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

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Date:   Thu, 15 Dec 2016 10:47:57 +0200
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Nicholas Mc Guire <hofrat@...dl.org>,
        Daniel Vetter <daniel.vetter@...el.com>
Cc:     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] drm/i915: use udelay for very small delays

On Thu, 15 Dec 2016, Nicholas Mc Guire <hofrat@...dl.org> wrote:
> 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 ?

This is in the modeset path, i.e. pretty slow anyway. In this case, the
point is not to try hard to minimize the wait, the point is to guarantee
"at least 0.5 us" has passed. If the CPU can do something else,
including dozing off, in the mean time, great. I think we should stick
with usleep_range().

I think the question is, how do we express this in code? IMO udelay() is
not the answer.

And why doesn't usleep_range() kernel-doc mention anything about the
ranges?


BR,
Jani.


>
> 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);

-- 
Jani Nikula, Intel Open Source Technology Center