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

[Mikko Perttunen]

On 05/15/2015 06:40 PM, Boris Brezillon wrote:
> 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 :-/).

I think I got it through linux-tegra anyway, but thanks :)

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

Yes, sounds good to me. If a driver uses the existing methods with too 
large frequencies, the issue is pretty discoverable anyway. I think 
"adjust_rate" sounds a bit too much like it sets the clock's rate, 
though; perhaps "adjust_rate_request" or something like that?

Thanks,
Mikko

> On Thu, 7 May 2015 09:37:02 +0200
> Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx> wrote:
>
> > Hi Stephen,
> >
> > On Wed, 6 May 2015 23:39:53 -0700
> > Stephen Boyd <sboyd@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
> > > > Tested-by: Heiko Stuebner <heiko@xxxxxxxxx>
> > > > Tested-by: Mikko Perttunen <mikko.perttunen@xxxxxxxx>
> > > > Reviewed-by: Heiko Stuebner <heiko@xxxxxxxxx>
> > >
> > > 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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/