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

From: Greg KH <gregkh@linuxfoundation.org>
To: Pawel Laszczak <pawell@cadence.com>
Cc: "felipe.balbi@linux.intel.com" <felipe.balbi@linux.intel.com>,
	"linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>,
	"rogerq@ti.com" <rogerq@ti.com>,
	"jbergsagel@ti.com" <jbergsagel@ti.com>,
	"nsekhar@ti.com" <nsekhar@ti.com>, "nm@ti.com" <nm@ti.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Jayshri Dajiram Pawar <jpawar@cadence.com>,
	Rahul Kumar <kurahul@cadence.com>,
	Sanket Parmar <sparmar@cadence.com>,
	Peter Chan <peter.chan@nxp.com>
Subject: Re: [PATCH] usb: cdns3: Fix: ARM core hang after connect/disconnect operation.
Date: Thu, 9 Jan 2020 10:38:12 +0100	[thread overview]
Message-ID: <20200109093812.GC44349@kroah.com> (raw)
In-Reply-To: <BYAPR07MB4709AA109700B4BCAD1C1ED8DD390@BYAPR07MB4709.namprd07.prod.outlook.com>

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