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

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Message-ID: <71130307-1c5c-468b-b876-799c434002cc@rowland.harvard.edu>
Date:   Fri, 21 Jul 2023 13:26:22 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Dingyan Li <18500469033@....com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        sebastian.reichel@...labora.com, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: Re: Re: [PATCH] USB: add usbfs ioctl to get specific
 superspeedplus rates

On Fri, Jul 21, 2023 at 11:43:29PM +0800, Dingyan Li wrote:
> Okay, got it. The motivation should come from real-world needs.
> 
> Just like I mentioned above, currently in libusb ioctl USBDEVFS_GET_SPEED
> is still used, especially where sysfs is not supported. My original idea
> was exactly trying to add this new ioctl into libusb. So in order to get 20Gbps
> speed, we need extra information. The basic workflow is like below:
> 
> // This is pretty much how libusb does it, get 10Gbps at most
> ret = ioctl(USBDEVFS_GET_SPEED)
> if (ret == USB_SPEED_SUPER_PLUS) then
>     speed = 10Gbps
>     // With this new ioctl, we can get 20Gbps now
>     ret = ioctl(USBDEVFS_GET_SSP_RATE)
>     if (ret == USB_SSP_GEN_2x2)
>         speed = 20Gbps
> 
> libusb can be a good place to document the usage of this new ioctl if similar patch
> can be accepted into it. And I can't think of other real-world users now. Of course,
> like you've explained, it seems quite unnecessary when sysfs is supported.

For what it's worth, many of the other ioctls in usbfs are documented 
(very incompletely) in Documentation/driver-api/usb/usb.rst.  That's 
probably the best place to add any documentation for new APIs.

Alan Stern