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From:   "james qian wang (Arm Technology China)" <james.qian.wang@arm.com>
To:     Arnd Bergmann <arnd@arndb.de>
CC:     Liviu Dudau <Liviu.Dudau@arm.com>,
        Brian Starkey <Brian.Starkey@arm.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        "Lowry Li (Arm Technology China)" <Lowry.Li@arm.com>,
        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        nd <nd@arm.com>
Subject: Re: [PATCH] drm/komeda: fix 32-bit komeda_crtc_update_clock_ratio
Thread-Topic: [PATCH] drm/komeda: fix 32-bit komeda_crtc_update_clock_ratio
Thread-Index: AQHVJXpNE3N21cdpKkuGT3soJLEwQQ==
Date:   Tue, 18 Jun 2019 02:05:35 +0000
Message-ID: <20190618020529.GB32081@james-ThinkStation-P300>
References: <20190617125121.1414507-1-arnd@arndb.de>
In-Reply-To: <20190617125121.1414507-1-arnd@arndb.de>
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On Mon, Jun 17, 2019 at 02:51:04PM +0200, Arnd Bergmann wrote:
> clang points out a bug in the clock calculation on 32-bit, that leads
> to the clock_ratio always being zero:
>=20
> drivers/gpu/drm/arm/display/komeda/komeda_crtc.c:31:36: error: shift coun=
t >=3D width of type [-Werror,-Wshift-count-overflow]
>         aclk =3D komeda_calc_aclk(kcrtc_st) << 32;
>=20
> Move the shift into the division to make it apply on a 64-bit
> variable. Also use the more expensive div64_u64() instead of div_u64()
> to account for pxlclk being a 64-bit integer.
>=20
> Fixes: a962091227ed ("drm/komeda: Add engine clock requirement check for =
the downscaling")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  drivers/gpu/drm/arm/display/komeda/komeda_crtc.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
>=20
> diff --git a/drivers/gpu/drm/arm/display/komeda/komeda_crtc.c b/drivers/g=
pu/drm/arm/display/komeda/komeda_crtc.c
> index cafb4457e187..3f222f464eb2 100644
> --- a/drivers/gpu/drm/arm/display/komeda/komeda_crtc.c
> +++ b/drivers/gpu/drm/arm/display/komeda/komeda_crtc.c
> @@ -28,10 +28,9 @@ static void komeda_crtc_update_clock_ratio(struct kome=
da_crtc_state *kcrtc_st)
>  	}
> =20
>  	pxlclk =3D kcrtc_st->base.adjusted_mode.clock * 1000;
> -	aclk =3D komeda_calc_aclk(kcrtc_st) << 32;
> +	aclk =3D komeda_calc_aclk(kcrtc_st);
> =20
> -	do_div(aclk, pxlclk);
> -	kcrtc_st->clock_ratio =3D aclk;
> +	kcrtc_st->clock_ratio =3D div64_u64(aclk << 32, pxlclk);
>  }
> =20
>  /**
> --=20
> 2.20.0

Hi Arnd:

Thank you for the patch.

Reviewed-by: James Qian Wang (Arm Technology China) <james.qian.wang@arm.co=
m>