[PATCH v3 0/4] Rate-limit shadow-FB-to-console-update to screen refresh

[Sam Ravnborg]

Hi Thomas.


On Thu, Dec 05, 2019 at 05:01:38PM +0100, Thomas Zimmermann wrote:
> A full-screen memcpy() moves the console's shadow buffer to hardware; with
> possibly significant runtime overhead. [1] Synchronizing the screen update
> with VBLANK events can hopefully reduce the number of screen updates.
> 
> The patchset first adds vblank support to mgag200 as the problem was
> initially reported for Matrox hardware.
> 
> The console's dirty worker now waits for the vblank to rate limit the
> output frequency. Screen output can pile up while waiting and there's a
> chance that multiple screen updates can be handled with a single memcpy().
> Note that this has no effect on tearing: while the dirty worker updates
> the hardware buffer, new data can still arrive in the shadow buffer. This
> can create a tearing effcet, even though console output is synchronized
> to vblank.
> 
> In version 2 of this patchset, fbdev emulation missed the first vblank
> event because mgag200 initialized the fbdev console before the irq.
> Initializing fbdev last fixes this problem. If other drivers now start
> reporting a missed vblank during boot, this might be the reason.
> 
> The patchset was tested by running fbdev emulation and Gnome (X11) on
> mgag200 HW.
> 
> v3:
> 	* fbdev: hold helper->mutex
> 	* fbdev: acquire DRM master so other masters cannot interfere
> 	* mgag200: init fbdev after irq avoids missing first vblank
> 	* mgag200: trigger irq at <vdisplay> + 1 matches vblank
> v2:
> 	* remove locking from fbdev patch
> 	* use constants for mgag200 register names and fields
> 	* double-check that VLINE irq is active on mgag200
> 	* only signal vblank on CRTC 0 of mgag200
> 	* coding-style fixes
> 
> [1] https://lists.freedesktop.org/archives/dri-devel/2019-July/228663.html
> 
> Thomas Zimmermann (4):
>   drm/mgag200: Create fbdev console after registering device
>   drm/mgag200: Rename constant MGAREG_Status to MGAREG_STATUS
>   drm/mgag200: Add vblank support
>   drm/fb-helper: Synchronize dirty worker with vblank
Whole series is:
Acked-by: Sam Ravnborg <sam at ravnborg.org>

The code all looked good, but  I am not familiar enough
with the code to say I reviewed it all.
A few nits in one patch sent in a separate mail.

	Sam