linux-kernel - [PATCH 3/3] drm: arcpgu: Allow some clock deviation in crtc->mode

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Date:   Mon, 17 Jul 2017 13:08:27 +0100
From:   Jose Abreu <Jose.Abreu@...opsys.com>
To:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Cc:     Jose Abreu <Jose.Abreu@...opsys.com>,
        Carlos Palminha <CARLOS.PALMINHA@...opsys.com>,
        Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Dave Airlie <airlied@...il.com>
Subject: [PATCH 3/3] drm: arcpgu: Allow some clock deviation in crtc->mode_valid() callback

Currently we expect that clock driver produces the exact same value
as we are requiring. There can, and will, be some deviation
however so we need to take that into account instead of just
rejecting the mode.

According to the HDMI spec we have a max of +-0.5% for the pixel clock
frequency deviation. Lets take that into an advantage and use it to
calculate how much deviation we can support.

Signed-off-by: Jose Abreu <joabreu@...opsys.com>
Acked-by: Alexey Brodkin <abrodkin@...opsys.com>
Cc: Carlos Palminha <palminha@...opsys.com>
Cc: Alexey Brodkin <abrodkin@...opsys.com>
Cc: Daniel Vetter <daniel.vetter@...el.com>
Cc: Dave Airlie <airlied@...il.com>
---
 drivers/gpu/drm/arc/arcpgu_crtc.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/arc/arcpgu_crtc.c b/drivers/gpu/drm/arc/arcpgu_crtc.c
index 1859dd3..55c5d5b 100644
--- a/drivers/gpu/drm/arc/arcpgu_crtc.c
+++ b/drivers/gpu/drm/arc/arcpgu_crtc.c
@@ -69,12 +69,13 @@ static enum drm_mode_status arc_pgu_crtc_mode_valid(struct drm_crtc *crtc,
 {
 	struct arcpgu_drm_private *arcpgu = crtc_to_arcpgu_priv(crtc);
 	long rate, clk_rate = mode->clock * 1000;
+	long diff = clk_rate / 200; /* +-0.5% allowed by HDMI spec */
 
 	rate = clk_round_rate(arcpgu->clk, clk_rate);
-	if (rate != clk_rate)
-		return MODE_NOCLOCK;
+	if ((max(rate, clk_rate) - min(rate, clk_rate) < diff) && (rate > 0))
+		return MODE_OK;
 
-	return MODE_OK;
+	return MODE_NOCLOCK;
 }
 
 static void arc_pgu_crtc_mode_set_nofb(struct drm_crtc *crtc)
-- 
1.9.1