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References: <20190403185310.8437-1-matwey@sai.msu.ru>
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From:   "Matwey V. Kornilov" <matwey.kornilov@gmail.com>
Date:   Wed, 24 Apr 2019 18:42:30 +0300
Message-ID: <CAJs94EZsDOo6wp2rZmsETLs7uPE3CCv1bNOsewxehyw8R52scA@mail.gmail.com>
Subject: Re: [PATCH 0/6] musb: Improve performance for hub-attached webcams
To:     Bin Liu <b-liu@ti.com>, Greg KH <gregkh@linuxfoundation.org>
Cc:     open list <linux-kernel@vger.kernel.org>,
        "open list:MUSB MULTIPOINT HIGH SPEED DUAL-ROLE CONTROLLER" 
        <linux-usb@vger.kernel.org>
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=D1=81=D1=80, 3 =D0=B0=D0=BF=D1=80. 2019 =D0=B3. =D0=B2 21:53, Matwey V. Ko=
rnilov <matwey@sai.msu.ru>:
>
> 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 IS=
O 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
>
> 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 | 114 +++++++++++++++++++++++++++----------=
------
>  1 file changed, 71 insertions(+), 43 deletions(-)
>
> --
> 2.16.4
>


--=20
With best regards,
Matwey V. Kornilov