On Wed, Jul 08, 2020 at 11:22:00AM +0200, Christian König wrote: > Am 07.07.20 um 20:35 schrieb Chris Wilson: > > Quoting lepton (2020-07-07 19:17:51) > > > On Tue, Jul 7, 2020 at 10:20 AM Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote: > > > > Quoting lepton (2020-07-07 18:05:21) > > > > > On Tue, Jul 7, 2020 at 9:00 AM Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote: > > > > > > If we assign obj->filp, we believe that the create vgem bo is native and > > > > > > allow direct operations like mmap() assuming it behaves as backed by a > > > > > > shmemfs inode. When imported from a dmabuf, the obj->pages are > > > > > > not always meaningful and the shmemfs backing store misleading. > > > > > > > > > > > > Note, that regular mmap access to a vgem bo is via the dumb buffer API, > > > > > > and that rejects attempts to mmap an imported dmabuf, > > > > > What do you mean by "regular mmap access" here? It looks like vgem is > > > > > using vgem_gem_dumb_map as .dumb_map_offset callback then it doesn't call > > > > > drm_gem_dumb_map_offset > > > > As I too found out, and so had to correct my story telling. > > > > > > > > By regular mmap() access I mean mmap on the vgem bo [via the dumb buffer > > > > API] as opposed to mmap() via an exported dma-buf fd. I had to look at > > > > igt to see how it was being used. > > > Now it seems your fix is to disable "regular mmap" on imported dma buf > > > for vgem. I am not really a graphic guy, but then the api looks like: > > > for a gem handle, user space has to guess to find out the way to mmap > > > it. If user space guess wrong, then it will fail to mmap. Is this the > > > expected way > > > for people to handle gpu buffer? > > You either have a dumb buffer handle, or a dma-buf fd. If you have the > > handle, you have to use the dumb buffer API, there's no other way to > > mmap it. If you have the dma-buf fd, you should mmap it directly. Those > > two are clear. > > > > It's when you import the dma-buf into vgem and create a handle out of > > it, that's when the handle is no longer first class and certain uAPI > > [the dumb buffer API in particular] fail. > > > > It's not brilliant, as you say, it requires the user to remember the > > difference between the handles, but at the same time it does prevent > > them falling into coherency traps by forcing them to use the right > > driver to handle the object, and have to consider the additional ioctls > > that go along with that access. > > Yes, Chris is right. Mapping DMA-buf through the mmap() APIs of an importer > is illegal. > > What we could maybe try to do is to redirect this mmap() API call on the > importer to the exporter, but I'm pretty sure that the fs layer wouldn't > like that without changes. We already do that, there's a full helper-ified path from I think shmem helpers through prime helpers to forward this all. Including handling buffer offsets and all the other lolz back&forth. Of course there's still the problem that many drivers don't forward the cache coherency calls for begin/end cpu access, so in a bunch of cases you'll cache cacheline dirt soup. But that's kinda standard procedure for dma-buf :-P But yeah trying to handle the mmap as an importer, bypassing the export: nope. The one exception is if you have some kind of fancy gart with cpu-visible pci bar (like at least integrated intel gpus have). But in that case the mmap very much looks&acts like device access in every way. Cheers, Daniel > Regards, > Christian. > > > > -Chris > -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/intel-gfx
