[PATCH 3/6] clk: bcm2835: enable clocks that have been enabled by firmware

[Martin Sperl]

> On 17.03.2016, at 18:39, Eric Anholt <eric at anholt.net> wrote:
> 
> Eric Anholt <eric at anholt.net> writes:
> 
>> kernel at martin.sperl.org writes:
>> 
>>> From: Martin Sperl <kernel at martin.sperl.org>
>>> 
>>> If a clock that has been enabled by the firmware gets disabled
>>> by a driver this may right now result in a crash of the system
>>> as then also the corresponding PLL_dividers as well as PLLs
>>> get disabled (if not used) - some of which are used by the
>>> VideoCore GPU (which also runs the firmware)
>>> 
>>> This patch prepares/enables those clocks that have been
>>> configured by the firmware.
>>> 
>>> Whenever the clock framework implements either
>>> CLK_IS_CRITICAL or HAND_OFF this can get changed to use this
>>> new mechanism.
>>> 
>>> For this to be completely successful (i.e not missing a clock
>>> and subsequently a pll) it is recommended to add all the known
>>> clocks of the soc so that this can get applied to all clocks.
>> 
>> I think this makes sense to have, for now at least.
>> 
>> Reviewed-by: Eric Anholt <eric at anholt.net>
> 
> Scratch that, this breaks display.  Since the clkgen clocks are flagged
> as needing to be gated in order to change dividers, it means you can't
> set clock rates for anything that was turned on at boot.

See that separate thread that triggered this:
  serial: clk: bcm2835: Strange effects when using aux-uart in console
and this patch fixes this issue.

To summarize the situation figured in the thread:
* you load the module

* you start using the tty (say by using "stty -F /dev/ttyAMA0")
* this opens the device
* this prepare the relevant clock (usage = 1)
* this prepares the parent pll-divider (usage = 1)
* this prepares the parent pll (usage = 1)

* you stop using the tty (stty closes the device)
* this release the clock
*   usage count drops to 0, so disable the clock
* this releases the parent pll-divider
*   usage count drops to 0, so disable the pll-div
* this releases the parent pll (and disables it as usage = 0)
*   usage count drops to 0, so disable the pll-div

* system crashes (with a bit of delay)

The prepare should just increase the usage so it never gets to a count of 0.

Maybe we need to use those "CLK_IS_CRITICAL” “HANDS_OFF” flags instead?
(when/if they become available)

How do you want to solve that - I have not got a DSI display, 
but HDMI continues to work...

Martin