Re: [PATCH v3 1/2] drivers: usb/core/urb: Add URB_FREE_COHERENT

[Vincent MAILHOL <mailhol.vincent@wanadoo.fr>]

On Tue. 2 Aug. 2022 at 02:48, Shuah Khan <skhan@linuxfoundation.org> wrote:
> On 6/30/22 8:10 PM, Shuah Khan wrote:
> > On 6/27/22 7:35 PM, Alan Stern wrote:
> >> On Mon, Jun 27, 2022 at 04:54:17PM -0600, Shuah Khan wrote:
> >>> On 6/24/22 12:07 PM, Alan Stern wrote:
> >>>> In the future people will want to make other changes to
> >>>> include/linux/usb.h and they will not be aware that those changes will
> >>>> adversely affect usbip, because there is no documentation saying that
> >>>> the values defined in usb.h are part of a user API.  That will be a
> >>>> problem, because those changes may be serious and important ones, not
> >>>> just decorative or stylistic as in this case.
> >>>>
> >>>
> >>> How often do these values change based on our past experience with these
> >>> fields?
> >>
> >> I don't know.  You could check the git history to find out for certain.
> >> My guess would be every eight or ten years.
> >>
> >>>> I agree with Hongren that values defined in include/linux/ should not be
> >>>> part of a user API.  There are two choices:
> >>>>
> >>>
> >>> I agree with this in general. I don't think this is an explicit decision
> >>> to make them part of API. It is a consequence of simply copying the
> >>> transfer_flags. I am with you both on not being able to recognize the
> >>> impact until as this is rather obscure usage hidden away in the packets.
> >>> These defines aren't directly referenced.
> >>>
> >>>>     Move the definitions into include/uapi/linux/, or
> >>>>
> >>>
> >>> Wouldn't this be easier way to handle the change? With this option
> >>> the uapi will be well documented.
> >>>
> >>>>     Add code to translate the values between the numbers used in
> >>>>     userspace and the numbers used in the kernel.  (This is what
> >>>>     was done for urb->transfer_flags in devio.c:proc_do_submiturb()
> >>>>     near line 1862.)
> >>>>
> >>>
> >>> I looked at the code and looks simple enough. I am okay going this route
> >>> if we see issues with the option 1.
> >>
> >> It's up to you; either approach is okay with me.  However, I do think
> >> that the second option is a little better; I don't see any good reason
> >> why the kernel should be forced to use the same numeric values for these
> >> flags forever.  Especially since the only user program that needs to
> >> know them is usbip, which is fairly closely tied to the kernel; if there
> >> were more programs using those values then they would constitute a good
> >> reason for choosing the first option.
> >>
> >
> > Thank you Alan and Hongren for your help with this problem. Since there
> > are no changes to the flags for the time being, I am comfortable going
> > with the second option.
> >
> > I will send a patch soon.
> >
>
> Patch is almost ready to be sent out. Changes aren't bad at all. Hoping to
> get this done sooner - summer vacations didn't cooperate.
>
> Just an update that I haven't forgotten and it will taken care of.
> thanks,

Thanks for keeping this under your radar. I also have on my TODO list
to send a new version of my patch to add the `URB_FREE_COHERENT' flag
but this time adding an `allocated_length' field to struct urb. I will
wait for your patch to go first. By the way, I will be out for summer
holiday for the next couple of weeks so I wasn't planning to submit
anything soon regardless.

Yours sincerely,
Vincent Mailhol