Re: [v5] clk: add si5351 i2c common clock driver

[Sebastian Hesselbarth]

On 04/08/2013 07:36 PM, Mike Turquette wrote:
> Quoting Sebastian Hesselbarth (2013-04-08 08:38:44)
> > On Mon, Apr 8, 2013 at 4:54 PM, Guenter Roeck<linux@xxxxxxxxxxxx>  wrote:
> > > On Mon, Apr 08, 2013 at 08:11:38AM +0200, Sebastian Hesselbarth wrote:
> > > > On 04/08/2013 02:17 AM, Guenter Roeck wrote:
> > > > > On Mon, Apr 08, 2013 at 01:49:24AM +0200, Sebastian Hesselbarth wrote:
> > > > > > On 04/08/2013 12:50 AM, Guenter Roeck wrote:
> > > > > > > On Fri, Apr 05, 2013 at 05:23:35AM -0000, Sebastian Hesselbarth wrote:
> > > > > > > > This patch adds a common clock driver for Silicon Labs Si5351a/b/c
> > > > > > > > i2c programmable clock generators. Currently, the driver supports
> > > > > > > > DT kernels only and VXCO feature of si5351b is not implemented. DT
> > > > > > > > bindings selectively allow to overwrite stored Si5351 configuration
> > > > > > > > which is very helpful for clock generators with empty eeprom
> > > > > > > > configuration. Corresponding device tree binding documentation is
> > > > > > > > also added.
> > > > > > > >
> > > > > > > > Signed-off-by: Sebastian Hesselbarth<sebastian.hesselbarth@xxxxxxxxx>
> > > > > > > > Tested-by: Daniel Mack<zonque@xxxxxxxxx>
> > > > > > > [ ... ]
> > > > > > >
> > > > > > > > +static inline void _si5351_msynth_set_pll_master(
> > > > > > > > + struct si5351_driver_data *drvdata, unsigned char num, int is_master)
> > > > > > > > +{
> > > > > > > > + unsigned long flags;
> > > > > > > > +
> > > > > > > > + if (num>    8 ||
> > > > > > > > +     (drvdata->variant == SI5351_VARIANT_A3&&    num>    3))
> > > > > > > > +         return;
> > > > > > > > +
> > > > > > > > + flags = __clk_get_flags(drvdata->msynth[num].hw.clk);
> > > > > > > > + if (is_master)
> > > > > > > > +         flags |= CLK_SET_RATE_PARENT;
> > > > > > > > + else
> > > > > > > > +         flags&= ~CLK_SET_RATE_PARENT;
> > > > > > > > + __clk_set_flags(drvdata->msynth[num].hw.clk, flags);
> > > > > > > > +}
> > > > > > > > +
> > > > > > > Unless I am missing something, neither __clk_get_flags() nor the new
> > > > > > > __clk_set_flags is exported.
> > > > > > >
> > > > > > > Did you try to build and load the driver as module ?
> > > > > >
> > > > > > Well, good catch. I didn't try to build v5 as a module, but I guess it
> > > > > > will fail. But I consider this as something that has to be addressed in
> > > > > > clk framework itself, not in this patch. There will be other
> > > > > > clk-providers built as module in the future for sure.
> > > > > Sure, but you provided the patch to make __clk_set_flags global. To avoid
> > > > > build failures, I would suggest to either submit a patch to export the
> > > > > missing functions, or to remove the ability to build the driver as module.
> > > >
> > > > Actually, I knew that __clk_set_flags patch will not be accepted
> > > > before posting it ;)
> > > Ah, but part of that is to get you to think about it again, and to defend it if
> > > it is really needed. After all, "it can be abused" applies to pretty much every
> > > API.
> >
> > Guenter,
> >
> > I already thought about it a lot and I consider clk api broken in a way here.
> >
> > > Key question is if you _really_ need run-time flag modifications, or if you can
> > > live with initialization-time settings. If you really need it, you'll have to
> > > explain the reasons.
> >
> > The question is not if _I_ really need run-time flags but why the api allows to
> > perform run-time modifications of the clock hierarchy without allowing different
> > flags? There is clk_set_parent() so I guess clk api knows about run-time changes
> > already, but you cannot have different flags per parent. And with
> > __clk_set_flags()
> > rejected, you are not allowed to change the flags.
> >
> > I understand that some flags are permanent and required at registration, but
> > CLK_SET_PARENT_RATE is not. It is not limited by hardware but limited by api.
> > One way would be a more generic clk-mux with per parent flags, but for
> > the current
> > implementation, I cannot see how clk-mux can be exploited here.
>
> There are a couple of ways to get past this issue.  One is removal of
> some of the flags.  I have never liked CLK_SET_RATE_PARENT, since I
> think the ability to propagate a rate-change request up to the parent
> should be enabled for all clocks.  This is in the interest of the driver
> author who does not care to know intimate details of the clock
> hierarchy.
>
> When I developed the rate-change propagation code I was likely too hasty
> in defining a per-clock flag for that behavior.  It might have been
> enough to simply compare the&best_parent_rate value from .round_rate
> and compare it against clk->parent->rate.  This means that no flag is
> necessary.
>
> This assumes that the .round_rate implementation has enough knowledge to
> know whether or not to propagate the rate-change request up to the
> parent.  This may not not true for the common divider type.
>
> I'll make some tests on removing this flag (and potentially other flags)
> to see how painful it is.

Mike,

another good thing would be to have implementation independent building 
blocks for clk drivers. I think clk-fixed-rate, clk-gate, clk-mux and
some clk-synth should be enough. The current core clk templates in the
api are almost all special implementations.

Sebastian
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