On Tue, Feb 12, 2019 at 04:28:13PM +0100, Paul Kocialkowski wrote: > Hi, > > On Mon, 2019-02-11 at 15:41 +0100, Maxime Ripard wrote: > > The current calculation for the video start delay in the current DSI driver > > is that it is the total vertical size, minus the front porch and sync length, > > plus 1. This equals to the active vertical size plus the back porch plus 1. > > > > That 1 is coming in the Allwinner BSP from an variable that is set to 1. > > However, if we look at the Allwinner BSP more closely, and especially in > > the "legacy" code for the display (in drivers/video/sunxi/legacy/), we can > > see that this variable is actually computed from the porches and the sync > > minus 10, clamped between 8 and 100. > > For future reference, it look like there is a version of the BSP where > this is clamped between 8 and 31 in the legacy ebios code: > https://github.com/Icenowy/linux-kernel-lichee-a33/blob/master/drivers/video/sunxi/legacy/disp/de_bsp/de/ebios/de_dsi.c#L687 > > As well as in the A23 code: > https://github.com/Icenowy/linux-kernel-lichee-a33/blob/master/drivers/video/sunxi/disp/de/lowlevel_sun8iw3/de_dsi.c#L686 > > > This fixes the start delay symptom we've seen on some panels (vblank > > timeouts with vertical white stripes at the bottom of the panel). > > > > Signed-off-by: Maxime Ripard <maxime.ripard@xxxxxxxxxxx> > > --- > > drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c | 4 +++- > > 1 file changed, 3 insertions(+), 1 deletion(-) > > > > diff --git a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c > > index 380fc527a707..9471fa695ec7 100644 > > --- a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c > > +++ b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c > > @@ -357,7 +357,9 @@ static void sun6i_dsi_inst_init(struct sun6i_dsi *dsi, > > static u16 sun6i_dsi_get_video_start_delay(struct sun6i_dsi *dsi, > > struct drm_display_mode *mode) > > { > > - return mode->vtotal - (mode->vsync_end - mode->vdisplay) + 1; > > + u16 start = clamp(mode->vtotal - mode->vdisplay - 10, 8, 100); > > + > > + return mode->vtotal - (mode->vsync_end - mode->vdisplay) + start; > > Looking at the formula from the BSP, I get a different result from > yours. > > From what I can see, the BSP formula is: > start_delay = vtotal - vfp + start > vfp = vtotal - y - vbp > > With vbp counting the back porch and sync length. vbp is only the back porch. Maxime -- Maxime Ripard, Bootlin Embedded Linux and Kernel engineering https://bootlin.com
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