Re: [PATCH 07/25] serial: sh-sci: Convert from clk_get() to devm_clk_get()

Laurent Pinchart
2015-12-14T01:36:48Z

Hi Geert,

On Thursday 10 December 2015 13:30:18 Geert Uytterhoeven wrote:
> On Thu, Nov 19, 2015 at 9:38 PM, Laurent Pinchart wrote:
> > On Thursday 19 November 2015 19:38:46 Geert Uytterhoeven wrote:
> >> Transfer clock cleanup handling to the core device management code.
> >>
> >> Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
> >> ---
> >>
> >> drivers/tty/serial/sh-sci.c | 8 +++-----
> >> 1 file changed, 3 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
> >> index cba51da604253db6..9442961a198378c7 100644
> >> --- a/drivers/tty/serial/sh-sci.c
> >> +++ b/drivers/tty/serial/sh-sci.c
> >> @@ -2216,7 +2216,7 @@ static struct uart_ops sci_uart_ops = {
> >>
> >> static int sci_init_clocks(struct sci_port *sci_port, struct device
> >> *dev)
> >> {
> >>
> >> /* Get the SCI functional clock. It's called "fck" on ARM. */
> >>
> >> - sci_port->fclk = clk_get(dev, "fck");
> >> + sci_port->fclk = devm_clk_get(dev, "fck");
> >
> > Have you tested what happens if you unbind the device from the driver
> > while userspace has the serial port open ?
>
> Yes I have. And I didn't notice any user-visible behavioral differences.
>
> There is a small ordering difference, though: with clk_get() and clk_put(),
> the explicit clk_put() is done before removing the device from its PM
> Domain (which involces another call to clk_put() on the module clock):
>
> device_release_driver
> __device_release_driver
> platform_drv_remove
> sci_remove
> sci_cleanup_single
> __clk_put
> genpd_dev_pm_detach
> pm_genpd_remove_device
> pm_clk_destroy
> __pm_clk_remove
> __clk_put
>
> When using devm_clk_get(), the managed cleanup is done after the device has
> been removed from its PM Domain:
>
> device_release_driver
> __device_release_driver
> platform_drv_remove
> genpd_dev_pm_detach
> pm_genpd_remove_device
> pm_clk_destroy
> __pm_clk_remove
> __clk_put
> release_nodes
> __clk_put
>
> This shouldn't make a difference, and applies to all other drivers using
> devm_*(), and devices that are part of a PM Domain.

I was more concerned about the clk_disable_unprepare() calls. The function is
called through the uart pm handler which is called when closing the uart
(uart_close()). It's a bit difficult to follow the dynamic code paths as I'm
not very familiar with the framework, but I was concerned that the would end
up calling clk_disable_unprepare() on a clock for which we have no reference
anymore. What prevents clk_disable_unprepare() from being called in the close
path after the device has been unbound ? And what ensures that the clocks get
unprepared when unbinding the device if the port is held open by a userspace
process ?

--
Regards,

Laurent Pinchart