Re: [PATCH 1/2] drm/shmem-helpers: revert "Redirect mmap for imported dma-buf"

From: Daniel Vetter <daniel@ffwll.ch>
To: "Christian König" <christian.koenig@amd.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	 Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	 "moderated list:DMA BUFFER SHARING FRAMEWORK"
	<linaro-mm-sig@lists.linaro.org>, Linux MM <linux-mm@kvack.org>,
	 "open list:DMA BUFFER SHARING FRAMEWORK"
	<linux-media@vger.kernel.org>
Subject: Re: [PATCH 1/2] drm/shmem-helpers: revert "Redirect mmap for imported dma-buf"
Date: Tue, 15 Sep 2020 13:07:18 +0200	[thread overview]
Message-ID: <CAKMK7uHDuzzh_w=7w7m3EoHyx+3HOVawoj2QbJL+9F1+1LULbw@mail.gmail.com> (raw)
In-Reply-To: <92b77d29-ae9b-b9cd-4e03-f193aa386cae@gmail.com>

On Tue, Sep 15, 2020 at 1:03 PM Christian König
<ckoenig.leichtzumerken@gmail.com> wrote:
>
> Am 15.09.20 um 12:39 schrieb Daniel Vetter:
> > On Mon, Sep 14, 2020 at 3:29 PM Christian König
> > <ckoenig.leichtzumerken@gmail.com> wrote:
> >> This reverts commit 26d3ac3cb04d171a861952e89324e347598a347f.
> >>
> >> We need to figure out if dma_buf_mmap() is valid or not first.
> >>
> >> Signed-off-by: Christian König <christian.koenig@amd.com>
> > The trouble is that doing dma-buf mmap by looking at the struct pages
> > behind the sg list and just inserting those into userspace doesn't
> > really work any better. You still won't get the unmap_mapping_range
> > and hence pte shoot-down. So maybe dma_buf_mmap forwarding doesn't
> > work, but this doesn't make it any better.
>
> Good point. Question is what should we do? Return -EPERM?

IIrc there's userspace which expects this to work, but I think it's
also only trying to do this with simpler drivers that don't need
unmap_mapping_range to work correctly. Or it's simply that no one ever
reported the bugs. It's a bit a mess :-/
-Daniel

>
> >
> > Also commit message should probably explain a bit the context here,
> > not a lot of people have been in our private discussion on this.
>
> Well, that's certain true.
>
> Christian.
>
> > -Daniel
> >
> >> ---
> >>   drivers/gpu/drm/drm_gem_shmem_helper.c | 3 ---
> >>   1 file changed, 3 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> >> index 0a952f27c184..cd727343f72b 100644
> >> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> >> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> >> @@ -594,9 +594,6 @@ int drm_gem_shmem_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
> >>          /* Remove the fake offset */
> >>          vma->vm_pgoff -= drm_vma_node_start(&obj->vma_node);
> >>
> >> -       if (obj->import_attach)
> >> -               return dma_buf_mmap(obj->dma_buf, vma, 0);
> >> -
> >>          shmem = to_drm_gem_shmem_obj(obj);
> >>
> >>          ret = drm_gem_shmem_get_pages(shmem);
> >> --
> >> 2.17.1
> >>
> >> _______________________________________________
> >> dri-devel mailing list
> >> dri-devel@lists.freedesktop.org
> >> https://lists.freedesktop.org/mailman/listinfo/dri-devel
> >
> >
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch