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

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Date:   Fri, 16 Dec 2016 02:59:20 +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() settings

udelay_range(2, 3) 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.

Link: http://lkml.org/lkml/2016/12/15/127
Fixes: commit 37ab0810c9b7 ("drm/i915/bxt: DSI enable for BXT")
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.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_dsi.c b/drivers/gpu/drm/i915/intel_dsi.c
index 5b72c50..92b96fa 100644
--- a/drivers/gpu/drm/i915/intel_dsi.c
+++ b/drivers/gpu/drm/i915/intel_dsi.c
@@ -379,7 +379,8 @@ static void bxt_dsi_device_ready(struct intel_encoder *encoder)
 		val &= ~ULPS_STATE_MASK;
 		val |= (ULPS_STATE_ENTER | DEVICE_READY);
 		I915_WRITE(MIPI_DEVICE_READY(port), val);
-		usleep_range(2, 3);
+		/* at least 2us - relaxed for hrtimer subsystem optimization */
+		usleep_range(10, 50);
 
 		/* 3. Exit ULPS */
 		val = I915_READ(MIPI_DEVICE_READY(port));
-- 
2.1.4