On Tue, Jun 22, 2021 at 4:33 PM Christoph Hellwig <hch@xxxxxx> wrote: > > On Fri, Jun 18, 2021 at 01:44:08PM +0900, Tomasz Figa wrote: > > > Well, dma_alloc_coherent users want a non-cached mapping. And while > > > some architectures provide that using a vmap with "uncached" bits in the > > > PTE to provide that, this: > > > > > > a) is not possibly everywhere > > > b) even where possible is not always the best idea as it creates mappings > > > with differnet cachability bets > > > > I think this could be addressed by having a dma_vmap() helper that > > does the right thing, whether it's vmap() or dma_common_pages_remap() > > as appropriate. Or would be this still insufficient for some > > architectures? > > It can't always do the right thing. E.g. for the case where uncached > memory needs to be allocated from a special boot time fixed pool. > Fair enough. Thanks for elaborating. > > > And even without that dma_alloc_noncoherent causes less overhead than > > > dma_alloc_noncontigious if you only need a single contiguous range. > > > > > > > Given that behind the scenes dma_alloc_noncontiguous() would also just > > call __dma_alloc_pages() for devices that need contiguous pages, would > > the overhead be basically the creation of a single-entry sgtable? > > In the best case: yes. > > > > So while I'm happy we have something useful for more complex drivers like > > > v4l I think the simple dma_alloc_coherent API, including some of the less > > > crazy flags for dma_alloc_attrs is the right thing to use for more than > > > 90% of the use cases. > > > > One thing to take into account here is that many drivers use the > > existing "simple" way, just because there wasn't a viable alternative > > to do something better. Agreed, though, that we shouldn't optimize for > > the rare cases. > > While that might be true for a few drivers, it is absolutely not true > for the wide majority. I think you media people are a little special, > with only the GPU folks contending for "specialness" :) (although > media handles it way better, gpu folks just create local hacks that > can't work portably). I don't have the evidence to argue, so let's just leave it at "time will tell". I think it's great that we have the possibility to do the more special things and we can see where it goes from now on. :) Best regards, Tomasz
