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

[Dingyan Li]

At 2023-07-25 23:12:01, "Greg KH" <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>On Tue, Jul 25, 2023 at 10:40:10PM +0800, Dingyan Li wrote:
>> 
>> At 2023-07-25 22:08:44, "Oliver Neukum" <oneukum@xxxxxxxx> wrote:
>> >On 25.07.23 15:54, Dingyan Li wrote:
>> >
>> >> If we can't "deprecate" ioctls, can we change the returned contents of existing ones?
>> >
>> >No. Absolutely not. That is totally unacceptable. It would be much
>> >worse than just removing the support.
>> >
>> >	Regards
>> >		Oliver
>> 
>> Got it, I guess this is for backward-compatibility.
>> 
>> I also have another thought. USBDEVFS_CONNINFO_EX is kind of special and
>> used to retrieve contents of variable length. If you check proc_conninfo_ex(),
>> I think maybe we can append a new member to "struct usbdevfs_conninfo_ex"
>> without breaking backward-compatibility.
>
>How exactly would that work?  Remember, new kernels still need to work
>with old userspace code.
>
>greg k-h

In proc_conninfo_ex(), the number of returned bytes is determined by
the smaller number between sizeof(struct usbdevfs_conninfo_ex) and a
user specified size. So if we only append new members to the end of
struct usbdevfs_conninfo_ex, it won't impact the bytes in the beginning.

For example, current sizeof(struct usbdevfs_conninfo_ex) is 24 bytes.
Let's assume there is already some code using ioctl USBDEVFS_CONNINFO_EX
and requesting a full size, which is 24 bytes. Now we append a new __u32
member called ssp_rate in the end of struct usbdevfs_conninfo_ex. For the old
code, the meaning of the beginning 24 bytes is still the same. But for new code,
we can now request a full size of 28 bytes.


Regards,
Dingyan