Re: [PATCH v2 1/2] clk: change clk_ops' ->round_rate() prototype

[Boris Brezillon <boris.brezillon@free-electrons.com>]

Hi Stephen,

Adding Mikko in the loop (after all, he was the one complaining about
this signed long limitation in the first place, and I forgot to add
him in the Cc list :-/).

Mikko, are you okay with the approach proposed by Stephen (adding a
new method) ?

On Thu, 7 May 2015 09:37:02 +0200
Boris Brezillon <boris.brezillon@free-electrons.com> wrote:

> Hi Stephen,
> 
> On Wed, 6 May 2015 23:39:53 -0700
> Stephen Boyd <sboyd@codeaurora.org> wrote:
> 
> > On 04/30, Boris Brezillon wrote:
> > > Clock rates are stored in an unsigned long field, but ->round_rate()
> > > (which returns a rounded rate from a requested one) returns a long
> > > value (errors are reported using negative error codes), which can lead
> > > to long overflow if the clock rate exceed 2Ghz.
> > > 
> > > Change ->round_rate() prototype to return 0 or an error code, and pass the
> > > requested rate as a pointer so that it can be adjusted depending on
> > > hardware capabilities.
> > > 
> > > Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
> > > Tested-by: Heiko Stuebner <heiko@sntech.de>
> > > Tested-by: Mikko Perttunen <mikko.perttunen@kapsi.fi>
> > > Reviewed-by: Heiko Stuebner <heiko@sntech.de>
> > 
> > This patch is fairly invasive, and it probably doesn't even
> > matter for most of these clock providers to be able to round a
> > rate above 2GHz.
> 
> Fair enough.
> 
> > I've been trying to remove the .round_rate op
> > from the framework by encouraging new features via the
> > .determine_rate op.
> 
> Oh, I wasn't aware of that (BTW, that's a good thing).
> Maybe this should be clearly stated (both in the struct clk_ops
> kerneldoc header and in Documentation/clk.txt).
> 
> > Sadly, we still have to do a flag day and
> > change all the .determine_rate ops when we want to add things.
> 
> Yes, but the number of clk drivers implementing ->determine_rate() is
> still quite limited compared to those implementing ->round_rate().
> 
> > 
> > What if we changed determine_rate ops to take a struct
> > clk_determine_info (or some better named structure) instead of
> > the current list of arguments that it currently takes? Then when
> > we want to make these sorts of framework wide changes we can just
> > throw a new member into that structure and be done.
> 
> I really like this idea, especially since I was wondering if we could
> pass other 'clk rate requirements' like the rounding policy (down,
> closest, up), or the maximum clk inaccuracy.
> 
> > 
> > It doesn't solve the unsigned long to int return value problem
> > though. We can solve that by gradually introducing a new op and
> > handling another case in the rounding path. If we can come up
> > with some good name for that new op like .decide_rate or
> > something then it makes things nicer in the long run. I like the
> > name .determine_rate though :/

Okay, if you want a new method, how about this one:

struct clk_adjust_rate_req {
	/* fields filled by the caller */
	unsigned long rate; /* rate is updated by the clk driver */
	unsigned long min;
	unsigned long max;

	/* fields filled by the clk driver */
	struct clk_hw *best_parent;
	unsigned long best_parent_rate;

	/*
	 * new fields I'd like to add at some point:
	 * unsigned long max_inaccuracy;
	 * something about the power consumption constraints :-)
	 */
};

int (*adjust_rate)(struct clk_hw *hw, struct clk_adjust_rate_req *req);

> 
> Why not changing the ->determine_rate() prototype. As said above, the
> number of clk drivers implementing this function is still quite
> limited, and I guess we can have an ack for all of them.
> 
> > 
> > The benefit of all this is that we don't have to worry about
> > finding the random clk providers that get added into other
> > subsystems and fixing them up. If drivers actually care about
> > this problem then they'll be fixed to use the proper op. FYI,
> > last time we updated the function signature of .determine_rate we
> > broke a couple drivers along the way.
> > 
> 
> Hm, IMHO, adding a new op is not a good thing. I agree that it eases
> the transition, but ITOH you'll have to live with old/deprecated ops in
> your clk_ops structure with people introducing new drivers still using
> the old ops (see the number of clk drivers implementing ->round_rate()
> instead of ->determine_rate()).
> 
> Best Regards,
> 
> Boris
> 



-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com