linux-kernel - Re: [PATCH] usb: gadget: uvc: Improve error checking and tagging

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Message-ID: <Zgu3pkbaQztXxrmc@pengutronix.de>
Date: Tue, 2 Apr 2024 09:45:42 +0200
From: Michael Grzeschik <mgr@...gutronix.de>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Daniel Scally <dan.scally@...asonboard.com>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: gadget: uvc: Improve error checking and tagging

On Tue, Mar 26, 2024 at 10:32:48AM +0100, Greg Kroah-Hartman wrote:
>On Mon, Mar 25, 2024 at 12:32:30AM +0100, Michael Grzeschik wrote:
>> Right now after one transfer was completed with EXDEV the currently
>> encoded frame will get the UVC_STREAM_ERR tag attached. Since the
>> complete and encode path are handling separate requests from different
>> threads, there is no direct correspondence between the missed transfer
>> of one request and the currently encoded request which might already
>> belong to an completely different frame.
>>
>> When queueing requests into the hardware by calling ep_queue the
>> underlying ringbuffer of the usb driver will be filled. However when
>> one of these requests will have some issue while transfer the hardware
>> will trigger an interrupt but will continue transferring the pending
>> requests in the ringbuffer. This interrupt-latency will make it
>> impossible to react in time to tag the fully enqueued frame with the
>> UVC_STREAM_ERR in the header.
>>
>> This patch is also addressing this particular issue by delaying the
>> transmit of the EOF/ERR tagged header by waiting for the last enqueued
>> buffer of the frame to be completed. This way it is possible to react to
>> send the EOF/ERR tag depending on the whole frame transfer status.
>>
>> As this is patch is adding latency to the enqueuing path of the frames
>> we make this errorcheck optional by adding an extra module parameter.
>>
>> Signed-off-by: Michael Grzeschik <m.grzeschik@...gutronix.de>
>> ---
>>  drivers/usb/gadget/function/f_uvc.c     |  4 ++
>>  drivers/usb/gadget/function/uvc.h       |  3 ++
>>  drivers/usb/gadget/function/uvc_video.c | 69 +++++++++++++++++++++++++++++----
>>  3 files changed, 68 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/usb/gadget/function/f_uvc.c b/drivers/usb/gadget/function/f_uvc.c
>> index 929666805bd23..6a7ca8ccaf360 100644
>> --- a/drivers/usb/gadget/function/f_uvc.c
>> +++ b/drivers/usb/gadget/function/f_uvc.c
>> @@ -33,6 +33,10 @@ unsigned int uvc_gadget_trace_param;
>>  module_param_named(trace, uvc_gadget_trace_param, uint, 0644);
>>  MODULE_PARM_DESC(trace, "Trace level bitmask");
>>
>> +bool uvc_gadget_errorcheck_param = true;
>> +module_param_named(errorcheck, uvc_gadget_errorcheck_param, bool, 0644);
>> +MODULE_PARM_DESC(errorcheck, "Check and mark errors in the transfer of a frame");
>
>I really really really really hate adding new module parameters as they
>do not scale nor work properly for multiple devices and really, it's not
>the 1990's anymore.
>
>Any way to make this debugging thing part of a debugfs interface or
>worst case, a sysfs entry instead?
>
>Or why not just make it a tracing thing instead?
>
>But wait, you are fixing a real issue here, why is it an option at all?
>Shouldn't this always be the case and the driver should always recover
>in this way?  Why would you not want this to be the default and only way
>it operates?

Yes and no. I added the thange to be optional, since it is changing
the amount of frames we will be able to transfer at once. However
I have another set of changes in the queue, that is also taking
the expected size and fps into the account of requst_size and
therefor the real amount of bandwidth. That will indirectly will have
the same result.

So this makes the option obsolete anyway. I will remove it in v2.

Thanks,
Michael

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