On Tue, Aug 06, 2019 at 09:23:51AM -0700, Rob Clark wrote: > On Tue, Aug 6, 2019 at 8:50 AM Christoph Hellwig <h...@lst.de> wrote: > > > > On Tue, Aug 06, 2019 at 07:11:41AM -0700, Rob Clark wrote: > > > Agreed that drm_cflush_* isn't a great API. In this particular case > > > (IIUC), I need wb+inv so that there aren't dirty cache lines that drop > > > out to memory later, and so that I don't get a cache hit on > > > uncached/wc mmap'ing. > > > > So what is the use case here? Allocate pages using the page allocator > > (or CMA for that matter), and then mmaping them to userspace and never > > touching them again from the kernel? > > Currently, it is pages coming from tmpfs. Ideally we want pages that > are swappable when unpinned.
tmpfs is basically a (complicated) frontend for alloc pages as far as page allocation is concerned. > CPU mappings are *mostly* just mapping to userspace. There are a few > exceptions that are vmap'd (fbcon, and ringbuffer). And those use the same backend? > (Eventually I'd like to support pages passed in from userspace.. but > that is down the road.) Eww. Please talk to the iommu list before starting on that. > > > Tying it in w/ iommu seems a bit weird to me.. but maybe that is just > > > me, I'm certainly willing to consider proposals or to try things and > > > see how they work out. > > > > This was just my through as the fit seems easy. But maybe you'll > > need to explain your use case(s) a bit more so that we can figure out > > what a good high level API is. > > Tying it to iommu_map/unmap would be awkward, as we could need to > setup cpu mmap before it ends up mapped to iommu. And the plan to > support per-process pagetables involved creating an iommu_domain per > userspace gl context.. some buffers would end up mapped into multiple > contexts/iommu_domains. > > If the cache operation was detached from iommu_map/unmap, then it > would seem weird to be part of the iommu API. > > I guess I'm not entirely sure what you had in mind, but this is why > iommu seemed to me like a bad fit. So back to the question, I'd like to understand your use case (and maybe hear from the other drm folks if that is common): - you allocate pages from shmem (why shmem, btw? if this is done by other drm drivers how do they guarantee addressability without an iommu?) - then the memory is either mapped to userspace or vmapped (or even both, althrough the lack of aliasing you mentioned would speak against it) as writecombine (aka arm v6+ normal uncached). Does the mapping live on until the memory is freed? - as you mention swapping - how do you guarantee there are no aliases in the kernel direct mapping after the page has been swapped in? - then the memory is potentially mapped to the iommu. Is it using a long-living mapping, or does get unmapped/remapped repeatedly?
