Re: [PATCH 0/2] usb: gadget: User space URBs for FunctionFS

[Ingo Rohloff <ingo.rohloff@lauterbach.com>]

Hi Greg,

> The gadget code has always used AIO since the very beginning, this is
> nothing new (decades old).  While it might feel "odd", I recommend
> working with it first before trying to create new kernel apis that
> duplicate existing functionality for the only reason being that AIO is
> "different".

Forget my patch: You are right: I am now convinced, that using the AIO
of the kernel should really provide the same.

For me it was the other way round: I wrote code talking to "devio.c" on
the USB Host and using "ioctl"; so that sounded natural to me.
I think "devio.c" does not support async I/O ?
So when starting, I was not aware at all that the gadget side supports
AIO; and then I got misled by the POSIX aio interface (which did hide
the native Linux aio system calls from me.)

The "bug" I found is a bug in the example code (not in the kernel)
as far as I can tell.

The other thing I want in the future:
> > > The final goal here is to be able to directly let user space
> > > provide data buffers (via mmap I guess), which are then
> > > transferred via USB; but this is the next step.    
> 
> > Isn't that kind of what the AIO inteface provides today?  :)  
>
> I think my explanation was not clear at all:
> What I want to have is a "zero copy" transfer.

I now think this can be implemented within the already existing
AIO framework in f_fs.c by implementing a suitable mmap call.

But before doing any of that I need test code.

So my plan right now:
- Write a working (fast) echo example using libaio

- Write a working (fast) echo example using liburing
  (https://github.com/axboe/liburing)
  because this should result in even faster AIO.
  Another big reason for not doing extra ioctls;
  using the existing AIO framework in the kernel
  should allow to use liburing :)

- Once I have done that I will look into extending
  f_fs.c with a mmap call so that complete zero copy
  transfers to/from USB bulk endpoints should become possible.

Then I should be able to do some performance tests via USB 3.0
to see how much this helps.

I will post again once I have some working code.

Thank you for your time and comments; 
that really helps me to find a better solution :-)

so long
  Ingo


PS: Not important:
> >   https://pagure.io/libaio 
> >   version 0.3.111
>
> I do not know if this is the latest one or not, sorry.  Ask your Linux
> distro about this, or use the "raw" kernel aio syscalls.

Thanks to gentoo, I found out there is 
  https://releases.pagure.org/libaio/
and this one has the 0.3.112 release; and of course there is always the
git repository (which unfortunately does not include tags for anything
more recent than 0.3.110).