Re: [PATCH V2] usb: xhci: add support for performing fake doorbell

From: Mathias Nyman
Date: Mon Nov 21 2016 - 10:31:12 EST

On 21.11.2016 09:57, RafaÅ MiÅecki wrote:
Hi Mathias,

On 17 October 2016 at 22:30, RafaÅ MiÅecki <zajec5@xxxxxxxxx> wrote:
From: RafaÅ MiÅecki <rafal@xxxxxxxxxx>

Broadcom's Northstar XHCI controllers seem to need a special start
procedure to work correctly. There isn't any official documentation of
this, the problem is that controller doesn't detect any connected
devices with default setup. Moreover connecting USB device to controller
that doesn't run properly can cause SoC's watchdog issues.

A workaround that was successfully tested on multiple devices is to
perform a fake doorbell. This patch adds code for doing this and enables
it on BCM4708 family.

Signed-off-by: RafaÅ MiÅecki <rafal@xxxxxxxxxx>
V2: Enable quirk for brcm,bcm4708 machines instead of adding separated binding
for it. Thanks Rob for your comment on this.

Do you think you can pick & push this one? V2 follows Rob's suggestion
and he has some DT knowledge for sure, so I guess it should be OK.

Is there some more background information on this?

I don't have any contacts to Broadcom myself, adding the BMC Kernel Feedback list to CC.
Maybe someone over there has an errata, documentation or just general feedback.

How was this workaround even figured out? ringing the doorbell for the first
device doesn't seem like something found by trial and error, especially when
xhci specs state that:

"Software shall not write the Doorbell of an endpoint until after it has issued a
Configure Endpoint Command for the endpoint and received a successful Command
Completion Event."

The whole workaround is a bit intrusive, allocating a fake device, ring a doorbell for a
fake device in the wrong state, clearing off HSE (host system error) which should only be set
when things really go bad, some random usleeps, and possible calling xhci_start() twice.

I can't take this as is without some more info.