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

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Date:   Thu, 25 Jul 2019 10:00:54 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Christoph Hellwig <hch@....de>
Cc:     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 2019-07-25 5:50 a.m., Christoph Hellwig wrote:
> 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.


Yup, that was my intention and I mentioned that in the commit message.

Logan