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

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Message-ID: <1466956.fL4LZVx6Ru@avalon>
Date:	Sun, 13 Dec 2015 21:39:02 +0200
From:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Geert Uytterhoeven <geert+renesas@...der.be>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Simon Horman <horms@...ge.net.au>,
	Magnus Damm <magnus.damm@...il.com>,
	Yoshinori Sato <ysato@...rs.sourceforge.jp>,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	Linux-sh list <linux-sh@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>
Subject: Re: [PATCH 07/25] serial: sh-sci: Convert from clk_get() to devm_clk_get()

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@...der.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

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