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

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Date:   Thu, 3 Aug 2023 11:10:01 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Dingyan Li <18500469033@....com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Hans de Goede <hdegoede@...hat.com>,
        Xiaofan Chen <xiaofanc@...il.com>,
        Oliver Neukum <oneukum@...e.com>,
        Tormod Volden <lists.tormod@...il.com>,
        sebastian.reichel@...labora.com, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: Re: [PATCH] USB: add usbfs ioctl to get specific superspeedplus
 rates

On Thu, Aug 03, 2023 at 02:13:33PM +0800, Dingyan Li wrote:
> 
> At 2023-07-26 22:39:32, "Hans de Goede" <hdegoede@...hat.com> wrote:

> >Right, so the reason why IOCTL USBDEVFS_GET_SPEED was added is so 
> >that a confined qemu process which gets just a fd for a /dev/bus/usb/ 
> >device passed by a more privileged process can still get the speed 
> >despite it not having sysfs access. This is necessary for correct 
> >pass-through of USB devices.
> >
> >Since USBDEVFS_GET_SPEED now no longer tells the full story I believe 
> >that the proposed USBDEVFS_GET_SSP_RATE ioctl makes sense.
> >
> >The current patch however misses moving the enum usb_ssp_rate 
> >declaration from include/linux/usb/ch9.h to 
> >include/uapi/linux/usb/ch9.h so that needs to be fixed in a version 
> >2. Assuming that with the above explanation of why this is necessary 
> >Greg and Alan are ok with adding the ioctl.
> >
> >Regards,
> >
> >Hans
> >
> 
> Hi Greg and Alan,
> 
> Could you please share your opinions about Hans' justification?

Instead of adding a new ioctl or modifying an existing one, how about 
increasing the set of constants in enum usb_device_speed?  Then the 
existing ioctls could return the newly defined values when appropriate, 
with no other changes necessary.

(This doesn't mean just moving the definition of usb_ssp_rate from one 
header file to the other.  The usb_device_speed enumeration should be 
extended with the new members.  Perhaps omitting USB_SSP_GEN_UNKNOWN 
since we already have USB_SPEED_SUPER_PLUS, or setting the first equal 
to the second.)

I don't think there was ever a requirement in the API that the set of 
values in usb_device_speed could never increase (and in fact it has 
increased in the past).  Such a requirement wouldn't make any sense, 
given how the USB-IF keeps defining newer and faster USB bus 
implementations.

Hans, would that play well with libusb?

Alan Stern