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

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Message-ID: <87shpons3p.fsf@intel.com>
Date:   Fri, 16 Dec 2016 11:25:14 +0200
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Nicholas Mc Guire <hofrat@...dl.org>,
        Daniel Vetter <daniel.vetter@...el.com>
Cc:     Shashank Sharma <shashank.sharma@...el.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: Re: [PATCH V2] drm/i915: relax uncritical udelay_range()

On Fri, 16 Dec 2016, Nicholas Mc Guire <hofrat@...dl.org> wrote:
> 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>

Pushed to drm-intel-next-queued, thanks for the patch.

BR,
Jani.

> ---
>
> 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);

-- 
Jani Nikula, Intel Open Source Technology Center