Re: [PATCH 1/2] drm/vgem: Do not allocate backing shmemfs file for an import dmabuf object

[lepton <ytht.net@gmail.com>]

On Tue, Jul 7, 2020 at 10:20 AM Chris Wilson <chris@chris-wilson.co.uk> wrote:
>
> Quoting lepton (2020-07-07 18:05:21)
> > On Tue, Jul 7, 2020 at 9:00 AM Chris Wilson <chris@chris-wilson.co.uk> 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?
> -Chris