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

[Shuah Khan <skhan@linuxfoundation.org>]

On 8/1/22 12:28 PM, Vincent MAILHOL wrote:
> 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.
> 

Sounds good. I now have the patch ready to be sent out. I will wait for
the merge window to close before I send it out.

thanks,
-- Shuah