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From:   Felipe Balbi <balbi@kernel.org>
To:     Maxim Devaev <mdevaev@gmail.com>
Cc:     gregkh@linuxfoundation.org, ruslan.bilovol@gmail.com,
        mika.westerberg@linux.intel.com, jj251510319013@gmail.com,
        maze@google.com, linux-usb@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] usb: gadget: f_hid: optional SETUP/SET_REPORT mode
Date:   Tue, 24 Aug 2021 10:05:52 +0300
In-reply-to: <20210821134004.363217-1-mdevaev@gmail.com>
Message-ID: <877dgb8gps.fsf@kernel.org>
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Maxim Devaev <mdevaev@gmail.com> writes:

> f_hid provides the OUT Endpoint as only way for receiving reports
> from the host. SETUP/SET_REPORT method is not supported, and this causes
> a number of compatibility problems with various host drivers, especially
> in the case of keyboard emulation using f_hid.
>
>   - Some hosts do not support the OUT Endpoint and ignore it,
>     so it becomes impossible for the gadget to receive a report
>     from the host. In the case of a keyboard, the gadget loses
>     the ability to receive the status of the LEDs.
>
>   - Some BIOSes/UEFIs can't work with HID devices with the OUT Endpoint
>     at all. This may be due to their bugs or incomplete implementation
>     of the HID standard.
>     For example, absolutely all Apple UEFIs can't handle the OUT Endpoint
>     if it goes after IN Endpoint in the descriptor and require the reverse
>     order (OUT, IN) which is a violation of the standard.
>     Other hosts either do not initialize gadgets with a descriptor
>     containing the OUT Endpoint completely (like some HP and DELL BIOSes
>     and embedded firmwares like on KVM switches), or initialize them,
>     but will not poll the IN Endpoint.
>
> This patch adds configfs option no_out_endpoint=1 to disable
> the OUT Endpoint and allows f_hid to receive reports from the host
> via SETUP/SET_REPORT.
>
> Previously, there was such a feature in f_hid, but it was replaced
> by the OUT Endpoint [1] in the commit 99c515005857 ("usb: gadget: hidg:
> register OUT INT endpoint for SET_REPORT"). So this patch actually
> returns the removed functionality while making it optional.
> For backward compatibility reasons, the OUT Endpoint mode remains
> the default behaviour.
>
>   - The OUT Endpoint mode provides the report queue and reduces
>     USB overhead (eliminating SETUP routine) on transmitting a report
>     from the host.
>
>   - If the SETUP/SET_REPORT mode is used, there is no report queue,
>     so the userspace will only read last report. For classic HID devices
>     like keyboards this is not a problem, since it's intended to transmit
>     the status of the LEDs and only the last report is important.
>     This mode provides better compatibility with strange and buggy
>     host drivers.
>
> Both modes passed USBCV tests. Checking with the USB protocol analyzer
> also confirmed that everything is working as it should and the new mode
> ensures operability in all of the described cases.
>
> Signed-off-by: Maxim Devaev <mdevaev@gmail.com>
> Link: https://www.spinics.net/lists/linux-usb/msg65494.html [1]

Acked-by: Felipe Balbi <balbi@kernel.org>

-- 
balbi