linux-kernel - Re: [PATCH 14/14] PCI/P2PDMA: Introduce pci_p2pdma_[un]map

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Message-ID: <20190725115038.GC31065@lst.de>
Date:   Thu, 25 Jul 2019 13:50:38 +0200
From:   Christoph Hellwig <hch@....de>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Christoph Hellwig <hch@....de>, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-rdma@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
        Christian Koenig <Christian.Koenig@....com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Dan Williams <dan.j.williams@...el.com>,
        Eric Pilmore <epilmore@...aio.com>,
        Stephen Bates <sbates@...thlin.com>
Subject: Re: [PATCH 14/14] PCI/P2PDMA: Introduce
 pci_p2pdma_[un]map_resource()

On Wed, Jul 24, 2019 at 10:06:22AM -0600, Logan Gunthorpe wrote:
> Yes. This is the downside of dealing only with a phys_addr_t: we have to
> look up against it. Unfortunately, I believe it's possible for different
> BARs on a device to be in different windows, so something like this is
> necessary unless we already know the BAR the phys_addr_t belongs to. It
> might probably be sped up a bit by storing the offsets of each bar
> instead of looping through all the bridge windows, but I don't think it
> will get you *that* much.
> 
> As this is an example with no users, the answer here will really depend
> on what the use-case is doing. If they can lookup, ahead of time, the
> mapping type and offset then they don't have to do this work on the hot
> path and it means that pci_p2pdma_map_resource() is simply not a
> suitable API.

Ok.  So lets just keep this out as an RFC and don't merge it until an
actual concrete user shows up.