linux-kernel - Re: [PATCH] usb: dwc3: gadget: don't rely on jiffies while holding spinlock

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Date:	Tue, 16 Aug 2016 10:12:23 +0300
From:	Felipe Balbi <balbi@...nel.org>
To:	Nicolas Saenz Julienne <nicolassaenzj@...il.com>
Cc:	gregkh@...uxfoundation.org, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: dwc3: gadget: don't rely on jiffies while holding spinlock


Hi,

Nicolas Saenz Julienne <nicolassaenzj@...il.com> writes:
> __dwc3_gadget_wakeup() is called while holding a spinlock, then depends on
> jiffies in order to timeout while polling the USB core for a link state
> update. In the case the wakeup failed, the timeout will never happen and
> will also cause the cpu to stall until rcu_preempt kicks in.
>
> This switches to a "decrement variable and wait" timeout scheme.
>
> Signed-off-by: Nicolas Saenz Julienne <nicolassaenzj@...il.com>
> ---
>  drivers/usb/dwc3/gadget.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
> index 8f8c215..d0c711f 100644
> --- a/drivers/usb/dwc3/gadget.c
> +++ b/drivers/usb/dwc3/gadget.c
> @@ -1433,7 +1433,7 @@ static int dwc3_gadget_get_frame(struct usb_gadget *g)
>  
>  static int __dwc3_gadget_wakeup(struct dwc3 *dwc)
>  {
> -	unsigned long		timeout;
> +	int			retries;
>  
>  	int			ret;
>  	u32			reg;
> @@ -1484,14 +1484,16 @@ static int __dwc3_gadget_wakeup(struct dwc3 *dwc)
>  	}
>  
>  	/* poll until Link State changes to ON */
> -	timeout = jiffies + msecs_to_jiffies(100);
> +	retries = 20000;
>  
> -	while (!time_after(jiffies, timeout)) {
> +	while (retries--) {
>  		reg = dwc3_readl(dwc->regs, DWC3_DSTS);
>  
>  		/* in HS, means ON */
>  		if (DWC3_DSTS_USBLNKST(reg) == DWC3_LINK_STATE_U0)
>  			break;
> +
> +		udelay(5);

let's just drop this udelay() here. It's very likely this will switch
the link state much quicker than 100ms anyway. Other than that, nice
catch :-)

-- 
balbi

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