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

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Date:   Wed, 9 Nov 2016 12:54:18 -0800
From:   Doug Anderson <dianders@...gle.com>
To:     William Wu <wulf@...k-chips.com>,
        Heiko Stübner <heiko@...ech.de>
Cc:     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

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 ...)

-Doug