linux-kernel - RE: [PATCH] fsl/usb: Workarourd for USB erratum-A005697

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Date:   Thu, 24 Nov 2016 11:53:52 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Sriram Dash <sriram.dash@....com>
cc:     Jerry Huang <jerry.huang@....com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "julia.lawall@...6.fr" <julia.lawall@...6.fr>,
        Ramneek Mehresh <ramneek.mehresh@....com>,
        Suresh Gupta <suresh.gupta@....com>
Subject: RE: [PATCH] fsl/usb: Workarourd for USB erratum-A005697

On Thu, 24 Nov 2016, Sriram Dash wrote:

> >From: Changming Huang [mailto:jerry.huang@....com]
> >As per USB specification, in the Suspend state, the status bit does not change until
> >the port is suspended. However, there may be a delay in suspending a port if there
> >is a transaction currently in progress on the bus.
> >
> >In the USBDR controller, the PORTSCx[SUSP] bit changes immediately when the
> >application sets it and not when the port is actually suspended
> >
> >Workaround for this issue involves waiting for a minimum of 10ms to allow the
> >controller to go into SUSPEND state before proceeding ahead

The USB core guarantees that there won't be any data transactions in 
progress when a root hub is suspended.  There might possibly be an SOF 
transaction, but that doesn't take more than a few microseconds at 
most.  Certainly nowhere near 10 ms!

Given that we already perform a 150-us delay, is this change really 
needed?

Alan Stern