Re: [PATCH V3 01/10] clk: clk-divider: add CLK_DIVIDER_ZERO_GATE clk support

[Dong Aisheng <dongas86@gmail.com>]

On Tue, Jan 23, 2018 at 12:03:46PM +0100, Jerome Brunet wrote:
> On Fri, 2018-01-19 at 21:11 +0800, Dong Aisheng wrote:
> > diff --git a/include/linux/clk-provider.h b/include/linux/clk-provider.h
> > index f711be6..68ccd36 100644
> > --- a/include/linux/clk-provider.h
> > +++ b/include/linux/clk-provider.h
> > @@ -360,6 +360,7 @@ struct clk_div_table {
> >   * @shift:     shift to the divider bit field
> >   * @width:     width of the divider bit field
> >   * @table:     array of value/divider pairs, last entry should have div = 0
> > + * @cached_val: cached div hw value used for CLK_DIVIDER_ZERO_GATE
> >   * @lock:      register lock
> >   *
> >   * Clock with an adjustable divider affecting its output frequency.  Implements
> > @@ -388,6 +389,12 @@ struct clk_div_table {
> >   * CLK_DIVIDER_MAX_AT_ZERO - For dividers which are like CLK_DIVIDER_ONE_BASED
> >   *     except when the value read from the register is zero, the divisor is
> >   *     2^width of the field.
> > + * CLK_DIVIDER_ZERO_GATE - For dividers which are like CLK_DIVIDER_ONE_BASED
> 
> Unless I missed something in your patch, this comment says that, like
> CLK_DIVIDER_MAX_AT_ZERO, CLK_DIVIDER_ZERO_GATE behave as a CLK_DIVIDER_ONE_BASED
> clock
> 
> However, I don't see anything special done in _get_val() around
> CLK_DIVIDER_ZERO_GATE which means that calling _get_val() with div=2 would give
> val=1. This is more like a regular divider (when CLK_DIVIDER_ONE_BASED is not
> set)
> 
> Also, when looking for the best divider, CCF could find that the best div is 1.
> On a non-CLK_DIVIDER_ONE_BASED, this would translate to value 0 and
> (accidentally) gate the clock .
> 
> all the occurrences of CLK_DIVIDER_ZERO_GATE I have seen in patch 9 are combined
> with CLK_DIVIDER_ONE_BASED, which is probably why this potential issue has gone
> unnoticed.
> 

Yes, this feature only works with CLK_DIVIDER_ONE_BASED in current design.
Probably we should state more clearly in the code comments?

> I think CLK_DIVIDER_ZERO_GATE should just means that value 0 gate the clock, and
> just that. It should not imply what the rest of values mean.
> 

It did not imply what the reset of values mean. User needs to specify the
correct divider types. For current case, it should be CLK_DIVIDER_ONE_BASED
only.
e.g.
000b - Clock disabled
001b - Divide by 1
010b - Divide by 2

If anymore divider type want to use it, then we need extend the support
accordingly.

Theoretically any type of divider gets a 0 val (register value) is invalid for
ZERO_GATE feature. We should avoid it.

> In a more general way, I'd love to see a feature such as CLK_DIVIDER_ZERO_GATE
> added to the divider but I'm bit concerned of all the quirks we are slowly
> adding to the generic divider. It seems we are all trying re-use the algorithm
> of clk_divider_bestdiv() with different 'val-to-div' transfer function. Not too
> sure what the best solution could be though.
> 

IMHO CLK_DIVIDER_ZERO_GATE only indicates the 0 val means clk gate.
It does not assume divider types. That looks like a generic way and is exactly
what this patch intends to do. Does it make sense?

Regards
Dong Aisheng

> > + *     when the value read from the register is zero, it means the divisor
> > + *     is gated. For this case, the cached_val will be used to store the
> > + *     intermediate div for the normal rate operation, like set_rate/get_rate/
> > + *     recalc_rate. When the divider is ungated, the driver will actually
> > + *     program the hardware to have the requested divider value.
> >   */
> 
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