linux-kernel - Re: [PATCH] phy: rockchip-inno-usb2: correct 480MHz output clock stable time

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Date:   Thu, 10 Nov 2016 10:21:51 +0100
From:   Heiko Stübner <heiko@...ech.de>
To:     wlf <wulf@...k-chips.com>
Cc:     Doug Anderson <dianders@...gle.com>,
        Kishon Vijay Abraham I <kishon@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Wang <frank.wang@...k-chips.com>,
        黄涛 <huangtao@...k-chips.com>,
        Brian Norris <briannorris@...gle.com>,
        Guenter Roeck <groeck@...gle.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        linux-clk <linux-clk@...r.kernel.org>
Subject: Re: [PATCH] phy: rockchip-inno-usb2: correct 480MHz output clock stable time

Am Donnerstag, 10. November 2016, 10:54:49 schrieb wlf:
> Hi Doug,
> 
> 在 2016年11月10日 04:54, Doug Anderson 写道:
> > Hi,
> > 
> > On Mon, Nov 7, 2016 at 5:00 AM, William Wu <wulf@...k-chips.com> wrote:
> >> We found that the system crashed due to 480MHz output clock of
> >> USB2 PHY was unstable after clock had been enabled by gpu module.
> >> 
> >> Theoretically, 1 millisecond is a critical value for 480MHz
> >> output clock stable time, so we try to change the delay time
> >> to 1.2 millisecond to avoid this issue.
> >> 
> >> Signed-off-by: William Wu <wulf@...k-chips.com>
> >> ---
> >> 
> >>   drivers/phy/phy-rockchip-inno-usb2.c | 2 +-
> >>   1 file changed, 1 insertion(+), 1 deletion(-)
> >> 
> >> diff --git a/drivers/phy/phy-rockchip-inno-usb2.c
> >> b/drivers/phy/phy-rockchip-inno-usb2.c index ecfd7d1..8f2d2b6 100644
> >> --- a/drivers/phy/phy-rockchip-inno-usb2.c
> >> +++ b/drivers/phy/phy-rockchip-inno-usb2.c
> >> @@ -267,7 +267,7 @@ static int rockchip_usb2phy_clk480m_enable(struct
> >> clk_hw *hw)>> 
> >>                          return ret;
> >>                  
> >>                  /* waitting for the clk become stable */
> >> 
> >> -               mdelay(1);
> >> +               udelay(1200);
> > 
> > Several people who have seen this patch have expressed concern that a
> > 1.2 ms delay is pretty long for something that's supposed to be
> > "atomic" like a clk_enable().  Consider that someone might call
> > clk_enable() while interrupts are disabled and that a 1.2 ms interrupt
> > latency is not so great.
> > 
> > It seems like this clock should be moved to be enabled in "prepare"
> > and the "enable" should be a no-op.  This is a functionality change,
> > but I don't think there are any real users for this clock at the
> > moment so it should be fine.
> > 
> > (of course, the 1 ms latency that existed before this patch was still
> > pretty bad, but ...)
> 
> Thanks a lot for your suggestion.
> I agree with you.  clk_enable() will call spin_lock_irqsave() to disable
> interrupt, and we add
> more than 1ms in clk_enable may cause big latency.
> 
> And according to clk_prepare() description:
>   In a simple case, clk_prepare can be used instead of clk_enable to
> ungate a clk if the
>   operation may sleep.  One example is a clk which is accessed over I2c.
> 
> So maybe we can remove the clock to clk_prepare.
> 
> Hi Heiko, Frank,
>         What  do you think of it?

moving to clk_prepare sounds sensible. That way you can switch from delay to 
sleep functions as well.


Heiko