Re: [PATCH v2 0/6] musb: Improve performance for hub-attached webcams

[Bin Liu]

Matwey,

On Tue, Jul 02, 2019 at 08:29:03PM +0300, Matwey V. Kornilov wrote:
> Ping?

I was offline and just got back. I will review it soon. Sorry for the
delay.

-Bin.

> 
> ÐÑ, 14 ÐÑÐ. 2019 Ð. Ð 19:47, Matwey V. Kornilov <matwey@xxxxxxxxxx>:
> >
> > The series is concerned to issues with isochronous transfer while
> > streaming the USB webcam data. I discovered the issue first time
> > when attached PWC USB webcam to AM335x-based BeagleBone Black SBC.
> > It appeared that the root issue was in numerous missed IN requests
> > during isochronous transfer where each missing leaded to the frame
> > drop. Since every IN request is triggered in MUSB driver
> > individually, it is important to queue the send IN request as
> > earlier as possible when the previous IN completed. At the same
> > time the URB giveback handler of the device driver has also to be
> > called there, that leads to arbitrarily delay depending on the
> > device driver performance. The details with the references are
> > described in [1].
> >
> > The issue has two parts:
> >
> >   1) peripheral driver URB callback performance
> >   2) MUSB host driver performance
> >
> > It appeared that the first part is related to the wrong memory
> > allocation strategy in the most USB webcam drivers. Non-cached
> > memory is used in assumption that coherent DMA memory leads to
> > the better performance than non-coherent memory in conjunction with
> > the proper synchronization. Yet the assumption might be valid for
> > x86 platforms some time ago, the issue was fixed for PWC driver in:
> >
> >     1161db6776bd ("media: usb: pwc: Don't use coherent DMA buffers for ISO transfer")
> >
> > that leads to 3.5x performance gain. The more generic fix for this
> > common issue are coming for the rest drivers [2].
> >
> > The patch allowed successfully running full-speed USB PWC webcams
> > attached directly to BeagleBone Black USB port.
> >
> > However, the second part of the issue is still present for
> > peripheral device attached through the high-speed USB hub due to
> > its 125us frame time. The patch series is intended to reorganize
> > musb_advance_schedule() to allow host to send IN request quicker.
> >
> > The patch series is organized as the following. First three patches
> > improve readability of the existing code in
> > musb_advance_schedule(). Patches 4 and 5 introduce updated
> > signature for musb_start_urb(). The last patch introduce new
> > code-path in musb_advance_schedule() which allows for faster
> > response.
> >
> > References:
> >
> > [1] https://www.spinics.net/lists/linux-usb/msg165735.html
> > [2] https://www.spinics.net/lists/linux-media/msg144279.html
> >
> > Changes since v1:
> >  - Patch 6 was redone to keep URB giveback order and stop transmission at
> >    erroneous URB.
> >
> > Matwey V. Kornilov (6):
> >   usb: musb: Use USB_DIR_IN when calling musb_advance_schedule()
> >   usb: musb: Introduce musb_qh_empty() helper function
> >   usb: musb: Introduce musb_qh_free() helper function
> >   usb: musb: Rename musb_start_urb() to musb_start_next_urb()
> >   usb: musb: Introduce musb_start_urb()
> >   usb: musb: Decrease URB starting latency in musb_advance_schedule()
> >
> >  drivers/usb/musb/musb_host.c | 132 ++++++++++++++++++++++++++++---------------
> >  drivers/usb/musb/musb_host.h |   1 +
> >  2 files changed, 86 insertions(+), 47 deletions(-)
> >
> > --
> > 2.16.4
> >
> 
> 
> -- 
> With best regards,
> Matwey V. Kornilov