linux-kernel - [PATCH v6 2/5] drm/vc4: Avoid using vrefresh==0 mode in DSI htotal math.

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Message-Id: <20170815234722.20700-2-eric@anholt.net>
Date:   Tue, 15 Aug 2017 16:47:19 -0700
From:   Eric Anholt <eric@...olt.net>
To:     dri-devel@...ts.freedesktop.org,
        Archit Taneja <architt@...eaurora.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Thierry Reding <thierry.reding@...il.com>
Cc:     linux-kernel@...r.kernel.org, Eric Anholt <eric@...olt.net>
Subject: [PATCH v6 2/5] drm/vc4: Avoid using vrefresh==0 mode in DSI htotal math.

The incoming mode might have a missing vrefresh field if it came from
drmModeSetCrtc(), which the kernel is supposed to calculate using
drm_mode_vrefresh().  We could either use that or the adjusted_mode's
original vrefresh value.

However, we can maintain a more exact vrefresh value (not just the
integer approximation), by scaling by the ratio of our clocks.

v2: Use math suggested by Andrzej Hajda instead.
v3: Simplify math now that adjusted_mode->clock isn't padded.

Signed-off-by: Eric Anholt <eric@...olt.net>
---
 drivers/gpu/drm/vc4/vc4_dsi.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/vc4/vc4_dsi.c b/drivers/gpu/drm/vc4/vc4_dsi.c
index eb787eed8abe..2516cd3a1d87 100644
--- a/drivers/gpu/drm/vc4/vc4_dsi.c
+++ b/drivers/gpu/drm/vc4/vc4_dsi.c
@@ -862,7 +862,8 @@ static bool vc4_dsi_encoder_mode_fixup(struct drm_encoder *encoder,
 	adjusted_mode->clock = pixel_clock_hz / 1000;
 
 	/* Given the new pixel clock, adjust HFP to keep vrefresh the same. */
-	adjusted_mode->htotal = pixel_clock_hz / (mode->vrefresh * mode->vtotal);
+	adjusted_mode->htotal = (adjusted_mode->clock * mode->htotal /
+				 mode->clock);
 	adjusted_mode->hsync_end += adjusted_mode->htotal - mode->htotal;
 	adjusted_mode->hsync_start += adjusted_mode->htotal - mode->htotal;
 
-- 
2.14.1