On Fri, Nov 30, 2018 at 02:22:08PM +0100, Christoph Hellwig wrote: > Error reporting for the dma_map_single and dma_map_page operations is > currently a mess. Both APIs directly return the dma_addr_t to be used for > the DMA, with a magic error escape that is specific to the instance and > checked by another method provided. This has a few downsides: > > - the error check is easily forgotten and a __must_check marker doesn't > help as the value always is consumed anyway > - the error checking requires another indirect call, which have gotten > incredibly expensive > - a lot of code is wasted on implementing these methods > > The historical reason for this is that people thought DMA mappings would > not fail when the API was created, which sounds like a really bad > assumption in retrospective, and then we tried to cram error handling > onto it later on. > > There basically are two variants: the error code is 0 because the > implementation will never return 0 as a valid DMA address, or the error > code is all-F as the implementation won't ever return an address that > high. The old AMD GART is the only one not falling into these two camps > as it picks sort of a relative zero relative to where it is mapped. > > The 0 return doesn't work for direct mappings that have ram at address > zero and a lot of IOMMUs that start allocating bus space from address > zero, so we can't consolidate on that, but I think we can move everyone > to all-Fs, which the patch here does. The reason for that is that > there is only one way to ever get this address: by doing a 1-byte long, > 1-byte aligned mapping, but all our mappings are not only longer but > generally aligned, and the mappings have to keep at least the basic > alignment. > > A git tree is also available here: > > git://git.infradead.org/users/hch/misc.git dma-mapping-error.3 Hi Chris, For patches 1, 3 and 23, Acked-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxx> Thanks. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up
