Re: [PATCH v10 04/27] clocksource: Add a new timer-ingenic driver

[Paul Cercueil]

Hi Thierry,

Le ven. 8 mars 2019 à 7:22, Thierry Reding <thierry.reding@xxxxxxxxx> 
a écrit :
> On Mon, Mar 04, 2019 at 07:13:05PM +0100, Paul Cercueil wrote:
> >  Hi Thierry,
> >
> >  On Mon, Mar 4, 2019 at 1:22 PM, Thierry Reding 
> > <thierry.reding@xxxxxxxxx>
> >  wrote:
> >  > On Sat, Mar 02, 2019 at 08:33:50PM -0300, Paul Cercueil wrote:
> >  > [...]
> >  > >  diff --git a/drivers/clocksource/ingenic-timer.c
> >  > > b/drivers/clocksource/ingenic-timer.c
> [...]
> >  > >  +static u64 notrace ingenic_tcu_timer_read(void)
> >  > >  +{
> >  > >  +	unsigned int channel = ingenic_tcu->cs_channel;
> >  > >  +	u16 count;
> >  > >  +
> >  > >  +	count = readw(ingenic_tcu_base + TCU_REG_TCNTc(channel));
> >  >
> >  > Can't yo do this via the regmap?
> >
> >  I could, but for the sched_clock to be precise the function must 
> > return
> >  as fast as possible. That's the rationale behind the use of readw() 
> > here.
> >
> >  That's also the reason why ingenic_tcu_base is global and exported, 
> > so
> >  that the OS Timer driver can use it as well.
>
> Is the path through the regmap really significantly slower than the
> direct register access? Anyway, if you need the global anyway, might 
> as
> well use it.

About 10% slower.

> [...]
> >  > >  +	// Register the sched_clock at the very end as there's no 
> > way to
> >  > > undo it
> >  > >  +	rate = clk_get_rate(tcu->cs_clk);
> >  > >  +	sched_clock_register(ingenic_tcu_timer_read, 16, rate);
> >  >
> >  > Oh wow... so you managed to nicely encapsulate everything and now 
> > this
> >  > seems to be the only reason why you need to rely on global 
> > variables.
> >  >
> >  > That's unfortunate. I suppose we could go and add a void *data 
> > parameter
> >  > to sched_clock_register() and pass that to the read() function. 
> > That way
> >  > you could make this completely independent of global variables, 
> > but
> >  > there are 73 callers of sched_clock_register() and they are 
> > spread all
> >  > over the place, so sounds a bit daunting to me.
> >
> >  Yes, that's the main reason behind the use of a global variables.
> >  Is there a way we could introduce another callback, e.g. 
> > .read_value(),
> >  that would receive a void *param? Then the current .read() callback
> >  can be deprecated and the 73 callers can be migrated later.
>
> Yeah, I suppose that would be possible. I'll defer to Daniel and 
> Thomas
> on this. They may not consider this important enough.
>
> >  > >  +
> >  > >  +	return 0;
> >  > >  +
> >  > >  +err_tcu_clocksource_cleanup:
> >  > >  +	ingenic_tcu_clocksource_cleanup(tcu);
> >  > >  +err_tcu_clk_cleanup:
> >  > >  +	ingenic_tcu_clk_cleanup(tcu, np);
> >  > >  +err_tcu_intc_cleanup:
> >  > >  +	ingenic_tcu_intc_cleanup(tcu);
> >  > >  +err_clk_disable:
> >  > >  +	clk_disable_unprepare(tcu->clk);
> >  > >  +err_clk_put:
> >  > >  +	clk_put(tcu->clk);
> >  > >  +err_free_regmap:
> >  > >  +	regmap_exit(tcu->map);
> >  > >  +err_iounmap:
> >  > >  +	iounmap(base);
> >  > >  +	release_mem_region(res.start, resource_size(&res));
> >  > >  +err_free_ingenic_tcu:
> >  > >  +	kfree(tcu);
> >  > >  +	return ret;
> >  > >  +}
> >  > >  +
> >  > >  +TIMER_OF_DECLARE(jz4740_tcu_intc,  "ingenic,jz4740-tcu",
> >  > > ingenic_tcu_init);
> >  > >  +TIMER_OF_DECLARE(jz4725b_tcu_intc, "ingenic,jz4725b-tcu",
> >  > > ingenic_tcu_init);
> >  > >  +TIMER_OF_DECLARE(jz4770_tcu_intc,  "ingenic,jz4770-tcu",
> >  > > ingenic_tcu_init);
> >  > >  +
> >  > >  +
> >  > >  +static int __init ingenic_tcu_probe(struct platform_device 
> > *pdev)
> >  > >  +{
> >  > >  +	platform_set_drvdata(pdev, ingenic_tcu);
> >  >
> >  > Then there's also this. Oh well... nevermind then.
> >
> >  The content of ingenic_tcu_platform_init() could be moved inside
> >  ingenic_tcu_init(). But can we get a hold of the struct device 
> > before the
> >  probe function is called? That would allow to set the drvdata and 
> > regmap
> >  without relying on global state.
>
> I'm not sure if the driver core is ready at this point. It's likely it
> isn't, otherwise there'd be no need for TIMER_OF_DECLARE(), really. I
> also vaguely recall looking into this a few years ago because of some
> similar work I was but I eventually gave up because I couldn't find a
> way that would allow both the code to execute early enough and still
> use the regular driver model.
>
> Platform device become available at arch_initcall_sync (3s), while the
> TIMER_OF_DECLARE code runs way earlier than any of the initcalls, so I
> don't think there's a way to do it. Unless perhaps if the timers can
> be initialized later. I'm not sure if that's possible, and might not 
> be
> worth it just for the sake of being pedantic.
>
> Thierry

I think for the time being I can't really go around using the global 
variables.
Hopefully that's something that can be fixed in the future.

-Paul