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From:   Xiaofan Chen <xiaofanc@gmail.com>
Date:   Wed, 26 Jul 2023 09:37:01 +0800
Message-ID: <CAGjSPUCQ892adFdYm7zCuMpWujwzwoQtYimvp3xXrnnCCyN47w@mail.gmail.com>
Subject: Re: [PATCH] USB: add usbfs ioctl to get specific superspeedplus rates
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     Oliver Neukum <oneukum@suse.com>, Dingyan Li <18500469033@163.com>,
        stern@rowland.harvard.edu, sebastian.reichel@collabora.com,
        linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org
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On Tue, Jul 25, 2023 at 10:23=E2=80=AFPM Greg KH <gregkh@linuxfoundation.or=
g> wrote:
>
> On Mon, Jul 24, 2023 at 11:47:49AM +0200, Oliver Neukum wrote:
> > On 21.07.23 16:51, Greg KH wrote:
>> ...
> > > > 4. I found in libusb, there are two ways to get speed value for a d=
evice.
> > > >      One way is via sysfs, which has supported 20Gbps now. Another =
way is
> > > >      to use ioctl USBDEVFS_GET_SPEED. This is when I found this ioc=
tl can only
> > > >      return USB_SPEED_SUPER_PLUS at most, it cannot determine curre=
nt ssp rate
> > > >      further, no matter Gen1x2(10Gbps), Gen2x1(10Gbps) or Gen2x2(20=
Gbps). So I
> > > >      thought maybe it's good to provide a similar way like ioctl US=
BDEVFS_GET_SPEED
> > > >      in order to get ssp rates.
> > >
> > > If libusb doesn't need this ioctl, who would use it?  We only add api=
s
> > > that are actually going to be used.
> > >
> > > So if libusb doesn't use it, we need a real-world user for us to be a=
ble
> > > to add this.
> >
> > I am sorry, but that looks pretty much like a question of API design to=
 me.
> > To what extent is libusb supposed to be functional without sysfs? There=
 is
> > no technical answer to this. It is a question of design goals.
> >
> > If we follow the precedent of c01b244ad848a
> > ("USB: add usbfs ioctl to retrieve the connection speed")
> > then we should apply an updated version of Dingyan Li's patch, preferab=
ly
> > coupled with a patch for libusb or we go and deprecate some ioctls.
>
> We can never "deprecate" ioctls, sorry.
>
> So unless there is some actual need from userspace tools like libusb (or
> anything else?) that requires this new ioctl, let's not add it otherwise
> we are signing ourselves up to support it for forever.

Interestingly there is PR in libusb now, which uses sysfs for 20Gbps.
Maybe this new usbfs IOCTL is indeed good to have if we can not extend
the existing IOCTL USBDEVFS_GET_SPEED (but why not?).
https://github.com/libusb/libusb/pull/1298


--=20
Xiaofan