linux-kernel - Re: [PATCH v2 2/2] platform/chrome: cros_ec

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Message-ID: <aHXwOtrFpn-yRFvs@google.com>
Date: Tue, 15 Jul 2025 06:07:54 +0000
From: Tzung-Bi Shih <tzungbi@...nel.org>
To: Radu Vele <raduvele@...gle.com>
Cc: Benson Leung <bleung@...omium.org>,
	Abhishek Pandit-Subedi <abhishekpandit@...omium.org>,
	Jameson Thies <jthies@...gle.com>,
	Andrei Kuchynski <akuchynski@...omium.org>,
	chrome-platform@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] platform/chrome: cros_ec_typec: Add lock per-port

On Mon, Jul 14, 2025 at 10:32:03AM +0200, Radu Vele wrote:
> On Fri, Jul 11, 2025 at 6:12 AM Tzung-Bi Shih <tzungbi@...nel.org> wrote:
> >
> > On Fri, Jul 11, 2025 at 12:35:02AM +0000, Radu Vele wrote:
> > > Add a lock associated to each port to protect port data against
> > > concurrent access. Concurrency may result from sysfs commands
> > > and ec events.
> >
> > I realized the critical sections are way too large.  What exactly data the
> > lock tries to protect?  Is the race possibility introduced by any previous
> > commits?  Please provide more context.
> 
> With the implementation of the role swap operations from the previous
> commit (and also enter usb mode from another recent commit) we
> introduce the possibility of concurrent access to the cros_ec_typec port
> data from the userspace (e.g. trigger a power role swap from sysfs) vs
> from EC events (e.g. partner triggered a role swap that we accept).
> This is the main reason to propose a per-port lock. This way we ensure
> we protect the state of each port in the cros_ec_typec driver.

To make sure I understand, did you mean the lock tries to prevent from
sending multiple commands to EC at a time?  If yes, does it still need
if the underlying ec_dev is guranteed that [1]?

[1] https://elixir.bootlin.com/linux/v6.15/source/drivers/platform/chrome/cros_ec_proto.c#L661

By taking the following hunk the patch adds as an example:

@@ -54,6 +56,7 @@  static int cros_typec_enter_usb_mode(struct typec_port *tc_port, enum usb_mode m
 		.mode_to_enter = CROS_EC_ALTMODE_USB4
 	};
 
+	guard(mutex)(&port->lock);
 	return cros_ec_cmd(port->typec_data->ec, 0, EC_CMD_TYPEC_CONTROL,
 			  &req, sizeof(req), NULL, 0);

It seems the lock doesn't protect any data but the command transfer.