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

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Date:   Wed, 26 Jul 2023 11:38:34 +0200
From:   Oliver Neukum <oneukum@...e.com>
To:     Xiaofan Chen <xiaofanc@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>
Cc:     Oliver Neukum <oneukum@...e.com>, 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 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.

	Regards
		Oliver