linux-kernel - Re: [PATCH v7 03/13] PCI/P2PDMA: Add PCI p2pmem DMA mappings to adjust the bus offset

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Message-ID: <1537896805.11137.25.camel@acm.org>
Date:   Tue, 25 Sep 2018 10:33:25 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Logan Gunthorpe <logang@...tatee.com>,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-rdma@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-block@...r.kernel.org
Cc:     Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        Keith Busch <keith.busch@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Christian König <christian.koenig@....com>,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH v7 03/13] PCI/P2PDMA: Add PCI p2pmem DMA mappings to
 adjust the bus offset

On Tue, 2018-09-25 at 10:22 -0600, Logan Gunthorpe wrote:
> +int pci_p2pdma_map_sg(struct device *dev, struct scatterlist *sg, int nents,
> +		      enum dma_data_direction dir)
> +{
> +	struct dev_pagemap *pgmap;
> +	struct scatterlist *s;
> +	phys_addr_t paddr;
> +	int i;
> +
> +	/*
> +	 * p2pdma mappings are not compatible with devices that use
> +	 * dma_virt_ops. If the upper layers do the right thing
> +	 * this should never happen because it will be prevented
> +	 * by the check in pci_p2pdma_add_client()
> +	 */
> +	if (WARN_ON_ONCE(IS_ENABLED(CONFIG_DMA_VIRT_OPS) &&
> +			 dev->dma_ops == &dma_virt_ops))
> +		return 0;

Are you assuming that the compiler will optimize out the dev->dma_ops
== &dma_virt_ops test if CONFIG_DMA_VIRT_OPS=n such that no reference to the
dma_virt_ops symbol appears in the object file? Are you sure all compilers
and compiler versions that are used to build the Linux kernel will do that?

Thanks,

Bart.