linux-kernel - Re: [PATCH v4 2/2] usb: overridable hub bInterval by device node

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Date:   Thu, 5 Dec 2019 15:26:41 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Ikjoon Jang <ikjn@...omium.org>
Cc:     Johan Hovold <johan@...nel.org>, linux-usb@...r.kernel.org,
        GregKroah-Hartman <gregkh@...uxfoundation.org>,
        RobHerring <robh+dt@...nel.org>,
        MarkRutland <mark.rutland@....com>,
        AlanStern <stern@...land.harvard.edu>,
        SuwanKim <suwan.kim027@...il.com>,
        "GustavoA . R . Silva" <gustavo@...eddedor.com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Nicolas Boichat <drinkcat@...omium.org>
Subject: Re: [PATCH v4 2/2] usb: overridable hub bInterval by device node

On Thu, Dec 05, 2019 at 03:32:38PM +0800, Ikjoon Jang wrote:
> On Wed, Dec 4, 2019 at 3:55 PM Johan Hovold <johan@...nel.org> wrote:

> > But related to my question above, why do you need to do this during
> > enumeration? Why not just set the lower interval value in the hub
> > driver?
> 
> Because I want device tree's bInterval to be checked against the same rules
> defined in usb_parse_endpoint(). e.g. although hardware says its maximum
> is 255, but the practical limit is still 0 to 16, so the code can
> print warnings when bInterval from device node is too weird.

But that could be handled refactoring the code in question or similar.

The fundamental problem here is that you're using devicetree, which is
supposed to only describe the hardware, to encode policy which should be
deferred to user space.

So I think you need to figure out an interface that allows user space to
set the polling interval for any hub at runtime instead.

Johan