Re: drm_gem_dumb_map_offset patch

[Daniel Vetter <daniel.vetter@ffwll.ch>]

On Mon, Jun 3, 2019 at 2:25 PM Pierre Yves MORDRET
<pierre-yves.mordret@st.com> wrote:
>
> Hi all,
>
> Many thanks for your valuable support and answers.
>
> Since Dumb mmap is for buffer created using dumb_create ioctl we won't use it
> anymore. In place a mmap/ummap is called with DMA Buf FD.
> After some tests it seems working in our side.

Please note that with dma-buf you need to call the begin/end cpu
access ioctl, or depending upon driver/platform the mmap access might
be incoherent.
-Daniel

>
> Many thanks again.
> Regards.
>
> On 5/31/19 1:36 PM, Daniel Vetter wrote:
> > On Fri, May 31, 2019 at 1:28 PM Noralf Trønnes <noralf@tronnes.org> wrote:
> >>
> >> Hi,
> >>
> >> [add Daniel Vetter]
> >> [cc dri-devel]
> >>
> >> Den 29.05.2019 15.09, skrev Pierre Yves MORDRET:
> >>> Hello Noralf,
> >>>
> >>> Sorry for bothering you with question but I need to better understand the
> >>> rational about a patch you did in DRM/GEM.
> >>>
> >>> First of all, let me introduce myself.
> >>> I'm currently employee to STMicroelectronics company and in charge of GPU
> >>> integration within STM32 MPU (Cortex A7 + Cortex M4)
> >>>
> >>> On Cortex A7 is running a Linux Kernel 4.19 as for today.
> >>>
> >>> We came across some trouble when we switch from Kernel 4.14 to 4.19 for GPU
> >>> stack. On august you submit this commit :
> >>>
> >>>       90378e58919285637aa0f063c04ba0c6449d98b1
> >>>           drm/gem: drm_gem_dumb_map_offset(): reject dma-buf
> >>>
> >>>           Reject mapping an imported dma-buf since is's an invalid use-case.
> >>>
> >>> In Userland GPU stack we have such statements :
> >>>    bo_map_fd
> >>>        DRM_IOCTL_MODE_MAP_DUMB
> >>>        mmap (offset)
> >>>
> >>> With the patch above, ioctl returns an error EINVAL and prevents the nmap.
> >>> As for today we revert this patch and it works fine in our end.
> >
> > Link to source code would be good.
> >
> >>> Thus the questions are :
> >>>  - what is the rational behind this fix ?
> >>>  - What we doing wrong this situation ?
> >>>  - What do we need in such situation ?
> >>>
> >>
> >> I need to pass those on to Daniel Vetter (DRM maintainer) since he
> >> wanted the change. The details were never clear to me.
> >> Some of the discussion is here:
> >> https://patchwork.freedesktop.org/patch/172242/
> >
> > Dumb mmap is for buffer created using dumb_create ioctl, and nothing
> > else. Anything else really has at best undefined behaviour. E.g. for
> > dma-buf you really need to call the begin/end_cpu_access ioctl, and
> > dumb buffers simply have no provision for that. Hence why we can't
> > support this in general.
> >
> > Thanks, Daniel
> >



-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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