Query regarding initial mode setting by X server & /dev/dri/control

[Rob Clark]

On Fri, Jan 17, 2014 at 6:21 AM, Akash Hajari <akashhajari at gmail.com> wrote:
> Hi Rob,
>
> Thanks for your answer and your example
> (https://github.com/robclark/kmscube/blob/master/kmscube.c). I
> understood standalone KMS without x11 (as written by you @
> http://lists.freedesktop.org/archives/dri-devel/2012-September/027348.html
> ).
>
> But, looking at Linux Graphics stack
> (http://en.wikipedia.org/wiki/File:Linux_Graphics_Stack_2013.svg), I
> am confused. Does libGL calls always directly calls libDRM and never
> to x-server?

you have two cases:

1) direct rendering:  rendering commands (gl/gles) go directly to hw
(via kernel execbuf/pushbuf/submit type ioctl, usually utilizing
helpers/wrappers in libdrm_{intel,radeon,nouveau,freedreno,etc}.  But
presentation commands (egl/glx), ie. swapbuffers, etc, go via the
display server (x11, wayland, etc).

2) indirect rendering (gl + glx only), everything goes to xserver

In either case, the KMS APIs are used by the display server.  Direct
vs indirect rendering is only about the rendering part of things.

> If so, then how simple gl example ‘drawf.c’
> (http://www.glprogramming.com/red/chapter08.html) using glBitmap()
> work well when x11 server running?
>
>
> Also, when switching between multiple windows, x server running in
> backgroung, why drm_mode_addfb (DRM_IOCTL_MODE_ADDFB) getting called
> many times?
> (as I think, fb might have already created for multiple windows.)

no, that would be addfb getting called for every page flip.
Technically you could just create the fb once and re-use it as you
flip back to a previous buffer.  But addfb/rmfb is relatively
lightweight operation (the fb is just a wrapper for a GEM object,
which itself is not re-allocated each frame).

BR,
-R

> Thanks for your patience, Rob.
>
> Akash Hajari.
>
> On 1/14/14, Rob Clark <robdclark at gmail.com> wrote:
>> On Tue, Jan 14, 2014 at 5:15 AM, Akash Hajari <akashhajari at gmail.com>
>> wrote:
>>> Hi,
>>> I am new to DRM & KMS and I had seen your pdf named "why & how to use kms
>>> as
>>> your user space display api if choice".
>>> I have to understand initial mode setting  flow of x server. So is it
>>> located in /xorg-server/hw/xfree86/modes/ or some where else?
>>
>>
>> currently it is in each different DDX driver (for example,
>> xf86-video-intel, xf86-video-modesetting, etc)
>>
>> Perhaps either tests/modetest/modetest.c (in libdrm tree) or kmscube
>> (https://github.com/robclark/kmscube/blob/master/kmscube.c) might be a
>> simpler stand-alone example to look at (the former does only modeset,
>> the later does modeset + gl)
>>
>>> And, I don't know use control node. What's use of /dev/dri/control node?
>>
>> I don't think it is really used for much yet.. but mostly people just
>> use drmOpen() in libdrm and it figures out what device file to open.
>>
>> BR,
>> -R
>>
>>>
>>> Akash Hajari.
>>>
>>> _______________________________________________
>>> dri-devel mailing list
>>> dri-devel at lists.freedesktop.org
>>> http://lists.freedesktop.org/mailman/listinfo/dri-devel
>>>
>>