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 99c5150058 ("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.
Link: https://www.spinics.net/lists/linux-usb/msg65494.html [1]
Reviewed-by: Maciej Żenczykowski <zenczykowski@gmail.com>
Acked-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Maxim Devaev <mdevaev@gmail.com>
Link: https://lore.kernel.org/r/20210821134004.363217-1-mdevaev@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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