From: 15330273...@189.cn
> Sent: 16 May 2023 18:30
>
> From: Sui Jingfeng <suijingf...@loongson.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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