linux-kernel - [PATCH 2/2] drm/arm/hdlcd: Allow a bit of clock tolerance

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Message-Id: <47fb141ddbf4cf256951758d2e7f90afb6507ded.1558111042.git.robin.murphy@arm.com>
Date:   Fri, 17 May 2019 17:37:22 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     liviu.dudau@....com
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] drm/arm/hdlcd: Allow a bit of clock tolerance

On the Arm Juno platform, the HDLCD pixel clock is constrained to 250KHz
resolution in order to avoid the tiny System Control Processor spending
aeons trying to calculate exact PLL coefficients. This means that modes
like my oddball 1600x1200 with 130.89MHz clock get rejected since the
rate cannot be matched exactly. In practice, though, this mode works
quite happily with the clock at 131MHz, so let's relax the check to
allow a little bit of slop.

Signed-off-by: Robin Murphy <robin.murphy@....com>
---
 drivers/gpu/drm/arm/hdlcd_crtc.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/arm/hdlcd_crtc.c b/drivers/gpu/drm/arm/hdlcd_crtc.c
index ecac6fe0b213..a3efa28436ea 100644
--- a/drivers/gpu/drm/arm/hdlcd_crtc.c
+++ b/drivers/gpu/drm/arm/hdlcd_crtc.c
@@ -193,7 +193,8 @@ static enum drm_mode_status hdlcd_crtc_mode_valid(struct drm_crtc *crtc,
 	long rate, clk_rate = mode->clock * 1000;
 
 	rate = clk_round_rate(hdlcd->clk, clk_rate);
-	if (rate != clk_rate) {
+	/* 0.1% seems a close enough tolerance for the TDA19988 on Juno */
+	if (abs(rate - clk_rate) * 1000 > clk_rate) {
 		/* clock required by mode not supported by hardware */
 		return MODE_NOCLOCK;
 	}
-- 
2.21.0.dirty