Re: [PATCH] greybus: usb: fix response buffer size for fixed-length hub requests

[Aditya Chari S <adi25charis@gmail.com>]

Hi Dan,

Before I go through your questions point by point, I want to flag
something that affects the confidence of the whole patch.

My rationale for hardcoding fixed sizes per request type
(GetHubDescriptor/GetHubStatus/GetPortStatus) was based on reasoning
from rh_call_control() alone, without checking how the actual Project
Ara module-side firmware behaves. I went and found the firmware at
github.com/projectara/, and the relevant file at
github.com/projectara/nuttx/blob/master/nuttx/drivers/greybus/usb.c,
gb_usb_hub_control(). It doesn't support the assumption I was making.
The module allocates its own response buffer purely from wLength and
passes wLength straight through to the hardware call:

	response = gb_operation_alloc_response(operation,
						sizeof(*response) + wLength);
	...
	status = device_usb_hcd_hub_control(usbdev, typeReq, wValue,
					     wIndex, (char*) response->buf,
					     wLength);

So it's deferential to wLength on both ends. I don't see evidence here
that the module would ever send back more data than wLength implied,
which was the scenario I described as the "impact" of the original
FIXME.

Given that, and given this driver has been unreachable at runtime for
11 years per commit a96493560cd1, I'd rather get your read on whether
this is worth pursuing further before I go answer the rest of your
review and send a v2. Happy to keep digging if you think it's still
worthwhile, or fine leaving the FIXME as-is otherwise.

Thanks,
Aditya