linux-kernel - Re: [PATCH] usb: raw-gadget: fix gadget endpoint selection

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Date:   Wed, 29 Apr 2020 03:42:38 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     USB list <linux-usb@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Felipe Balbi <balbi@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] usb: raw-gadget: fix gadget endpoint selection

On Wed, Apr 29, 2020 at 3:14 AM Alan Stern <stern@...land.harvard.edu> wrote:
>
> On Tue, 28 Apr 2020, Andrey Konovalov wrote:
>
> > Hi Alan,
> >
> > This patch uses the approach that I mentioned in the discussion about
> > endpoint selection. Does this look acceptable?
>
> I haven't had time to look through it yet.
>
> > I'm not sure which endpoint limits it makes sense to expose via
> > USB_RAW_IOCTL_EPS_INFO. I'm more or less sure about maxpacket_limit
> > and max_streams, but I don't exactly know what maxburst is used for.
> > Maybe there are some others?
>
> maxburst is a USB-3 thing.  It mainly affects just throughput, not
> functionality, and it's handled pretty much entirely by the hardware.
> You shouldn't worry about it, at least, not now.

The question is whether it will be needed when/if I ever add proper
USB3 support. It would be good to figure out which endpoint attributes
we need to expose now, rather than having to add another ioctl later.

> > I also wonder if we need to expose ep0 limits via USB_RAW_IOCTL_EPS_INFO too.
> > expose ep0 parameters?
>
> I don't think there are any significant attributes for ep0.  In
> general, gadget drivers have to live with what the hardware supports --
> or else fail to run at all.  After all, the driver can't substitute a
> different endpoint for ep0.

This is a good point. No need to export ep0 attributes then I guess.

Thanks!