Re: [PATCH v4 02/10] clk: Always clamp the rounded rate

[Stephen Boyd <sboyd@kernel.org>]

Quoting Maxime Ripard (2022-02-21 08:18:21)
> Hi,
> 
> On Fri, Feb 18, 2022 at 03:15:06PM -0800, Stephen Boyd wrote:
> > Quoting Maxime Ripard (2022-01-25 06:15:41)
> > > The current core while setting the min and max rate properly in the
> > > clk_request structure will not make sure that the requested rate is
> > > within these boundaries, leaving it to each and every driver to make
> > > sure it is.
> > 
> > It would be good to describe why. Or decide that it was an oversight and
> > write that down here.
> > 
> > > Add a clamp call to make sure it's always done, and add a few unit tests
> > > to make sure we don't have any regression there.
> > 
> > I looked through the per-user constraint patch history on the list but I
> > couldn't really figure out why it was done this way. I guess we didn't
> > clamp the rate in the core because we wanted to give the clk providers
> > all the information, i.e. the rate that was requested and the boundaries
> > that the consumers have placed on the rate.
> 
> I'm not really sure we should really leave it to the users, something like:
> 
> clk_set_range_rate(clk, 1000, 2000);
> clk_set_rate(clk, 500);
> clk_get_rate(clk) # == 500
> 
> Is definitely weird, and would break the least surprise :)
> 
> We shouldn't leave that to drivers, especially since close to none of
> them handle this properly.

Ok.

> 
> > With the round_rate() clk_op the providers don't know the min/max
> > because the rate request structure isn't passed. I think my concern a
> > long time ago was that a consumer could call clk_round_rate() and get
> > one frequency and then call clk_set_rate() and get another frequency.
> 
> I'm not sure I follow you there.
> 
> The function affected is clk_core_determine_round_nolock(), which is
> called by clk_core_round_rate_nolock() and clk_calc_new_rates(). In
> turn, they will be part of clk(_hw_)_round_clock for the former, and
> clk_core_set_rate_nolock() (and thus clk_set_rate()) for the latter.
> 
> I don't see how you can get a discrepancy between clk_round_rate() and
> clk_set_rate().
> 
> And yeah, it's true that the round_rate op won't have the min and max
> passed to them, but i'd consider this an argument for doing this check
> here, since you don't have that option at all for those clocks.

When the range setting API was introduced the rounding logic and the
rate setting logic didn't use the same code paths. It looks like that
code got consolidated now though so we should be fine.