Locking in the clk API

Saravana Kannan skannan at codeaurora.org
Thu Jan 27 22:27:25 EST 2011

On 01/27/2011 12:43 PM, Russell King - ARM Linux wrote:
> On Thu, Jan 27, 2011 at 12:30:36PM -0800, Saravana Kannan wrote:
>> On 01/27/2011 12:54 AM, Russell King - ARM Linux wrote:
>>> On Wed, Jan 26, 2011 at 08:34:20PM -0800, Saravana Kannan wrote:
>>>> I'm not too familiar with serial/tty, does anyone know if the
>>>> .set_termios needs to be atmoic? If not, we could just change
>>>> cpm_uart/cpm_uart_core.c to use mutex instead of spinlock.
>>> The locking is there to protect against the interrupt handler accessing
>>> the port->* stuff (which seems to have been forgotten by the cpm driver).
>>> I don't see any reason why clk_set_rate() needs to be under the spinlock
>>> there - we need the reprogramming of the baud rate within the spinlock
>>> on 8250 because of DLAB bit hiding the data registers.  It's also a good
>>> idea that it _is_ within the spinlock along with uart_update_timeout()
>>> to ensure timeouts and the baud rate are updated together.
>> For internal tree purposes, does .set_termios need to be atomic? Can it
>> grab mutexes instead of spinlock?
> I think I already answered that question above where I said "protect
> against the interrupt handler accessing the port->* stuff".
>> Going back to the topic, how about CPU freq drivers possibly using
>> clk_set_rate() to change freq? Do you think that's not the case or a
>> concern?
> CPUfreq drivers probably should busy-wait until the CPU PLL has locked
> if the CPU is allowed to continue running while the PLL relocks.  Some
> implementations will halt the CPU while the PLL is transitioning and
> that's really not unreasonable to do.
> I think some even require the code to be running out of SRAM and SDRAM
> remain untouched while the PLL is transitioning (omap maybe?)

Looks like you are confident to consider clk_set_rate() as sleepable. 
Can we add a comment to clk.h that says so?

Otherwise, there is no point in the suggesting the 
clk_prepare/unprepare() APIs.

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