linux-kernel - [PATCH V2] drm/i915: relax uncritical udelay

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Date:   Fri, 16 Dec 2016 02:59:38 +0100
From:   Nicholas Mc Guire <hofrat@...dl.org>
To:     Daniel Vetter <daniel.vetter@...el.com>
Cc:     Shashank Sharma <shashank.sharma@...el.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        ymohanma <yogesh.mohan.marimuthu@...el.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 V2] drm/i915: relax uncritical udelay_range()

udelay_range(1, 2) is inefficient and as discussions with Jani Nikula
<jani.nikula@...ux.intel.com> unnecessary here. This replaces this
tight setting with a relaxed delay of min=20 and max=50 which helps
the hrtimer subsystem optimize timer handling.

Fixes: commit be4fc046bed3 ("drm/i915: add VLV DSI PLL Calculations") 
Link: http://lkml.org/lkml/2016/12/15/147
Signed-off-by: Nicholas Mc Guire <hofrat@...dl.org>
---

V2: use relaxed uslee_range() rather than udelay
    fix documentation of changed timings

Problem found by coccinelle:

Patch was compile tested with: x86_64_defconfig (implies CONFIG_DRM_I915)

Patch is against 4.9.0 (localversion-next is next-20161215)

 drivers/gpu/drm/i915/intel_dsi_pll.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_dsi_pll.c b/drivers/gpu/drm/i915/intel_dsi_pll.c
index 56eff60..d210bc4 100644
--- a/drivers/gpu/drm/i915/intel_dsi_pll.c
+++ b/drivers/gpu/drm/i915/intel_dsi_pll.c
@@ -156,8 +156,10 @@ static void vlv_enable_dsi_pll(struct intel_encoder *encoder,
 	vlv_cck_write(dev_priv, CCK_REG_DSI_PLL_CONTROL,
 		      config->dsi_pll.ctrl & ~DSI_PLL_VCO_EN);
 
-	/* wait at least 0.5 us after ungating before enabling VCO */
-	usleep_range(1, 10);
+	/* wait at least 0.5 us after ungating before enabling VCO,
+	 * allow hrtimer subsystem optimization by relaxing timing
+	 */
+	usleep_range(10, 50);
 
 	vlv_cck_write(dev_priv, CCK_REG_DSI_PLL_CONTROL, config->dsi_pll.ctrl);
 
-- 
2.1.4