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Date:   Wed, 24 Jul 2019 08:32:36 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Logan Gunthorpe <logang@deltatee.com>
Cc:     linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org,
        linux-nvme@lists.infradead.org, linux-rdma@vger.kernel.org,
        Bjorn Helgaas <bhelgaas@google.com>,
        Christoph Hellwig <hch@lst.de>,
        Christian Koenig <Christian.Koenig@amd.com>,
        Jason Gunthorpe <jgg@mellanox.com>,
        Sagi Grimberg <sagi@grimberg.me>,
        Keith Busch <kbusch@kernel.org>, Jens Axboe <axboe@fb.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Eric Pilmore <epilmore@gigaio.com>,
        Stephen Bates <sbates@raithlin.com>
Subject: Re: [PATCH 14/14] PCI/P2PDMA: Introduce
 pci_p2pdma_[un]map_resource()
Message-ID: <20190724063235.GC1804@lst.de>
References: <20190722230859.5436-1-logang@deltatee.com> <20190722230859.5436-15-logang@deltatee.com>
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> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index baf476039396..20c834cfd2d3 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -874,6 +874,91 @@ void pci_p2pdma_unmap_sg_attrs(struct device *dev, struct scatterlist *sg,
>  }
>  EXPORT_SYMBOL_GPL(pci_p2pdma_unmap_sg_attrs);
>  
> +static pci_bus_addr_t pci_p2pdma_phys_to_bus(struct pci_dev *dev,
> +		phys_addr_t start, size_t size)
> +{
> +	struct pci_host_bridge *bridge = pci_find_host_bridge(dev->bus);
> +	phys_addr_t end = start + size;
> +	struct resource_entry *window;
> +
> +	resource_list_for_each_entry(window, &bridge->windows) {
> +		if (window->res->start <= start && window->res->end >= end)
> +			return start - window->offset;
> +	}
> +
> +	return DMA_MAPPING_ERROR;

This does once again look very expensive for something called in the
hot path.