Hi Robin, On 22.10.2019 16:25, Robin Murphy wrote: > On 22/10/2019 13:55, Laurentiu Tudor wrote: >> From: Laurentiu Tudor <laurentiu.tu...@nxp.com> >> >> Introduce a new dma map op called dma_addr_to_phys_addr() that converts >> a dma address to the physical address backing it up and add wrapper for >> it. > > I'd really love it if there was a name which could encapsulate that this > is *only* for extreme special cases of constrained descriptors/pagetable > entries/etc. where there's simply no practical way to keep track of a > CPU address alongside the DMA address, and the only option is this > potentially-arbitrarily-complex operation (I mean, on some systems it > may end up taking locks and poking hardware). > > Either way it's tricky - much as I don't like adding an interface which > is ripe for drivers to misuse, I also really don't want hacks like > bdf95923086f shoved into other APIs to compensate, so on balance I'd > probably consider this proposal ever so slightly the lesser evil.
We had an internal discussion over these points you are raising and Madalin (cc-ed) came up with another idea: instead of adding this prone to misuse api how about experimenting with a new dma unmap and dma sync variants that would return the physical address by calling the newly introduced dma map op. Something along these lines: * phys_addr_t dma_unmap_page_ret_phys(...) * phys_addr_t dma_unmap_single_ret_phys(...) * phys_addr_t dma_sync_single_for_cpu_ret_phys(...) I'm thinking that this proposal should reduce the risks opened by the initial variant. Please let me know what you think. --- Thanks & Best Regards, Laurentiu
