Some questions about DRM(Direct Rendering Manager)

Rob Clark robdclark at
Wed Jun 22 09:07:06 EDT 2011

fwiw, drm_fb_helper_single_fb_probe() calls register_framebuffer() (if
that was the question?)


2011/6/22 InKi Dae <daeinki at>:
> It adds dri-devel at to this mail thread.
> Thank you.
> 2011년 6월 22일 오후 3:04, daeinki <inki.dae at>님의 말:
>> below is additional comments.
>> daeinki 쓴 글:
>>> Hi all,
>>> I'm writing Samsung SoC based DRM framework and this one includes FIMD
>>> and HDMI driver as hardware dependent modules. and for now, encoder,
>>> connector, crtc and fb module has been materialized almost. but I'm
>>> contending with  framebuffer setting issue(created fb_info should be
>>> registered to linux framebuffer through register_framebuffer() or not)as
>>> default framebuffer at booting time.
>>> at drm_fb_helper_single_fb_probe() of drm_fb_helper.c file, fb_helper's
>>> fb_probe callback is called and this one creates new framebuffer and
>>> returns a value more then 0 if true. internally, this process creates an
>>> fb_info object and drm_framebuffer and then drm_framebuffer would be
>>> added to mode_config.fb_list of the drm_device.
>> it's my mistake. return value is 0 if true, nonzero otherwise.
>>> a value returned, new_fb is used to decide that it calls
>>> register_framebuffer() or drm_fb_helper_set_par(). at this point, I am
>>> confused it's a good way to call register_framebuffer() otherwise
>>> drm_fb_helper_set_par(). if register_framebuffer() is called then I
>>> guess drm_fb_helper_set_par() or drm_crtc_helper_set_config() should be
>>> called somewhere subsequently to apply this one to real hardware because
>>> previous process is just for maintaining data logically.(not set up data
>>> to h/w)
>>> it's a right way to call register_framebuffer() and then
>>> drm_fb_helper_set_par() or drm_crtc_helper_set_config()? otherwise just
>>> only drm_fb_helper_set_par() or drm_crtc_helper_set_config() ignoring
>>> register_framebuffer()? and what is the purpose of using
>>> register_framebuffer()?
>> I understood that if fb_probe() callback is fail then fb_info object is
>> registered to linux framebuffer through register_framebuffer()
>> otherwise(if true) hardware configuration would be completed by
>> drm_fb_helper_set_par() so the reason of using register_framebuffer() is
>> that the case of failing fb_probe() callback, it is for drawing on only
>> linux framebuffer. is it right?
>>> In my case, first, register_framebuffer() is called and then if desired
>>> default crtc id is matched with drm_fb_helper->crtc_info[0 ~ n].crtc_id,
>>> it gets mode_set of drm_fb_helper->crtc_info[n] and then  it calls
>>> drm_crtc_helper_set_config(mode_set). at this time, all the hardware
>>> configurations would be completed.
>>> thank you in advance.
>>> Best Regards
>>> Inki Dae.
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