[Mesa-dev] [PATCH v1 0/7] Implement commont gralloc_handle_t in libdrm

[Tomasz Figa]

On Fri, Feb 2, 2018 at 11:51 PM, Tomasz Figa <tfiga at chromium.org> wrote:
> On Fri, Feb 2, 2018 at 11:00 PM, Rob Herring <robh at kernel.org> wrote:
>> On Fri, Feb 2, 2018 at 2:01 AM, Tomasz Figa <tfiga at chromium.org> wrote:
>>> Hi Rob,
>>>
>>> On Tue, Jan 30, 2018 at 9:36 PM, Robert Foss <robert.foss at collabora.com> wrote:
>>>>>>>>>       uint32_t (*get_fd)(buffer_handle_t handle, uint32_t plane);
>>>>>>>>>       uint64_t (*get_modifier)(buffer_handle_t handle, uint32_t
>>>>>>>>> plane);
>>>>>>>>>       uint32_t (*get_offsets)(buffer_handle_t handle, uint32_t plane);
>>>>>>>>>       uint32_t (*get_stride)(buffer_handle_t handle, uint32_t plane);
>>>>>>>>>       ...
>>>>>>>>> } gralloc_funcs_t;
>>>>>
>>>>>
>>>>> These ones? >
>>>>> Yeah, if we could retrieve such function pointer struct using perform
>>>>> or any equivalent (like the implementation-specific methods in
>>>>> gralloc1, but not sure if that's going to be used in practice
>>>>> anywhere), it could work for us.
>>>>
>>>>
>>>> So this is where you and Rob Herring lose me, I don't think I understand
>>>> quite how the gralloc1 call would be used, and how it would tie into this
>>>> handle struct. I think I could do with some guidance on this.
>>>
>>> This would be very similar to gralloc0 perform call. gralloc1
>>> implementations need to provide getFunction() callback [1], which
>>> returns a pointer to given function. The list of standard functions is
>>> defined in the gralloc1.h header [2], but we could take some random
>>> big number and use it for our function that fills in provided
>>> gralloc_funcs_t struct with necessary pointers.
>>>
>>> [1] https://android.googlesource.com/platform/hardware/libhardware/+/master/include/hardware/gralloc1.h#300
>>> [2] https://android.googlesource.com/platform/hardware/libhardware/+/master/include/hardware/gralloc1.h#134
>>
>> This is a deadend because it won't work with a HIDL based
>> implementation (aka gralloc 2.0). You can't set function pointers (or
>> any pointers) because gralloc runs in a different process. Yes,
>> currently gralloc is a pass-thru HAL, but AIUI that will go away.
>
> Part of it. I can't see IMapper being implemented by a separate
> process. You can't map a buffer into one process from another process.
>
> But anyway, it's a good point, thanks, I almost forgot about its
> existence. I'll do further investigation.

Okay, so IMapper indeed breaks the approach I suggested. I'm not sure
at the moment what we could do about it. (The idea of a dynamic
library of a pre-defined name, exporting functions we specify, might
still work, though.)

Note that the DRM_GRALLOC_GET_FD used currently by Mesa will also be
impossible to implement with IAllocator/IMapper. (Although I still
think Mesa and Gralloc are free to have separate logic for choosing
the DRM device to use.)

Best regards,
Tomasz