrupts per
> CPU due to the send and receive virtqueues.
>
The use-case here is that one needs to feed the MSI-X index into the 
driver command structure. While we can extract that number trivially 
with scsi-mq, but for scsi-sq we don't have such means.

So if we start assigning interrupt affinity per default we need to 
figure out the msi-x index from a given SCSI command.
Currently most of these drivers keep an internal CPU map which I'd love 
to get rid of.
Hence this patch.

And before you complain: Yes, this patch is wrong; it returns the vector 
and not the index (which is what I'm after).
I found that on my test machine :-(

The main impetus of this RFC is to figure out if such a function would 
have a chance of getting upstream, or if I have to continue use cpumaps 
in the drivers.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
hare@...e.de			               +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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HRB 21284 (AG Nürnberg)