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() >> + */ >> + 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? 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 godbolt.org[1] and can't find any compiler version that doesn't work. Logan [1] https://godbolt.org/z/d5Mhee
