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

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Message-ID: <CACKy9TJvkx0Bi69pO187dxs8EUt3foc0seNYjn=vK4WCuTHYxQ@mail.gmail.com>
Date: Wed, 16 Jul 2025 13:39:53 +0200
From: Radu Vele <raduvele@...gle.com>
To: Tzung-Bi Shih <tzungbi@...nel.org>
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 Tue, Jul 15, 2025 at 8:07 AM Tzung-Bi Shih <tzungbi@...nel.org> wrote:
>
> 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]?

Not really, as I noticed that both the ec and Type-C connector class
use their own mutexes.

My intention with the mutexes is to avoid race conditions in the case
when a role swap is in progress but at the same time there is a
`cros_port_update` that modifies the state of the port. Another
example I have in mind is when the port is being unregistered and
a role swap is issued.

Please let me know what you think.

>
> [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.

Thanks for pointing that out, indeed I can remove the lock in this
function.