Re: [PATCH 0/1] uvc gadget sg performance issues

From: Michael Grzeschik
Date: Mon Sep 26 2022 - 16:15:59 EST

Hi Dan!

On Mon, Sep 26, 2022 at 02:53:06PM -0500, Dan Vacura wrote:

Hello uvc gadget developers,

I'm working on a 5.15.41 based kernel on a qcom chipset with the dwc3
controller and I'm encountering two problems related to the recent performance
improvement changes:

What's about that odd kernel number. UVC is under heavy development, if
you plan to work with this code, you should probably test top of tree.

https://patchwork.kernel.org/project/linux-usb/patch/20210628155311.16762-5-m.grzeschik@xxxxxxxxxxxxxx/ and
https://patchwork.kernel.org/project/linux-usb/patch/20210628155311.16762-6-m.grzeschik@xxxxxxxxxxxxxx/

If I revert these two changes, then I have much improved stability and a
transmission problem I'm seeing is gone. Has there been any success from
others on 5.15 with this uvc improvement and any recommendations for my
current problems? Those being:

1) a smmu panic, snippet here: 

<3>[  718.314900][  T803] arm-smmu 15000000.apps-smmu: Unhandled arm-smmu context fault from a600000.dwc3!
<3>[  718.314994][  T803] arm-smmu 15000000.apps-smmu: FAR    = 0x00000000efe60800
<3>[  718.315023][  T803] arm-smmu 15000000.apps-smmu: PAR    = 0x0000000000000000
<3>[  718.315048][  T803] arm-smmu 15000000.apps-smmu: FSR    = 0x40000402 [TF R SS ]
<3>[  718.315074][  T803] arm-smmu 15000000.apps-smmu: FSYNR0    = 0x5f0003
<3>[  718.315096][  T803] arm-smmu 15000000.apps-smmu: FSYNR1    = 0xaa02
<3>[  718.315117][  T803] arm-smmu 15000000.apps-smmu: context bank#    = 0x1b
<3>[  718.315141][  T803] arm-smmu 15000000.apps-smmu: TTBR0  = 0x001b0000c2a92000
<3>[  718.315165][  T803] arm-smmu 15000000.apps-smmu: TTBR1  = 0x001b000000000000
<3>[  718.315192][  T803] arm-smmu 15000000.apps-smmu: SCTLR  = 0x0a5f00e7 ACTLR  = 0x00000003
<3>[  718.315245][  T803] arm-smmu 15000000.apps-smmu: CBAR  = 0x0001f300
<3>[  718.315274][  T803] arm-smmu 15000000.apps-smmu: MAIR0   = 0xf404ff44 MAIR1   = 0x0000efe4
<3>[  718.315297][  T803] arm-smmu 15000000.apps-smmu: SID = 0x40
<3>[  718.315318][  T803] arm-smmu 15000000.apps-smmu: Client info: BID=0x5, PID=0xa, MID=0x2
<3>[  718.315377][  T803] arm-smmu 15000000.apps-smmu: soft iova-to-phys=0x0000000000000000

I can reduce this panic with the proposed patch, but it still happens until I
disable the "req->no_interrupt = 1" logic.

2) The frame is not fully transmitted in dwc3 with sg support enabled.

There seems to be a mapping limit I'm seeing where only the roughly first
70% of the total frame is sent. Interestingly, if I allocate a larger
size for the buffer upfront, in uvc_queue_setup(), like sizes[0] =
video->imagesize * 3. Then the issue rarely happens. For example, when I
do YUYV I see green, uninitialized data, at the bottom part of the
frame. If I do MJPG with smaller filled sizes, the transmission is fine.

+-------------------------+
| |
| |
| |
| Good data |
| |
| |
| |
+-------------------------+
|xxxxxxxxxxxxxxxxxxxxxxxxx|
|xxxx Bad data xxxxxxxxx|
|xxxxxxxxxxxxxxxxxxxxxxxxx|
+-------------------------+


Appreciate any thoughts or feedback related to these issues.

Anyway, this is probably due to the frames being given back to early to
the frameproducer. We have the following patches mainline now to fix this issue:

aef11279888c00e1841a3533a35d279285af3a51 usb: gadget: uvc: improve sg exit condition
9b969f93bcef9b3d9e92f1810e22bbd6c344a0e5 usb: gadget: uvc: giveback vb2 buffer on req complete

You might need some patches beforehand to get them applied on your
stack.

Regards,
Michael

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