Re: Locking in the clk API

From: Saravana Kannan
Date: Thu Jan 27 2011 - 15:30:46 EST

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?

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?


Do any one of your mach's control CPU freq using clk_set_rate() and does it need to be atomic? CPUfreq doesn't need it to be atomic. So, you will need clk_set_rate() to be atomic only if you try to use it to lower CPU freq very late during idle/suspend.


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