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

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Date:   Tue, 8 Nov 2016 16:11:50 +0100
From:   Christoph Hellwig <hch@....de>
To:     Hannes Reinecke <hare@...e.de>
Cc:     Christoph Hellwig <hch@....de>,
        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 Tue, Nov 08, 2016 at 04:08:51PM +0100, Hannes Reinecke wrote:
> 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.

> 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.

There should be no need for a cpumap, nor should there be any need
for a lookup.  Driver will need the vector index for some admin ops,
but it can store it in it's driver-private queue structure (e.g. take
a look at the cq_vector field in NVMe).  Drivers really should not need
this during I/O, but if for some weird reason they do that driver specific
field is trivially reachable through the hw_ctx which gets passed to
->queue_rq.