linux-kernel - [PATCH] fbdev/omap: fix max fclk divider for omap36xx

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Message-Id: <20191018124938.29313-1-aford173@gmail.com>
Date:   Fri, 18 Oct 2019 07:49:38 -0500
From:   Adam Ford <aford173@...il.com>
To:     linux-fbdev@...r.kernel.org
Cc:     linux-omap@...r.kernel.org, linux-kernel@...r.kernel.org,
        tomi.valkeinen@...com, adam.ford@...icpd.com,
        Adam Ford <aford173@...il.com>, stable@...r.kernel.org
Subject: [PATCH] fbdev/omap: fix max fclk divider for omap36xx

The OMAP36xx and AM/DM37x TRMs say that the maximum divider for DSS fclk
(in CM_CLKSEL_DSS) is 32. Experimentation shows that this is not
correct, and using divider of 32 breaks DSS with a flood or underflows
and sync losts. Dividers up to 31 seem to work fine.

There is another patch to the DT files to limit the divider correctly,
but as the DSS driver also needs to know the maximum divider to be able
to iteratively find good rates, we also need to do the fix in the DSS
driver.

Signed-off-by: Adam Ford <aford173@...il.com>
Cc: Tomi Valkeinen <tomi.valkeinen@...com>
Cc: stable@...r.kernel.org #linux-4.9.y+

diff --git a/drivers/video/fbdev/omap2/omapfb/dss/dss.c b/drivers/video/fbdev/omap2/omapfb/dss/dss.c
index 48c6500c24e1..4429ad37b64c 100644
--- a/drivers/video/fbdev/omap2/omapfb/dss/dss.c
+++ b/drivers/video/fbdev/omap2/omapfb/dss/dss.c
@@ -843,7 +843,7 @@ static const struct dss_features omap34xx_dss_feats = {
 };

 static const struct dss_features omap3630_dss_feats = {
-	.fck_div_max		=	32,
+	.fck_div_max		=	31,
 	.dss_fck_multiplier	=	1,
 	.parent_clk_name	=	"dpll4_ck",
 	.dpi_select_source	=	&dss_dpi_select_source_omap2_omap3,
-- 
2.17.1