linux-kernel - Re: [PATCH] usb: cdns3: Fix: ARM core hang after connect/disconnect operation.

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Date:   Thu, 9 Jan 2020 10:38:12 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Pawel Laszczak <pawell@...ence.com>
Cc:     "felipe.balbi@...ux.intel.com" <felipe.balbi@...ux.intel.com>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "rogerq@...com" <rogerq@...com>,
        "jbergsagel@...com" <jbergsagel@...com>,
        "nsekhar@...com" <nsekhar@...com>, "nm@...com" <nm@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jayshri Dajiram Pawar <jpawar@...ence.com>,
        Rahul Kumar <kurahul@...ence.com>,
        Sanket Parmar <sparmar@...ence.com>,
        Peter Chan <peter.chan@....com>
Subject: Re: [PATCH] usb: cdns3: Fix: ARM core hang after connect/disconnect
 operation.

On Thu, Jan 09, 2020 at 08:34:12AM +0000, Pawel Laszczak wrote:
> >
> >On Thu, Jan 09, 2020 at 06:27:02AM +0000, Pawel Laszczak wrote:
> >> >> +	writel(EP_CMD_EPRST, &priv_dev->regs->ep_cmd);
> >> >> +
> >> >> +	ret = readl_poll_timeout_atomic(&priv_dev->regs->ep_cmd, val,
> >> >> +					!(val & (EP_CMD_CSTALL | EP_CMD_EPRST)),
> >> >> +					1, 1000);
> >> >> +
> >> >> +	if (unlikely(ret))
> >> >
> >> >Unless you can measure the difference of using/not using a
> >> >unlikely/likely mark, NEVER use it.  The compiler and cpu can almost
> >> >always do better than you can, we have the tests to prove it.
> >> >
> >>
> >> The both of the above timeout should never occur. If they occurred it would be a
> >> critical controller bug. In this case driver can only inform  about this event.
> >
> >"Should never occur" is a fun thing to say :)
> >
> >If it can never occur, then don't even check for it.
> 
> Yes, on existing platforms it can never occur. 
> 
> >
> >If it can, then check for it and handle it properly.
> >
> >What about this controller in systems with removable busses (like PCI?)
> >What happens then (hint, I bet this could occur...)
> 
> It's good question.  Nobody from our customer currently use such system. 
> The only platform with PCI is used by me for testing purpose.  

So if you do have a PCI device, then you need to handle PCI reads
failing and returning all 1s.  Hopefully you can gracefully handle this :)

Adding timeout handling here, where it is totally obvious to do so,
would be a good thing.

thanks,

greg k-h