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)
