RFC: Issues using runtime API's with console uart

* RFC: Issues using runtime API's with console uart
@ 2011-07-12 11:44 ` Govindraj
  0 siblings, 0 replies; 5+ messages in thread
From: Govindraj @ 2011-07-12 11:44 UTC (permalink / raw)
  To: linux-omap
  Cc: Kevin Hilman, Paul Walmsley, Basak, Partha, Tony Lindgren,
	Sripathy, Vishwanath, linux-arm-kernel, Rajendra Nayak,
	Shilimkar, Santosh

Hi All,

With recent uart runtime conversion I am facing some issues with runtime
API usage with console.

With runtime adaptation the clock cutting is done aggressively with get and put.

as below with console_write API in uart code.

serial_omap_console_write(.....)
{
      get_sync();
      .
      .
      .
      put();
}

serial_omap_set_mctrl(..)
{
     get_sync();
      .
      .
      .
      put()
}

Now if debug messages(pr_debug) from hwmod and omap_device are enabled
then get_sync and put will internally have prints which get redirected to
console_write and will call get_sync.

so from a runtime API context I call another runtime API which leads
to power lock contention, since every runtime API entrant tries to get
a power lock.

To elaborate further from above code snippets consider set_mctrl
calling get_sync and printk from omap_hwmod calling console_write
which calls get_sync.

-> leads to get_sync calling get_sync.

similarly with put_sync having prints calling console driver calling get_sync
leading to power_lock contention.

As of today I don't see any clean approaches to be available.

Some of the approaches I am considering is

1.) let the clock gating for uart be handled with idle path as done today with
     prepare and resume calls.

2.) Aggressively binding all get and put with console lock to suppress
     prints from get/put and to printed during console_unlock.

    uart_port_enb(..)
    {
          if (console_trylock()) {
              get_sync();
              console_unlock();
          }
    }

    Even this approach doesn't seem fool proof, Similarly if I have port_disable
    console unlock(has not yet released the console lock)
    will have a print stating uart clock is disabled will call console_write
    which will call port_enb since above trylock fails and eventually
    might oops out trying
    to print the message as put has already disabled the uart clocks.

I don't see any clean method rather using approach[1] as of today.

So any suggestions on any other approach will be helpful.

---
Thanks,
Govindraj.R

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