Re: [Patch v2 1/2] 5200/mpc: improve i2c bus error recovery

["Albrecht Dreß" <albrecht.dress@arcor.de>]

Hi Joakim:

[snip]
> >   static void mpc_i2c_fixup(struct mpc_i2c *i2c)
> >   {
> > -   writeccr(i2c, 0);
> > -   udelay(30);
> > -   writeccr(i2c, CCR_MEN);
> > -   udelay(30);
> > -   writeccr(i2c, CCR_MSTA | CCR_MTX);
> > -   udelay(30);
> > -   writeccr(i2c, CCR_MSTA | CCR_MTX | CCR_MEN);
> > -   udelay(30);
> > -   writeccr(i2c, CCR_MEN);
> > -   udelay(30);
> > +   int k;
> > +   u32 delay_val =3D 1000000 / i2c->real_clk + 1;
> > +
> > +   if (delay_val < 2)
> > +      delay_val =3D 2;
> > +
> > +   for (k =3D 9; k; k--) {
> > +      writeccr(i2c, 0);
> > +      writeccr(i2c, CCR_MSTA | CCR_MTX | CCR_MEN);
> > +      udelay(delay_val);
> > +      writeccr(i2c, CCR_MEN);
> > +      udelay(delay_val << 1);
> > +   }
> >   }
>=20
> I am curious, didn't old method work with by just wrapping
> a for(k=3D9; k; k--) around it? How did the wave form look?

Sure does that work!  The waveform was somewhat "streched", mainly due to t=
he delays between some of the writeccr() calls which don't change the sda/s=
cl lines.  Unfortunately I didn't take shots from the scope.

However, for *one* cycle, the old code needed (only counting the udelay's) =
150 us.  For 9 cycles, it's 1.35 ms, which isn't really nice ;-).  At 375 k=
Hz real clock rate, delay_val is 3, i.e. each cycle consumes 9 us, or 81 us=
 for the whole fixup procedure.  If the clock is slower, the gain is of cou=
rse a lot smaller, and at 20.5 kHz each cycle again needs 150 us...

My feeling is that the delays used in the old code are just "some" values w=
hich work for sure, to if you like, my change is basically optimisation...

BTW, related to your earlier question, I checked the timings recorded with =
the scope at 100 and at 20 kHz against the nxp's "I2C bus specification and=
 user manual", rev. 03 - everything seems to be fine.

Thanks, Albrecht.

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