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From:   Guenter Roeck <groeck@google.com>
Date:   Wed, 3 Apr 2019 11:51:03 -0700
Message-ID: <CABXOdTfvRryiMrFAe4wmvt5mm2k==b1BShbzHQ68vp5C3NFETA@mail.gmail.com>
Subject: Re: [PATCH] mfd: cros_ec: check for NULL transfer function
To:     egranata@chromium.org
Cc:     Benson Leung <bleung@chromium.org>,
        Guenter Roeck <groeck@chromium.org>,
        Gwendal Grignou <gwendal@chromium.org>,
        Alexandru M Stan <amstan@chromium.org>,
        Enric Balletbo i Serra <enric.balletbo@collabora.com>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Wed, Apr 3, 2019 at 11:31 AM <egranata@chromium.org> wrote:
>
> From: Enrico Granata <egranata@chromium.org>
>
> As new transfer mechanisms are added to the EC codebase, they may
> not support v2 of the EC protocol.
>
> If the v3 initial handshake transfer fails, the kernel will try
> and call cmd_xfer as a fallback. If v2 is not supported, cmd_xfer
> will be NULL, and the code will end up causing a kernel panic.
>
> Add a check for NULL before calling the transfer function, along
> with a helpful comment explaining how one might end up in this
> situation.
>
> Signed-off-by: Enrico Granata <egranata@chromium.org>
> ---
>  drivers/platform/chrome/cros_ec_proto.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/drivers/platform/chrome/cros_ec_proto.c b/drivers/platform/chrome/cros_ec_proto.c
> index 97a068dff192d..953076ab401aa 100644
> --- a/drivers/platform/chrome/cros_ec_proto.c
> +++ b/drivers/platform/chrome/cros_ec_proto.c
> @@ -56,6 +56,16 @@ static int send_command(struct cros_ec_device *ec_dev,
>         else
>                 xfer_fxn = ec_dev->cmd_xfer;
>
> +       if (xfer_fxn == NULL) {
> +               /* This error can happen if a communication error happened and
> +                * the EC is trying to use protocol v2, on an underlying
> +                * communication mechanism that does not support v2.
> +                */

I am not personally a friend of networking-style multi-line comments.

> +               dev_err(ec_dev->dev,
> +                       "missing EC transfer API, cannot send command\n");

That message will be displayed each time a message is sent, ie in
practice for each message. Is there any value in that, other than
clogging the log ?

Guenter

> +               return -EIO;
> +       }
> +
>         ret = (*xfer_fxn)(ec_dev, msg);
>         if (msg->result == EC_RES_IN_PROGRESS) {
>                 int i;
> --
> 2.21.0.392.gf8f6787159e-goog
>