[PATCH v2] clk: shmobile: div6: Avoid changing divisor in .disable()

[PATCH v2] clk: shmobile: div6: Avoid changing divisor in .disable()

[Geert Uytterhoeven]

While DIV6 clocks require the divisor field to be non-zero when stopping
the clock, some clocks (e.g. ZB on sh73a0) fail to be re-enabled later
if the divisor field is changed when stopping the clock.
The reason for this is unknown.

To fix this, do not touch the divisor field if it's already non-zero.

On kzm9g, the smsc911x Ethernet controller is connected to the sh73a0
Bus State Controller, which is clocked by the ZB clock. Without this
fix, if the ZB clock is disabled during system suspend, and re-enabled
during resume, the kernel locks up when the smsc911x driver tries to
access the Ethernet registers.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
---
Mike, I can queue up this patch in a branch, together with other clk-related
patches for shmobile for 3.20, and ask you to pull later.
Is that OK for you?

Thanks!

v2:
  - Add a comment to the code
  - Add a note about our (lack of) understanding of the hardware.
---
 drivers/clk/shmobile/clk-div6.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/drivers/clk/shmobile/clk-div6.c b/drivers/clk/shmobile/clk-div6.c
index 639241e31e03ec24..efbaf6c81b7530b8 100644
--- a/drivers/clk/shmobile/clk-div6.c
+++ b/drivers/clk/shmobile/clk-div6.c
@@ -54,12 +54,19 @@ static int cpg_div6_clock_enable(struct clk_hw *hw)
 static void cpg_div6_clock_disable(struct clk_hw *hw)
 {
 	struct div6_clock *clock = to_div6_clock(hw);
+	u32 val;
 
-	/* DIV6 clocks require the divisor field to be non-zero when stopping
-	 * the clock.
+	val = clk_readl(clock->reg);
+	val |= CPG_DIV6_CKSTP;
+	/*
+	 * DIV6 clocks require the divisor field to be non-zero when stopping
+	 * the clock. However, some clocks (e.g. ZB on sh73a0) fail to be
+	 * re-enabled later if the divisor field is changed when stopping the
+	 * clock
 	 */
-	clk_writel(clk_readl(clock->reg) | CPG_DIV6_CKSTP | CPG_DIV6_DIV_MASK,
-		   clock->reg);
+	if (!(val & CPG_DIV6_DIV_MASK))
+		val |= CPG_DIV6_DIV_MASK;
+	clk_writel(val, clock->reg);
 }
 
 static int cpg_div6_clock_is_enabled(struct clk_hw *hw)
-- 
1.9.1

Re: [PATCH v2] clk: shmobile: div6: Avoid changing divisor in .disable()

[Laurent Pinchart]

Hi Geert,

Thank you for the patch.

On Wednesday 17 December 2014 15:30:07 Geert Uytterhoeven wrote:
> While DIV6 clocks require the divisor field to be non-zero when stopping
> the clock, some clocks (e.g. ZB on sh73a0) fail to be re-enabled later
> if the divisor field is changed when stopping the clock.
> The reason for this is unknown.
> 
> To fix this, do not touch the divisor field if it's already non-zero.
> 
> On kzm9g, the smsc911x Ethernet controller is connected to the sh73a0
> Bus State Controller, which is clocked by the ZB clock. Without this
> fix, if the ZB clock is disabled during system suspend, and re-enabled
> during resume, the kernel locks up when the smsc911x driver tries to
> access the Ethernet registers.
> 
> Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>

Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

By the way, have you noticed the issue on any platform other than sh73a0 ? 
There might be a chance to get feedback on the hardware on a new platform, but 
not on such an old one.

> ---
> Mike, I can queue up this patch in a branch, together with other clk-related
> patches for shmobile for 3.20, and ask you to pull later.
> Is that OK for you?
> 
> Thanks!
> 
> v2:
>   - Add a comment to the code
>   - Add a note about our (lack of) understanding of the hardware.
> ---
>  drivers/clk/shmobile/clk-div6.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/clk/shmobile/clk-div6.c
> b/drivers/clk/shmobile/clk-div6.c index 639241e31e03ec24..efbaf6c81b7530b8
> 100644
> --- a/drivers/clk/shmobile/clk-div6.c
> +++ b/drivers/clk/shmobile/clk-div6.c
> @@ -54,12 +54,19 @@ static int cpg_div6_clock_enable(struct clk_hw *hw)
>  static void cpg_div6_clock_disable(struct clk_hw *hw)
>  {
>  	struct div6_clock *clock = to_div6_clock(hw);
> +	u32 val;
> 
> -	/* DIV6 clocks require the divisor field to be non-zero when stopping
> -	 * the clock.
> +	val = clk_readl(clock->reg);
> +	val |= CPG_DIV6_CKSTP;
> +	/*
> +	 * DIV6 clocks require the divisor field to be non-zero when stopping
> +	 * the clock. However, some clocks (e.g. ZB on sh73a0) fail to be
> +	 * re-enabled later if the divisor field is changed when stopping the
> +	 * clock
>  	 */
> -	clk_writel(clk_readl(clock->reg) | CPG_DIV6_CKSTP | CPG_DIV6_DIV_MASK,
> -		   clock->reg);
> +	if (!(val & CPG_DIV6_DIV_MASK))
> +		val |= CPG_DIV6_DIV_MASK;
> +	clk_writel(val, clock->reg);
>  }
> 
>  static int cpg_div6_clock_is_enabled(struct clk_hw *hw)

-- 
Regards,

Laurent Pinchart

Re: [PATCH v2] clk: shmobile: div6: Avoid changing divisor in .disable()

[Geert Uytterhoeven]

Hi Laurent,

On Wed, Dec 17, 2014 at 3:40 PM, Laurent Pinchart
<laurent.pinchart@ideasonboard.com> wrote:
> On Wednesday 17 December 2014 15:30:07 Geert Uytterhoeven wrote:
>> While DIV6 clocks require the divisor field to be non-zero when stopping
>> the clock, some clocks (e.g. ZB on sh73a0) fail to be re-enabled later
>> if the divisor field is changed when stopping the clock.
>> The reason for this is unknown.
>>
>> To fix this, do not touch the divisor field if it's already non-zero.
>>
>> On kzm9g, the smsc911x Ethernet controller is connected to the sh73a0
>> Bus State Controller, which is clocked by the ZB clock. Without this
>> fix, if the ZB clock is disabled during system suspend, and re-enabled
>> during resume, the kernel locks up when the smsc911x driver tries to
>> access the Ethernet registers.
>>
>> Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
>
> Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

Thanks!

> By the way, have you noticed the issue on any platform other than sh73a0 ?
> There might be a chance to get feedback on the hardware on a new platform, but
> not on such an old one.

No, it doesn't seem to happen with e.g. the SD and MMC clocks on r8a7791,
which are also div6.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds