linux-kernel - Re: [PATCH] USB: add usbfs ioctl to get specific superspeedplus rates

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Message-ID: <CAGjSPUDtn-YMqmNrmku+hkyXCKys4XRue4wdzQwr+yGxe2cJSQ@mail.gmail.com>
Date:   Wed, 26 Jul 2023 11:20:24 +0800
From:   Xiaofan Chen <xiaofanc@...il.com>
To:     Oliver Neukum <oneukum@...e.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Tormod Volden <lists.tormod@...il.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Dingyan Li <18500469033@....com>, stern@...land.harvard.edu,
        sebastian.reichel@...labora.com, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] USB: add usbfs ioctl to get specific superspeedplus rates

On Wed, Jul 26, 2023 at 5:38 PM Oliver Neukum <oneukum@...e.com> wrote:
>
> On 26.07.23 03:37, Xiaofan Chen wrote:
> > On Tue, Jul 25, 2023 at 10:23 PM Greg KH <gregkh@...uxfoundation.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 otherwise
> >> 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 discussions
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 related
to sysfs vs usbfs.

An example commit from Hans in Sept 202 which is related to this discussion.
https://github.com/libusb/libusb/commit/f6068e83c4f5e5fba16b23b6a87f1f6d7ab7200a
++++++++++++++++
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=1871818
Reported-by: Gerd Hoffmann <kraxel@...hat.com>
Signed-off-by: Hans de Goede <hdegoede@...hat.com>
+++++++++++++++++

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


-- 
Xiaofan