linux-kernel - Re: [PATCH v2] usb: gadget: uvc: queue empty isoc requests if no video buffer is available

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Message-ID: <20230602211602.3b7rfa252wliiszp@synopsys.com>
Date:   Fri, 2 Jun 2023 21:16:05 +0000
From:   Thinh Nguyen <Thinh.Nguyen@...opsys.com>
To:     Avichal Rakesh <arakesh@...gle.com>
CC:     "laurent.pinchart@...asonboard.com" 
        <laurent.pinchart@...asonboard.com>,
        Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        "dan.scally@...asonboard.com" <dan.scally@...asonboard.com>,
        "etalvala@...gle.com" <etalvala@...gle.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "jchowdhary@...gle.com" <jchowdhary@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "stern@...land.harvard.edu" <stern@...land.harvard.edu>
Subject: Re: [PATCH v2] usb: gadget: uvc: queue empty isoc requests if no
 video buffer is available

Hi,

On Fri, Jun 02, 2023, Avichal Rakesh wrote:
> ISOC transfers expect a certain cadence of requests being queued. Not
> keeping up with the expected rate of requests results in missed ISOC
> transfers (EXDEV). The application layer is not required to produce video
> frames to match this expectation, so uvc gadget driver must not rely
> on data from application layer to maintain the ISOC cadence.
> 
> Currently, uvc gadget driver waits for new video buffer to become available
> before queuing up usb requests. With this patch the gadget driver queues up
> 0 length usb requests whenever there are no video buffers available. The
> USB controller's complete callback is used as the limiter for how quickly
> the 0 length packets will be queued. Video buffers are still queued as
> soon as they become available.
> 
> Link: https://urldefense.com/v3/__https://lore.kernel.org/CAMHf4WKbi6KBPQztj9FA4kPvESc1fVKrC8G73-cs6tTeQby9=w@mail.gmail.com/__;!!A4F2R9G_pg!e7A68uURThH2e8LnLybftBxZWaUVl5ewWfGl1wIDUqAvVQwWi_KE2rOUZFoDf__pq05pX3JXC3iGYiQjSInk$ 
> Signed-off-by: Avichal Rakesh <arakesh@...gle.com>
> ---
> Changelog:
> v2:
>   - Updated commit message to make it clear that userspace application is not
>     required to match the ISOC rate.
>   - Styling and comment revision based on review
> 


BTW, your previous version is already in Greg's usb-next branch. Any new
change should be rebased on his usb-next.

Thanks,
Thinh