linux-kernel - Re: [PATCH v2] extcon: fix extcon_get_extcon

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Date:   Tue, 23 Nov 2021 06:48:41 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Dan Carpenter <dan.carpenter@...cle.com>,
        MyungJoo Ham <myungjoo.ham@...sung.com>
Cc:     Chanwoo Choi <cw00.choi@...sung.com>,
        Sebastian Reichel <sre@...nel.org>,
        Chen-Yu Tsai <wens@...e.org>,
        Hans de Goede <hdegoede@...hat.com>,
        Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-usb@...r.kernel.org, linux-omap@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH v2] extcon: fix extcon_get_extcon_dev() error handling

On 11/23/21 12:43 AM, Dan Carpenter wrote:
> The extcon_get_extcon_dev() function returns error pointers on error,
> NULL when it's a -EPROBE_DEFER defer situation, and ERR_PTR(-ENODEV)
> when the CONFIG_EXTCON option is disabled.  This is very complicated for
> the callers to handle and a number of them had bugs that would lead to
> an Oops.
> 
> In real life, there are two things which prevented crashes.  First,
> error pointers would only be returned if there was bug in the caller
> where they passed a NULL "extcon_name" and none of them do that.
> Second, only two out of the eight drivers will build when CONFIG_EXTCON
> is disabled.
> 
> The normal way to write this would be to return -EPROBE_DEFER directly
> when appropriate and return NULL when CONFIG_EXTCON is disabled.  Then
> the error handling is simple and just looks like:
> 
> 	dev->edev = extcon_get_extcon_dev(acpi_dev_name(adev));
> 	if (IS_ERR(dev->edev))
> 		return PTR_ERR(dev->edev);
> 
> For the two drivers which can build with CONFIG_EXTCON disabled, then
> extcon_get_extcon_dev() will now return NULL which is not treated as an
> error and the probe will continue successfully.  Those two drivers are
> "typec_fusb302" and "max8997-battery".  In the original code, the
> typec_fusb302 driver had an 800ms hang in tcpm_get_current_limit() but
> now that function is a no-op.  For the max8997-battery driver everything
> should continue working as is.
> 
> Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>

Reviewed-by: Guenter Roeck <linux@...ck-us.net>