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@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