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Date:   Tue, 18 Oct 2022 17:28:19 +0200
From:   Michael Grzeschik <mgr@pengutronix.de>
To:     Dan Vacura <w36195@motorola.com>
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        Dan Scally <dan.scally@ideasonboard.com>,
        linux-usb@vger.kernel.org,
        Thinh Nguyen <Thinh.Nguyen@synopsys.com>,
        Jeff Vanhoof <qjv001@motorola.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Felipe Balbi <balbi@kernel.org>,
        Paul Elder <paul.elder@ideasonboard.com>,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org
Subject: Re: [PATCH v3 6/6] usb: gadget: uvc: add configfs option for sg
 support
Message-ID: <20221018152819.GA9097@pengutronix.de>
References: <20221017205446.523796-1-w36195@motorola.com>
 <20221017205446.523796-7-w36195@motorola.com>
 <78c6403a-22d9-903d-f0cf-4205e17962d3@ideasonboard.com>
 <Y065ASuFhM9bntvd@rowland.harvard.edu>
 <Y07C7hYKyByahNjL@p1g3>
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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@motorola.com>
>> >
>> >
>> > Nitpick: I would call it use_sg everywhere, but either way:
>> >
>> >
>> > Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
>> >
>> > Tested-by: Daniel Scally <dan.scally@ideasonboard.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/Doc=
umentation/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
>> > >   		=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D	=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D
>> > >   What:		/config/usb-gadget/gadget/functions/uvc.name/control
>> > > diff --git a/Documentation/usb/gadget-testing.rst b/Documentation/us=
b/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 it=
s function directory:
>> > >   	function_name       name of the interface
>> > >   	req_int_skip_div    divisor of total requests to aid in calculati=
ng
>> > >   			    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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