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

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Message-ID: <daa39dd3-4e8b-4d69-8ad3-5ec73fa12f8c@rowland.harvard.edu>
Date:   Fri, 4 Aug 2023 10:55:49 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Dingyan Li <18500469033@....com>
Cc:     Hans de Goede <hdegoede@...hat.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        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: Re: [PATCH] USB: add usbfs ioctl to get specific
 superspeedplus rates

On Fri, Aug 04, 2023 at 12:16:19PM +0800, Dingyan Li wrote:
> 
> At 2023-08-04 01:56:03, "Alan Stern" <stern@...land.harvard.edu> wrote:
> >On Fri, Aug 04, 2023 at 12:06:15AM +0800, Dingyan Li wrote:
> >> So after usb_device_speed is extended with Gen2x1, Gen1x2 and Gen2x2,
> >> it feels that enum usb_ssp_rate becomes useless. Is it okay to just delete it?
> >> I'm asking this since it is also used in several other source files so the fix may
> >> not be as trivial as it looks.
> >
> >As long as the file is being used by other source files, don't delete 
> >it.  If you want to fix up all those other places and then delete the 
> >file, that's fine.  But of course, it would have to be a separate set of 
> >patches.
> >
> >It will also be necessary to audit the places in the kernel that 
> >currently use usb_device_speed.  Some of them may need to be extended to 
> >handle the new entries properly.  (Including, obviously, the parts of 
> >the code that store the device's speed in the first place.)
> >
> 
> >Alan Stern
> 
> Another issue is that USB_SPEED_SUPER_PLUS has been widely used in
> so many files to execute conditional banches. Once we extend and store device's
> speed with new values in the first place, we might need to check all places where
> USB_SPEED_SUPER_PLUS is used in case of any regression.

Certainly.  That's part of auditing all the current users of 
usb_device_speed.

> I think maybe we can try to remove the dependency on enum usb_device_speed
> in usbfs and define a separate set of speed values similar to previous design
> at https://www.spinics.net/lists/linux-usb/msg157709.html
> By this way, in usbfs we get more freedom to determine how to explain
> usb_device_speed and usb_ssp_rate, without the risk of breaking anything
> elsewhere.
> 
> For example, define an USBDEVFS_SPEED_SUPER_PLUS to indicate
> USB_SPEED_SUPER_PLUS with ssp rates GEN_UNKNOWN, GEN_2x1 and
> GEN_1x2. They all stand for 10Gbps and we don't need to tell one from
> another, similar to how it works in sysfs. Then define an
> USBDEVFS_SPEED_SUPER_PLUS_BY2(maybe there is a more proper name)
> to indicate USB_SPEED_SUPER_PLUS with ssp rate GEN_2x2, which stands
> for 20Gbps.

You can't really do that.  The values returned by the USBDEVFS_GET_SPEED 
ioctl are the ones defined in include/uapi/linux/usb/ch9.h.  They are 
USB_SPEED_UNKNOWN, etc., not USBDEVFS_SPEED_UNKNOWN, etc.  The only way 
to extend them is by adding new entries to enum usb_device_speed.

For example, you must not do anything that would break a user program 
which does something like this:

#include <linux/usbdevfs.h>
#include <linux/usb/ch9.h>

...

	enum usb_device_speed s;

	s = ioctl(fd, USBDEVFS_GET_SPEED);
	if (s == USB_SPEED_HIGH) ...

Alan Stern