Re: [PATCH 12/21] ARM: tegra: clock: Add shared bus clock type

Colin Cross
2011-02-16T21:01:17Z

On Wed, Feb 16, 2011 at 12:34 PM, Stephen Boyd <sboyd@codeaurora.org> wrote:
> On 02/13/2011 01:40 AM, Colin Cross wrote:
>> +/* shared bus ops */
>> +/*
>> + * Some clocks may have multiple downstream users that need to request a
>> + * higher clock rate.  Shared bus clocks provide a unique shared_bus_user
>> + * clock to each user.  The frequency of the bus is set to the highest
>> + * enabled shared_bus_user clock, with a minimum value set by the
>> + * shared bus.
>> + */
>> +static void tegra_clk_shared_bus_update(struct clk *bus)
>> +{
>> +     struct clk *c;
>> +     unsigned long rate = bus->min_rate;
>> +
>> +     list_for_each_entry(c, &bus->shared_bus_list, u.shared_bus_user.node)
>> +             if (c->u.shared_bus_user.enabled)
>> +                     rate = max(c->u.shared_bus_user.rate, rate);
>> +
>> +     if (rate != clk_get_rate(bus))
>> +             clk_set_rate(bus, rate);
>
> What do you do if clk_set_rate() fails? Should you unwind all the state
> such as the rate and if it's enabled/disabled? Or is it safe to say
> clk_set_rate() can't fail unless the kernel is buggy in which case why
> aren't all those return -E* in the set rate functions just BUG_ONs?

In general, clk_set_rate can fail and return an error, but in this
case the failure may not be directly related to the driver that called
into tegra_clk_shared_bus_update. For example, if clk_disable is
called on a shared clock handle, the rate may drop to the rate
requested by another shared clock handle. clk_disable cannot fail, so
there's nothing that could be done with the return code, and the
problem was not caused by the driver that called clk_disable, so an
error would be meaningless.

I will modify tegra_clk_shared_bus_update to BUG on a failed
clk_set_rate, and modify tegra_clk_shared_bus_set_rate to
call clk_round_rate on the parent to ensure that the requested rate is valid.

>> +};
>> +
>> +static void tegra_clk_shared_bus_init(struct clk *c)
>> +{
>> +     c->max_rate = c->parent->max_rate;
>> +     c->u.shared_bus_user.rate = c->parent->max_rate;
>> +     c->state = OFF;
>> +#ifdef CONFIG_DEBUG_FS
>> +     c->set = 1;
>> +#endif
>> +
>> +     list_add_tail(&c->u.shared_bus_user.node,
>> +             &c->parent->shared_bus_list);
>> +}
>> +
>> +static int tegra_clk_shared_bus_set_rate(struct clk *c, unsigned long rate)
>> +{
>> +     c->u.shared_bus_user.rate = rate;
>> +     tegra_clk_shared_bus_update(c->parent);
>> +     return 0;
>> +}
>> +
>> +static long tegra_clk_shared_bus_round_rate(struct clk *c, unsigned long rate)
>> +{
>> +     return clk_round_rate(c->parent, rate);
>> +}
>> +
>> +static int tegra_clk_shared_bus_enable(struct clk *c)
>> +{
>> +     c->u.shared_bus_user.enabled = true;
>> +     tegra_clk_shared_bus_update(c->parent);
>> +     return 0;
>> +}
>
> Shouldn't you call clk_enable(c->parent)? And do you need to check for
> errors from clk_enable()?

clk_enable on the parent is handled by the clock op implementation in
mach-tegra/clock.c

>> +
>> +static void tegra_clk_shared_bus_disable(struct clk *c)
>> +{
>> +     c->u.shared_bus_user.enabled = false;
>> +     tegra_clk_shared_bus_update(c->parent);
>> +}
>
> And a similar clk_disable(c->parent) here.

Same for disable