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Date:   Tue, 25 Sep 2018 12:46:30 -0600
From:   Logan Gunthorpe <>
To:     Bart Van Assche <>,,,,,,
Cc:     Stephen Bates <>,
        Christoph Hellwig <>,
        Keith Busch <>,
        Sagi Grimberg <>,
        Bjorn Helgaas <>,
        Jason Gunthorpe <>,
        Max Gurtovoy <>,
        Dan Williams <>,
        Jérôme Glisse <>,
        Benjamin Herrenschmidt <>,
        Alex Williamson <>,
        Christian König <>,
        Jens Axboe <>
Subject: Re: [PATCH v7 03/13] PCI/P2PDMA: Add PCI p2pmem DMA mappings to
 adjust the bus offset

On 2018-09-25 11:33 a.m., Bart Van Assche wrote:
>> +	/*
>> +	 * 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()
>> +	 */
>> +			 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?

Hmm, I can't say for certain but I would definitely expect any sane
compiler to do so. It's a fairly obvious optimization and the kbuild
robot is happy with it.

I just did a test compile on 4.9 (which is the earliest compiler version
I have readily available) and it worked fine. The minimum gcc the kernel
supports is 4.6 right now.

I've also done a similar test on[1] and can't find any
compiler version that doesn't work.



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