bug in PL011 console
Uwe Kleine-König
u.kleine-koenig at pengutronix.de
Thu Dec 23 10:42:52 EST 2010
On Thu, Dec 23, 2010 at 03:08:41PM +0000, Russell King - ARM Linux wrote:
> On Thu, Dec 23, 2010 at 04:02:34PM +0100, Uwe Kleine-König wrote:
> > Steven told me on irc that sleeping was not allowed in the console write
> > callback. Maybe this didn't show up earlier because not all clk
> > implementations sleep as mxs' does.
> >
> > I think the only possible fix is to do the clk_enable in the setup
> > callback instead of per-write.
> >
> > Will send a patch as follow up.
>
> We really need to sort out what's expected from the CLK API. The drivers
> I write assume that it's absolutely fine to call clk_enable/clk_disable
> from IRQ context, and for the platforms I implemented the CLK API for,
> that's absolutely true.
The common struct clk patch[1] by Jeremy Kerr sleeps, too. And I think
most people who commented to this series thought that this is the right
behaviour.
> I'd lobby for it because it allows for proper power saving management of
> clocks for devices - PL011 only enables the clock when either the port is
> open or it's actually sending data out the port. So it's doing absolutely
> the best power management that can be done with UARTs.
Yeah, that makes fine-grained clk enabling harder/impossible. So
ideally we'd have something that only makes clk_enable sleep iff
that's sensible for that clk. And if you have a clock that can be
enabled "fast" it would not sleep.
Don't know if that works, maybe something like that:
int clk_enable(struct clk *clk)
{
spin_lock(something);
if (clk->flags & (SOME|FLAGS))
goto out_busy;
clk->flags |= ENABLING;
spin_unlock(something);
ret = clk->really_enable(...);
spin_lock(something);
clk->flags &= ~ENABLING;
spin_unlock(something);
}
Some things that need careful consideration are:
- clk->flags already has ENABLING when clk_enable is entered.
(needs to sleep/poll then?)
- clk->usecount already > 0
(early return unless clk->flags & DISABLING)
- do we need the irqsaving spinlock variants?
(I assume yes)
Probably there are more.
Best regards
Uwe
[1] last submission: http://thread.gmane.org/gmane.linux.kernel/1073751
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