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Date:   Wed, 23 May 2018 10:03:55 -0400 (EDT)
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Nicolas Boichat <drinkcat@chromium.org>
cc:     linux-usb@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Mathias Nyman <mathias.nyman@linux.intel.com>,
        Felipe Balbi <felipe.balbi@linux.intel.com>,
        Eugene Korenevsky <ekorenevsky@gmail.com>,
        Peter Chen <peter.chen@nxp.com>,
        Daniel Drake <drake@endlessm.com>,
        Joe Perches <joe@perches.com>, Johan Hovold <johan@kernel.org>,
        Richard Leitner <richard.leitner@skidata.com>,
        <linux-kernel@vger.kernel.org>, <groeck@chromium.org>
Subject: Re: [PATCH] usb: hub: Per-port setting to use old enumeration scheme
In-Reply-To: <20180523021656.122455-1-drinkcat@chromium.org>
Message-ID: <Pine.LNX.4.44L0.1805230958590.1481-100000@iolanthe.rowland.org>
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On Wed, 23 May 2018, Nicolas Boichat wrote:

> The "old" enumeration scheme is considerably faster (it takes
> ~294ms instead of ~439ms to get the descriptor).
> 
> It is currently only possible to use the old scheme globally
> (/sys/module/usbcore/parameters/old_scheme_first), which is not
> desirable as the new scheme was introduced to increase compatibility
> with more devices.
> 
> However, in our case, we care about time-to-active for a specific
> USB device (which we make the firmware for), on a specific port
> (that is pogo-pin based: not a standard USB port). This new
> sysfs option makes it possible to use the old scheme on a single
> port only.
> 
> Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
> ---
> 
> There are other "quirks" that we could add to reduce further
> enumeration time (e.g. reduce USB debounce time, reduce TRSTRCY
> to 10ms instead of 50ms as used currently), but the logic is quite
> similar, so it'd be good to have this reviewed first.

I'm not opposed to the idea in principle, although I don't like your
implementation because it breaks the original old_scheme_first
parameter.

Let's see what some other people think.

Yours is a rather special case, because you know exactly what device
will be attached to a specific port.  Still, I can see that sort of
thing happening in constrained and special-purpose settings.

How do you arrange to set the new quirk before the device is 
discovered?

Alan Stern