Re: [PATCH v8 7/8] i2c: rk3x: add i2c support for rk3399 soc

[David.Wu]

Hi Dougï

å 2016/5/12 9:08, David.Wu åé:
> Hi Doug,
>
> å 2016/5/12 1:37, Doug Anderson åé:
> > Hi,
> >
> > On Tue, May 10, 2016 at 12:31 PM, David Wu <david.wu@xxxxxxxxxxxxxx>
> > wrote:
> > >   static void rk3x_i2c_adapt_div(struct rk3x_i2c *i2c, unsigned long
> > > clk_rate)
> > >   {
> > >          struct i2c_timings *t = &i2c->t;
> > >          struct rk3x_i2c_calced_timings calc;
> > >          u64 t_low_ns, t_high_ns;
> > > +       u32 val;
> > >          int ret;
> > >
> > > -       ret = rk3x_i2c_calc_divs(clk_rate, t, &calc);
> > > +       ret = i2c->soc_data->calc_timings(clk_rate, t, &calc);
> > >          WARN_ONCE(ret != 0, "Could not reach SCL freq %u",
> > > t->bus_freq_hz);
> > >
> > > -       clk_enable(i2c->clk);
> > > +       clk_enable(i2c->pclk);
> > > +
> > >          i2c_writel(i2c, (calc.div_high << 16) | (calc.div_low &
> > > 0xffff),
> > >                     REG_CLKDIV);
> > > -       clk_disable(i2c->clk);
> > > +
> > > +       val = i2c_readl(i2c, REG_CON);
> > > +       val &= ~REG_CON_TUNING_MASK;
> > > +       val |= calc.tuning;
> > > +       i2c_writel(i2c, val, REG_CON);
> >
> > Another subtle bug here.  You need to be holding the spinlock here
> > since you're doing a read-modify-write of a register that is also
> > touched by the interrupt handler.  We never needed it before because
> > the previous register update wasn't touched by anyone else and it was
> > a single atomic write.
> >
> > Also: technically if we are midway through a transfer when all this
> > happens then there will be a very short period of time when the two
> > timing-related registers won't match with each other.  I have no idea
> > how much that would matter, but in the very least it seems wise to
> > minimize the time where they mismatch.  So I'd probably write:
> >
> >         spin_lock_irqsave(&i2c->lock, flags);
> >         val = i2c_readl(i2c, REG_CON);
> >         val &= ~REG_CON_TUNING_MASK;
> >         val |= calc.tuning;
> >         i2c_writel(i2c, val, REG_CON);
> >         i2c_writel(i2c, (calc.div_high << 16) | (calc.div_low & 0xffff),
> >                    REG_CLKDIV);
> >         spin_unlock_irqrestore(&i2c->lock, flags);
> >
> > ...if we really end up with on a system with a dynamically changing
> > clock that uses the new-style timing and we see real problems, we can
> > always try to come up with a way to avoid any problems.  Sound OK?
>
> Good, add spin_lock is very necessary for atomic write here, thanks for
> your advice.

I also found a subtle bug, it would clean the con register by use 
"i2c_writel(i2c, 0, REG_CON)" in rk3x_i2c_stop(). So i think it would be 
fixed by using read-modify-write way here in next version.

> > Otherwise, I think things look good to me.  Caesar's comments would
> > also be good to fix.
> >
> >
> > -Doug