linux-kernel - Re: [PATCH] mfd: cros_ec: check for NULL transfer function

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Message-ID: <CABXOdTeE5=iF24+2JxwjvGtVtFmneYJWC4yXgX1Ah4y+=+ecEg@mail.gmail.com>
Date:   Wed, 3 Apr 2019 12:53:40 -0700
From:   Guenter Roeck <groeck@...gle.com>
To:     Enrico Granata <egranata@...gle.com>
Cc:     Enrico Granata <egranata@...omium.org>,
        Benson Leung <bleung@...omium.org>,
        Guenter Roeck <groeck@...omium.org>,
        Gwendal Grignou <gwendal@...omium.org>,
        Alexandru M Stan <amstan@...omium.org>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mfd: cros_ec: check for NULL transfer function

On Wed, Apr 3, 2019 at 12:40 PM Enrico Granata <egranata@...gle.com> wrote:
>
> I can certainly add a "did_print_error" flag or some such, but in practice, if the transfer function is NULL, the initial handshake will fail, and this will in turn cause EC registration to fail, and no further communication should occur, so no further log entries will be printed.
>
Sorry, I am a bit lost. Why would dev_err_once() not work ?

Guenter

> Thanks
>
> Enrico Granata | egranata@...gle.com | ChromeOS | MTV1600
>
>
> On Wed, Apr 3, 2019 at 11:51 AM Guenter Roeck <groeck@...gle.com> wrote:
>>
>> On Wed, Apr 3, 2019 at 11:31 AM <egranata@...omium.org> wrote:
>> >
>> > From: Enrico Granata <egranata@...omium.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@...omium.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
>> >