[Intel-gfx] [PATCH 2/2] drm/i915: Simplify some icl pll calculations

[Chris Wilson]

Quoting Ville Syrjälä (2019-04-10 18:59:52)
> On Mon, Apr 08, 2019 at 06:05:06PM +0100, Chris Wilson wrote:
> > Quoting Ville Syrjälä (2019-04-08 17:06:01)
> > > On Mon, Apr 08, 2019 at 04:49:13PM +0100, Chris Wilson wrote:
> > > > Quoting Ville Syrjala (2019-04-08 16:27:02)
> > > > > -       /*
> > > > > -        * Adjust the original formula to delay the division by 2^22 in order to
> > > > > -        * minimize possible rounding errors.
> > > > > -        */
> > > > > -       tmp = (u64)m1 * m2_int * ref_clock +
> > > > > -             (((u64)m1 * m2_frac * ref_clock) >> 22);
> > > > > -       tmp = div_u64(tmp, 5 * div1 * div2);
> > > > > -
> > > > > -       return tmp;
> > > > > +       return div_u64(mul_u32_u32(ref_clock * m1, m2),
> > > > > +                      (5 * div1 * div2) << 22);
> > > > 
> > > > You say the denominator here is a u64, so do you not need to cast
> > > > (u64)(5 * d1 * d2) to ensure it doesn't overflow the shift?
> > > 
> > > It should fit into u32. The maximum value should be
> > > <= (5*0xf*0x7)<<22 based on the number of bits available
> > 3b * 4b * 3b = 10b. So just fits.
> > 
> > Is it worth asserting those limits? Feels like it is running close, and
> > will be subject to cargo-culting.
> 
> I suppose checking for it might be a good idea.
> 
> Just 'WARN_ON(5 * div1 * div2 >= 1 << 10)' maybe, or were you thinking
> of something fancier?

How about something like
struct {
	unsigned int div1 : 3;
	unsigned int div2 : 3;
} d;

then with a bit of luck smatch will spot an overflow, and people might
think twice when copying?

Even weirder,
add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-29 (-29)
Function                                     old     new   delta
intel_ddi_get_config                        2377    2348     -29

I dread to look into the function to see how that changed gcc's mind.
-Chris