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From:   Xiaofan Chen <xiaofanc@gmail.com>
Date:   Wed, 26 Jul 2023 11:20:24 +0800
Message-ID: <CAGjSPUDtn-YMqmNrmku+hkyXCKys4XRue4wdzQwr+yGxe2cJSQ@mail.gmail.com>
Subject: Re: [PATCH] USB: add usbfs ioctl to get specific superspeedplus rates
To:     Oliver Neukum <oneukum@suse.com>,
        Hans de Goede <hdegoede@redhat.com>,
        Tormod Volden <lists.tormod@gmail.com>
Cc:     Greg KH <gregkh@linuxfoundation.org>,
        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 Wed, Jul 26, 2023 at 5:38=E2=80=AFPM Oliver Neukum <oneukum@suse.com> wr=
ote:
>
> On 26.07.23 03:37, Xiaofan Chen wrote:
> > On Tue, Jul 25, 2023 at 10:23=E2=80=AFPM Greg KH <gregkh@linuxfoundatio=
n.org> wrote:
>
> Hi,
>
> >> 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 otherwi=
se
> >> we are signing ourselves up to support it for forever.
> >
> > Interestingly there is PR in libusb now, which uses sysfs for 20Gbps.
>
> True. Now would you write a patch for libusb?
> This looks to be turning into a chicken and egg problem.
>
> > Maybe this new usbfs IOCTL is indeed good to have if we can not extend
>
> Looking at the code of libusb you can see that libusb has two modes
> of operation. Either it finds sysfs, then it uses it, if not it
> goes for the ioctl.
>
> Now, how well shall it work without sysfs? That is a design decision
> and we should not be having this discussion again and again.
>
> BTW, that is not aimed at anybody personally, we are just trying to
> avoid a basic decision and it will come back.
>
> > the existing IOCTL USBDEVFS_GET_SPEED (but why not?).
>
> It does not include the lane count.
> And sort of fudging this into speed is a bad idea in the long
> run because we are likely to have collisions in the future.
>
> We have a basic issue here. Do we require libusb to use sysfs or not?

Adding Hans de Goede and Tormod Volder (libusb admins) here in the discussi=
ons
as I am more into the testing and support side of libusb and not a
real developer.

libusb does work with or without sysfs and there are multiple commits relat=
ed
to sysfs vs usbfs.

An example commit from Hans in Sept 202 which is related to this discussion=
.
https://github.com/libusb/libusb/commit/f6068e83c4f5e5fba16b23b6a87f1f6d7ab=
7200a
++++++++++++++++
linux: Fix libusb_get_device_speed() not working on wrapped devices

We don't have a sysfs_dir for wrapped devices, so we cannot read the speed
from sysfs.

The Linux kernel has supported a new ioctl to get the speed directly from
the fd for a while now, use that when we don't have sysfs access.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=3D1871818
Reported-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
+++++++++++++++++

To Hans and Tormod:
Discussion thread for reference:
https://lore.kernel.org/linux-usb/da536c80-7398-dae0-a22c-16e521be697a@suse=
.com/T/#t


--=20
Xiaofan