linux-kernel - RE: [PATCH] drm/drm_vblank.c: avoid unsigned int to signed int cast

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Message-ID: <f6bd362145124f34a1af800dd330f8e9@AcuMS.aculab.com>
Date:   Wed, 17 May 2023 10:59:14 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'15330273260@....cn'" <15330273260@....cn>,
        Sui Jingfeng <suijingfeng@...ngson.cn>,
        Li Yi <liyi@...ngson.cn>
CC:     Thomas Zimmermann <tzimmermann@...e.de>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "loongson-kernel@...ts.loongnix.cn" 
        <loongson-kernel@...ts.loongnix.cn>
Subject: RE: [PATCH] drm/drm_vblank.c: avoid unsigned int to signed int cast

From: 15330273260@....cn
> Sent: 16 May 2023 18:30
> 
> From: Sui Jingfeng <suijingfeng@...ngson.cn>
> 
> Both mode->crtc_htotal and mode->crtc_vtotal are u16 type,
> mode->crtc_htotal * mode->crtc_vtotal will results a unsigned type.

Nope, u16 gets promoted to 'signed int' and the result of the
multiply is also signed.

> Using a u32 is enough to store the result, but considering that the
> result will be casted to u64 soon after. We use a u64 type directly.
> So there no need to cast it to signed type and cast back then.
....
> -		int frame_size = mode->crtc_htotal * mode->crtc_vtotal;
> +		u64 frame_size = mode->crtc_htotal * mode->crtc_vtotal;
...
> -		framedur_ns = div_u64((u64) frame_size * 1000000, dotclock);
> +		framedur_ns = div_u64(frame_size * 1000000, dotclock);

The (u64) cast is there to extend the value to 64bits, not
because the original type is signed.
The compiler will detect that the old code is a 32x32 multiply
where a 64bit result is needed, that may not be true for the
changed code (it would need to track back as far as the u16s).

It is not uncommon to force a 64bit result from a multiply
by making the constant 64bit. As in:
	div_u64(frame_size * 1000000ULL, dotclock);

	David

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