linux-kernel - Re: [PATCH 2/2] usb: dwc3: gadget: restart the transfer if a isoc request is queued too late

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Message-ID: <45ce4107-7c52-48d2-90d1-dfe835024063@synopsys.com>
Date:   Wed, 13 Nov 2019 03:55:01 +0000
From:   Thinh Nguyen <Thinh.Nguyen@...opsys.com>
To:     Michael Olbrich <m.olbrich@...gutronix.de>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>
CC:     Felipe Balbi <balbi@...nel.org>,
        Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>
Subject: Re: [PATCH 2/2] usb: dwc3: gadget: restart the transfer if a isoc
 request is queued too late

Hi Michael,

Michael Olbrich wrote:
> Currently, most gadget drivers handle isoc transfers on a best effort
> bases: If the request queue runs empty, then there will simply be gaps in
> the isoc data stream.
>
> The UVC gadget depends on this behaviour. It simply provides new requests
> when video frames are available and assumes that they are sent as soon as
> possible.
>
> The dwc3 gadget currently works differently: It assumes that there is a
> contiguous stream of requests without any gaps. If a request is too late,
> then it is dropped by the hardware.
> For the UVC gadget this means that a live stream stops after the first
> frame because all following requests are late.

Can you explain little more how UVC gadget fails?
dwc3 controller expects a steady stream of data otherwise it will result 
in missed_isoc status, and it should be fine as long as new requests are 
queued. The controller doesn't just drop the request unless there's some 
other failure.

> This patch changes the behaviour of the dwc3 gadget driver to match the
> expectations. If a request arrives too late, then the transfer will be
> restart to create the needed gap.
>
> A request is late if:
> 1. There are currently no requests queued in the hardware
> 2. The current frame number provided by DSTS does not match the frame
>     number returned by the last transfer.
>
> If this happens transfers are stopped. The following XferNotReady provides
> the new correct frame number and restarts the transfer.
>
> Signed-off-by: Michael Olbrich <m.olbrich@...gutronix.de>
> ---
>   drivers/usb/dwc3/gadget.c | 8 ++++++++
>   1 file changed, 8 insertions(+)
>
> diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
> index ac4673d91939..eb7f09929f28 100644
> --- a/drivers/usb/dwc3/gadget.c
> +++ b/drivers/usb/dwc3/gadget.c
> @@ -1473,6 +1473,14 @@ static int __dwc3_gadget_ep_queue(struct dwc3_ep *dep, struct dwc3_request *req)
>   				return __dwc3_gadget_start_isoc(dep);
>   			}
>   		}
> +		if ((dep->flags & DWC3_EP_TRANSFER_STARTED) &&
> +		    list_empty(&dep->started_list) &&
> +		    ((dep->frame_number & 0x3fff) !=
> +		    __dwc3_gadget_get_frame(dwc))) {
> +			dwc3_stop_active_transfer(dep, true, true);
> +			dep->flags = DWC3_EP_ENABLED;
> +			return 0;
> +		}
>   	}
>   
>   	return __dwc3_gadget_kick_transfer(dep, false);

BR,
Thinh