linux-kernel - [PATCH] drm/i915: use udelay for very small delays

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Date:   Thu, 15 Dec 2016 05:03:29 +0100
From:   Nicholas Mc Guire <hofrat@...dl.org>
To:     Daniel Vetter <daniel.vetter@...el.com>
Cc:     ymohanma <yogesh.mohan.marimuthu@...el.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        David Airlie <airlied@...ux.ie>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, Nicholas Mc Guire <hofrat@...dl.org>
Subject: [PATCH] drm/i915: use udelay for very small delays

usleep_range() is intended for delays in the 10us to 10ms range that need
good precision. a useleep_range(1, will effectively be no more than an
imprecise udelay with some added cache disruption as it will fire more or
less immediately - use udelay() here.

Fixes: commit be4fc046bed3 ("drm/i915: add VLV DSI PLL Calculations")
Signed-off-by: Nicholas Mc Guire <hofrat@...dl.org>
---

Problem located by coccinelle

The requirement of waiting at least 0.5 us is assured with the udelay(1)
here which should be more effective than a usleep_range() - would 
ndelay(500) make sense here ?

Patch was compile tested with: x86_64_defconfig (implies CONFIG_DRM_I915)

Patch is against 4.9.0 (localvrsion-next is next-20161214)

 drivers/gpu/drm/i915/intel_dsi_pll.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_dsi_pll.c b/drivers/gpu/drm/i915/intel_dsi_pll.c
index 56eff60..0ec040e 100644
--- a/drivers/gpu/drm/i915/intel_dsi_pll.c
+++ b/drivers/gpu/drm/i915/intel_dsi_pll.c
@@ -157,7 +157,7 @@ static void vlv_enable_dsi_pll(struct intel_encoder *encoder,
 		      config->dsi_pll.ctrl & ~DSI_PLL_VCO_EN);
 
 	/* wait at least 0.5 us after ungating before enabling VCO */
-	usleep_range(1, 10);
+	udelay(1);
 
 	vlv_cck_write(dev_priv, CCK_REG_DSI_PLL_CONTROL, config->dsi_pll.ctrl);
 
-- 
2.1.4