On Tue, Sep 25, 2018 at 11:41:44AM -0600, Logan Gunthorpe wrote: > Hey, > > On 2018-09-25 11:11 a.m., Keith Busch wrote: > > Sorry if this was already discussed. Is there a reason the following > > pattern is not pushed to the generic dma_map_sg_attrs? > > > > if (is_pci_p2pdma_page(sg_page(sg))) > > pci_p2pdma_map_sg(dev, sg, nents, dma_dir); > > > > Beyond that, series looks good. > > Yes, this has been discussed. It comes down to a few reasons: > > 1) Intrusiveness on other systems: ie. not needing to pay the cost for > every single dma_map_sg call > > 2) Consistency: we can add the check to dma_map_sg, but adding similar > functionality to dma_map_page, etc is difficult seeing it's hard for the > unmap operation to detect if a dma_addr_t was P2P memory to begin with. > > 3) Safety for developers trying to use P2P memory: Right now developers > must be careful with P2P pages and ensure they aren't mapped using other > means (ie dma_map_page). Having them check the drivers that are handling > the pages to ensure the appropriate map function is always used is and > that P2P pages aren't being mixed with regular pages is better than > developers relying on magic in dma_map_sg() and getting things wrong. > > That being said, I think in the future everyone would like to move in > that direction but it means we will have to solve some difficult > problems with the existing infrastructure. Gotchya, thanks for jogging my memory.
