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    SubjectRe: [PATCH] usb: hub: Per-port setting to use old enumeration scheme
    On Fri, May 25, 2018 at 12:21 AM, Greg Kroah-Hartman
    <gregkh@linuxfoundation.org> wrote:
    > On Thu, May 24, 2018 at 07:42:00AM +0800, Nicolas Boichat wrote:
    >> On Thu, May 24, 2018 at 12:39 AM, Greg Kroah-Hartman
    >> <gregkh@linuxfoundation.org> wrote:
    >> > On Wed, May 23, 2018 at 10:03:55AM -0400, Alan Stern wrote:
    >> >> 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.
    >>
    >> I don't think it breaks the original parameter? I mean,
    >> /sys/module/usbcore/parameters/old_scheme_first is still a global
    >> default, while bit 0 of /sys/bus/usb/devices/x/y/z/quirks becomes a
    >> port-specific override.
    >>
    >> >> 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?
    >> >
    >> > Yeah, this last question is what I had when looking at this. Or does it
    >> > not matter at first boot and only matters for wake-up?
    >>
    >> It does not matter on boot, we have plenty of time to enumerate the
    >> device. We use USB (auto-)suspend and remote wake, so no
    >> re-enumeration there either. It only matters on unplug/replug where
    >> the device needs to be re-enumerated.
    >
    > How does this device get unplugged/replugged if it is connected directly
    > to the device?

    It is external. Essentially, this is a tablet with a detachable
    keyboard/touchpad. The interface between tablet and base is USB, over
    pogo pins. The port is non-standard (pogo, not normal USB), and we
    fully control the firmware on the base side as well, which allows us
    to take shortcuts like this: we know exactly what device will be
    connected on that port.

    >> Somewhere in an init script, we would do this (we know in advance that
    >> usb1 port2 is the bus/port where we have our pogo-pin USB interface,
    >> so we can hard-code the path):
    >> echo 1 > /sys/bus/usb/devices/usb1/1-0:1.0/usb1-port2/quirks
    >>
    >> We could try to add ACPI support (just like connect_type), but we
    >> don't strictly need it for our application.
    >
    > Isn't there an "internal" ACPI flag for USB ports, or is that
    > what connect_type is? Why wouldn't that work here instead?

    What we may need here is something like "external but non-standard, so
    we can ignore compatibility constraints when enumerating"...

    Thanks,

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