Re: drm_gem_dumb_map_offset patch

[Pierre Yves MORDRET <pierre-yves.mordret@st.com>]

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.

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
> 
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