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

From: Nicholas Mc Guire
Date: Wed Dec 14 2016 - 23:03:00 EST

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@xxxxxxxxx>

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);