linux-kernel - Re: [PATCH v3 6/6] usb: gadget: uvc: add configfs option for sg support

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Message-ID: <20221018152819.GA9097@pengutronix.de>
Date:   Tue, 18 Oct 2022 17:28:19 +0200
From:   Michael Grzeschik <mgr@...gutronix.de>
To:     Dan Vacura <w36195@...orola.com>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Dan Scally <dan.scally@...asonboard.com>,
        linux-usb@...r.kernel.org,
        Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        Jeff Vanhoof <qjv001@...orola.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan Corbet <corbet@....net>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Felipe Balbi <balbi@...nel.org>,
        Paul Elder <paul.elder@...asonboard.com>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v3 6/6] usb: gadget: uvc: add configfs option for sg
 support

Hi Dan!

On Tue, Oct 18, 2022 at 10:14:54AM -0500, Dan Vacura wrote:
>On Tue, Oct 18, 2022 at 10:32:33AM -0400, Alan Stern wrote:
>> On Tue, Oct 18, 2022 at 02:27:13PM +0100, Dan Scally wrote:
>> > On 17/10/2022 21:54, Dan Vacura wrote:
>> > > The scatter gather support doesn't appear to work well with some UDC hw.
>> > > Add the ability to turn on the feature depending on the controller in
>> > > use.
>> > >
>> > > Signed-off-by: Dan Vacura <w36195@...orola.com>
>> >
>> >
>> > Nitpick: I would call it use_sg everywhere, but either way:
>> >
>> >
>> > Reviewed-by: Daniel Scally <dan.scally@...asonboard.com>
>> >
>> > Tested-by: Daniel Scally <dan.scally@...asonboard.com>
>> >
>> > > ---
>> > > V1 -> V2:
>> > > - no change, new patch in serie
>> > > V2 -> V3:
>> > > - default on, same as baseline
>> > >
>> > >   Documentation/ABI/testing/configfs-usb-gadget-uvc | 1 +
>> > >   Documentation/usb/gadget-testing.rst              | 2 ++
>> > >   drivers/usb/gadget/function/f_uvc.c               | 2 ++
>> > >   drivers/usb/gadget/function/u_uvc.h               | 1 +
>> > >   drivers/usb/gadget/function/uvc_configfs.c        | 2 ++
>> > >   drivers/usb/gadget/function/uvc_queue.c           | 4 ++--
>> > >   6 files changed, 10 insertions(+), 2 deletions(-)
>> > >
>> > > diff --git a/Documentation/ABI/testing/configfs-usb-gadget-uvc b/Documentation/ABI/testing/configfs-usb-gadget-uvc
>> > > index 5dfaa3f7f6a4..839a75fc28ee 100644
>> > > --- a/Documentation/ABI/testing/configfs-usb-gadget-uvc
>> > > +++ b/Documentation/ABI/testing/configfs-usb-gadget-uvc
>> > > @@ -9,6 +9,7 @@ Description:	UVC function directory
>> > >   		streaming_interval	1..16
>> > >   		function_name		string [32]
>> > >   		req_int_skip_div	unsigned int
>> > > +		sg_supported		0..1
>> > >   		===================	=============================
>> > >   What:		/config/usb-gadget/gadget/functions/uvc.name/control
>> > > diff --git a/Documentation/usb/gadget-testing.rst b/Documentation/usb/gadget-testing.rst
>> > > index f9b5a09be1f4..8e3072d6a590 100644
>> > > --- a/Documentation/usb/gadget-testing.rst
>> > > +++ b/Documentation/usb/gadget-testing.rst
>> > > @@ -796,6 +796,8 @@ The uvc function provides these attributes in its function directory:
>> > >   	function_name       name of the interface
>> > >   	req_int_skip_div    divisor of total requests to aid in calculating
>> > >   			    interrupt frequency, 0 indicates all interrupt
>> > > +	sg_supported        allow for scatter gather to be used if the UDC
>> > > +			    hw supports it
>>
>> Why is a configuration option needed for this?  Why not always use SG
>> when the UDC supports it?  Or at least, make the decision automatically
>> (say, based on the amount of data to be transferred) with no need for
>> any user input?
>
>Patches for a fix and to select to use SG depending on amount of data
>are already submitted and under review. I agree, ideally we don't need
>this patch, but there have been several regressions uncovered with
>enabling this support and it takes time to root cause these issues.
>

>In my specific environment, Android GKI 2.0, changes need to get
>upstreamed first here before they're pulled into Android device
>software.

In fact this is actually a good policy, but adding workarounds mainline
to "hopefully" fix the real problems later are probably not what this
policy is about. Hopefully!

Michael

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