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

[Shuah Khan <skhan@linuxfoundation.org>]

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.

thanks,
-- Shuah