Mir in a virtual machine

Daniel van Vugt daniel.van.vugt at canonical.com
Tue Jan 28 02:02:30 UTC 2014

Before anyone corrects me... I know mir_buffer_usage_hardware is 
intentionally vague and not called mir_buffer_usage_opengl for a reason. 
The reason is that mir_buffer_usage_hardware is not limited to OpenGL. 
In XMir for example we use mir_buffer_usage_hardware and the various DDX 
drivers implement their own acceleration.

On 28/01/14 09:35, Daniel van Vugt wrote:
> Rian,
> The distinction between "software" and "hardware" buffers in Mir is
> confusing, I know, due to the vague names. What it actually means is:
> mir_buffer_usage_software: Client draws individual pixels and the server
> composites this (in hardware still). Presently glTexImage2D, I know. :/
> mir_buffer_usage_hardware: Client never has access to address individual
> pixels and must use OpenGL.
> So if you want to draw the pixels yourself then you must use
> mir_buffer_usage_software. If you feel glTexImage2D is too slow at that,
> then it's an enhancement for Mir later. But not something that's visible
> to the client.
> To see the various implementations just search for the "bind_to_texture"
> method:
> grep -r '::bind_to_texture' *
> - Daniel
> On 27/01/14 04:17, Rian Quinn wrote:
>> I went back and re-read the gbm code, and noticed that it only creates
>> the dumb buffer if you set the GBM_BO_USE_WRITE flag, which means that
>> you can use the gbm_bo_write command which takes a char*. So that makes
>> sense to me now. What I guess I was looking for, was something more like
>> DirectDraw, where a client could get access to device memory, and render
>> directly into for 2D.
>> In my case I’m not really interested in 3D, but instead my clients are
>> rendering using the VGA spec, 2D pixel data. I’m trying to figure out
>> the best way to reduce the number of memcpys. At the moment, I am going
>> to assume that I need to use a software buffer for the clients, and the
>> server will copy the pixel data to the scanout buffer.
>> That being the case, what is the best method for transferring this ARGB
>> pixel data from shared RAM, onto a scanout buffer? From what I
>> understand, glTexSubImage2D does a memcpy via the CPU. Ideally, I would
>> prefer this transfer to be done via DMA and not the CPU. Keep in mind
>> that each client is rendering as a VGA device, so I don’t really have
>> control of “when” the client decides to modify it’s char*.
>> After reading a lot of the compositor code in Mir, I so far have not
>> been able to locate how this is done for a software buffer vs. a
>> hardware buffer.
>> Also, is there an example of how someone would use Mir to composite
>> multiple clients. From what I can tell, all of the server examples just
>> call run_mir, and don’t really do anything.
>> Thanks
>> - Rian
>>> On Jan 26, 2014, at 6:10 AM, Alexandros Frantzis
>>> <alexandros.frantzis at canonical.com> wrote:
>>> On Sat, Jan 25, 2014 at 02:04:29PM -0800, Rian Quinn wrote:
>>>     At the moment I am trying to better understand how buffers are used
>>>     and shared in Mir. I have spent the past couple of days doing
>>> nothing
>>>     but reading the Mir source code, the DRM source code, and
>>> portions of
>>>     the Mesa source code. Here is what I think I have learned so far,
>>>     which will lead me to a question:
>>>     - The server creates the buffers. It can create either a hardware
>>>     buffer, which is a gbm allocated “dumb” buffer, or a software
>>> buffer,
>>>     which is nothing more than shared memory (in RAM).
>>> Using the mir_buffer_usage_hardware flag leads to the creation of a
>>> normal gbm buffer, i.e., one suited for direct usage by the GPU. This is
>>> *not* a gbm "dumb" buffer.
>>> You are correct that using mir_buffer_usage_software creates a shared
>>> memory (RAM) buffer.
>>>     - When a hardware buffer is created, it uses DRM prime
>>>     (drmPrimeHandleToFD) to create an FD for the “dumb" buffer.
>>> Correct, but as noted above, it's not a gbm "dumb" buffer, but a normal
>>> gbm buffer.
>>>     - The server then provides the client with a “ClientBuffer” which is
>>>     an abstraction of the hardware / software buffer containing the
>>>     information about the buffer (like format, stride, etc…) and its FD.
>>>     - To draw into this buffer, you need to
>>>     call mir_surface_get_graphics_region which through some indirection,
>>>     mmaps the buffer to a vaddr that can be written into.
>>> Not correct for hardware buffers, see below for more. This only works
>>> for software buffers, in which case the fd passed to the client is a
>>> shared memory fd which can be mapped. In the case of hardware buffers
>>> it's a "prime" fd which in general doesn't support sensible mapping of
>>> that kind.
>>>     If you look at the basic.c demo, it creates a hardware buffer,
>>> and if
>>>     you look at the fingerpaint.c demo, it creates a software buffer.
>>> If I
>>>     modify the basic.c demo to call mir_surface_get_graphics_region, it
>>>     fails on VMWare, saying that it could not mmap the buffer. It works
>>>     fine if I change the basic.c to use a software buffer.
>>>     Is this an issue with VMWare? Or am I fundamentally not
>>> understanding
>>>     something about how hardware buffer’s are used? If I am… why would a
>>>     client use hardware buffers if it cannot map the buffer to use it?
>>> In general, buffers created for direct GPU usage cannot be reasonably
>>> mmap-ed and their pixels accessed directly. Even if the mapping
>>> operation itself is supported by the driver, the contents of the mapped
>>> area are usually not laid out in memory in a linear fashion (i.e. they
>>> have some sort of tiling), and therefore not useful for pixel
>>> manipulation.
>>> The only way to draw into a hardware buffer in Mir is to go through one
>>> of the "accelerated" APIs supported by Mesa EGL (e.g. OpenGL). The
>>> example at examples/scroll.c shows how to do that.
>>> Bottom line: the failure to mmap is expected for hardware buffers, it's
>>> not a VMWare issue; the mir_surface_get_graphics_region call is only
>>> meaningful for Mir software buffers.
>>> Thanks,
>>> Alexandros

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