linux-kernel - Re: [PATCH 3/3] ARM: dts: imx6: rdu2: Limit USBH1 to Full Speed

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Message-ID: <CAFXsbZooc5=_8SQT6rUQ2cRnor_o0fpW8-C3hFtszuXK+PHGZQ@mail.gmail.com>
Date:   Wed, 22 May 2019 11:10:25 -0700
From:   Chris Healy <cphealy@...il.com>
To:     Andrey Smirnov <andrew.smirnov@...il.com>
Cc:     Lucas Stach <l.stach@...gutronix.de>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Fabio Estevam <festevam@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] ARM: dts: imx6: rdu2: Limit USBH1 to Full Speed

> > Am Mittwoch, den 22.05.2019, 00:12 -0700 schrieb Andrey Smirnov:
> > > Cabling used to connect devices to USBH1 on RDU2 does not meet USB
> > > spec cable quality and cable length requirements to operate at High
> > > Speed, so limit the port to Full Speed only.
> >
> > Really? I thought this issue is specific to the RDU1, but you've been
> > looking at this USB stuff for a lot longer than me.
> >
>
> I am merely a messenger here. I didn't personally verify this to be
> the case, so your knowledge is probably as good as mine. Chris
> reported this based on feedback from their EE team, so he should know
> all of the details better.

The issue is less about the internals of the device and more about the
cabling that the device is connected to.  In the target application of
this device, the USB cables are almost always longer than the spec
limit of the USB standard for high speed.  Given our use cases, we
don't have need for high speed so limiting the max speed to full speed
is the way we work around cables that are too long for stable high
speed operation.

We have validated that running with full speed does mitigate the
problems experienced when attempting to run with high speed on the
target application installations.

Reviewed-by: Chris Healy <cphealy@...il.com>