linux-kernel - Re: [PATCH] drm: hdlcd: Skip checking the clock provider when CRTC is disabled.

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Date:   Wed, 1 Nov 2017 14:42:31 +0000
From:   Brian Starkey <brian.starkey@....com>
To:     Liviu Dudau <Liviu.Dudau@....com>
Cc:     Mali DP Maintainers <malidp@...s.arm.com>,
        David Airlie <airlied@...ux.ie>,
        DRI-devel <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm: hdlcd: Skip checking the clock provider when CRTC
 is disabled.

On Wed, Nov 01, 2017 at 02:16:25PM +0000, Liviu Dudau wrote:
>Some clock providers (clk-vexpress-osc) trigger a WARN() when the
>requested rate falls outside its capabilities, as is the case when
>a CRTC gets disabled. Check if the CRTC's new state is enabled and
>skip the clk_round_rate() call if it is not.

In itself, this seems like a fine patch. However, the motivation
maybe isn't.

IMO the WARN() in clk-vexpress-osc is bogus and should be removed -
the clk_round_rate() API is intended to convert from a desired rate to
something the clock actually supports - so it's basically expected
that the input rate might not be achievable.

Spewing a WARN when the API is used as-intended seems a little off :-)

Cheers,
-Brian

>
>Reported-by: Vladimir Murzin <vladimir.murzin@....com>
>Signed-off-by: Liviu Dudau <Liviu.Dudau@....com>
>Tested-by: Vladimir Murzin <vladimir.murzin@....com>
>---
> drivers/gpu/drm/arm/hdlcd_crtc.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
>diff --git a/drivers/gpu/drm/arm/hdlcd_crtc.c b/drivers/gpu/drm/arm/hdlcd_crtc.c
>index 72b22b805412b..a1387a8bf78c4 100644
>--- a/drivers/gpu/drm/arm/hdlcd_crtc.c
>+++ b/drivers/gpu/drm/arm/hdlcd_crtc.c
>@@ -193,6 +193,10 @@ static int hdlcd_crtc_atomic_check(struct drm_crtc *crtc,
> 	struct drm_display_mode *mode = &state->adjusted_mode;
> 	long rate, clk_rate = mode->clock * 1000;
>
>+	/* if the crtc is disabled, skip the clock check */
>+	if (!state->enable)
>+		return 0;
>+
> 	rate = clk_round_rate(hdlcd->clk, clk_rate);
> 	if (rate != clk_rate) {
> 		/* clock required by mode not supported by hardware */
>-- 
>2.14.1
>