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

[David Laight]

From: 15330273260@xxxxxx
> Sent: 16 May 2023 18:30
> 
> From: Sui Jingfeng <suijingfeng@xxxxxxxxxxx>
> 
> 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

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)