Re: drm_gem_dumb_map_offset patch

From: Daniel Vetter <daniel.vetter@ffwll.ch>
To: "Noralf Trønnes" <noralf@tronnes.org>
Cc: DRI Development <dri-devel@lists.freedesktop.org>,
	Pierre Yves MORDRET <pierre-yves.mordret@st.com>
Subject: Re: drm_gem_dumb_map_offset patch
Date: Fri, 31 May 2019 13:36:16 +0200	[thread overview]
Message-ID: <CAKMK7uH=Lo+Ewc6UpjF_pEPB+O0NR4VOeDtM1mdpoAdV4HR6xQ@mail.gmail.com> (raw)
In-Reply-To: <2a6c0803-b386-c8cc-78f8-26faa230ac44@tronnes.org>

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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