linux-kernel - [PATCH v2] drm/mxsfb: fix pixel clock polarity

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Date:   Wed, 14 Dec 2016 12:48:09 -0800
From:   Stefan Agner <stefan@...er.ch>
To:     marex@...x.de
Cc:     airlied@...ux.ie, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, Stefan Agner <stefan@...er.ch>
Subject: [PATCH v2] drm/mxsfb: fix pixel clock polarity

The DRM subsystem specifies the pixel clock polarity from a
controllers perspective: DRM_BUS_FLAG_PIXDATA_NEGEDGE means
the controller drives the data on pixel clocks falling edge.
That is the controllers DOTCLK_POL=0 (Default is data launched
at negative edge).

Also change the data enable logic to be high active by default
and only change if explicitly requested via bus_flags. With
that defaults are:
- Data enable: high active
- Pixel clock polarity: controller drives data on negative edge

Signed-off-by: Stefan Agner <stefan@...er.ch>
---
Changes since v1:
- Improved comments/fixed typo

 drivers/gpu/drm/mxsfb/mxsfb_crtc.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/mxsfb/mxsfb_crtc.c b/drivers/gpu/drm/mxsfb/mxsfb_crtc.c
index 3770dd2..5556e53 100644
--- a/drivers/gpu/drm/mxsfb/mxsfb_crtc.c
+++ b/drivers/gpu/drm/mxsfb/mxsfb_crtc.c
@@ -195,9 +195,16 @@ static void mxsfb_crtc_mode_set_nofb(struct mxsfb_drm_private *mxsfb)
 		vdctrl0 |= VDCTRL0_HSYNC_ACT_HIGH;
 	if (m->flags & DRM_MODE_FLAG_PVSYNC)
 		vdctrl0 |= VDCTRL0_VSYNC_ACT_HIGH;
-	if (bus_flags & DRM_BUS_FLAG_DE_HIGH)
+	/* Make sure Data Enable is high active by default */
+	if (!(bus_flags & DRM_BUS_FLAG_DE_LOW))
 		vdctrl0 |= VDCTRL0_ENABLE_ACT_HIGH;
-	if (bus_flags & DRM_BUS_FLAG_PIXDATA_NEGEDGE)
+	/*
+	 * DRM_BUS_FLAG_PIXDATA_ defines are controller centric,
+	 * controllers VDCTRL0_DOTCLK is display centric.
+	 * Drive on positive edge       -> display samples on falling edge
+	 * DRM_BUS_FLAG_PIXDATA_POSEDGE -> VDCTRL0_DOTCLK_ACT_FALLING
+	 */
+	if (bus_flags & DRM_BUS_FLAG_PIXDATA_POSEDGE)
 		vdctrl0 |= VDCTRL0_DOTCLK_ACT_FALLING;
 
 	writel(vdctrl0, mxsfb->base + LCDC_VDCTRL0);
-- 
2.10.2