linux-kernel - Re: [PATCH v2] base/platform: return success when probe function is NULL

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Date:	Tue, 1 Dec 2015 21:30:24 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	martin.wilck@...fujitsu.com
Cc:	gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
	tpmdd-devel@...ts.sourceforge.net
Subject: Re: [PATCH v2] base/platform: return success when probe function is
 NULL

Hello,

On Mon, Nov 30, 2015 at 12:50:05PM +0100, martin.wilck@...fujitsu.com wrote:
> From: Martin Wilck <Martin.Wilck@...fujitsu.com>
> 
> Since b8b2c7d845d5, platform_drv_probe() is called for all platform
> devices. If drv->probe is NULL, and dev_pm_domain_attach() fails,
> platform_drv_probe() will return the error code from dev_pm_domain_attach().
> 
> This causes real_probe() to enter the "probe_failed" path and set
> dev->driver to NULL. Before b8b2c7d845d5, real_probe() would assume
> success if both dev->bus->probe and drv->probe were missing. As a result,
> a device and driver could be "bound" together just by matching their names;
> this doesn't work any more after b8b2c7d845d5.
> 
> This may cause problems later for certain usage of platform_driver_register()
> and platform_device_register_simple(). I observed a panic while loading
> the tpm_tis driver with parameter "force=1" (i.e. registering tpm_tis as
> a platform driver), because tpm_tis_init's assumption that the device
> returned by platform_device_register_simple() was bound didn't hold any more
> (tpmm_chip_alloc() dereferences chip->pdev->driver, causing panic).

I'm a bit uncertain if I should be happy with this wording or not. While
b8b2c7d845d5 has a bug which made the tpm_tis driver provoke a panic and
which needs fixing, this doesn't mean that the tpm_tis driver was right.

So "This may cause problems later ..." isn't a correct justification
because even in the presence of the bug introduced by b8b2c7d845d5, the
tpm_tis driver must not assume that it's device is bound on return of
platform_device_register_simple.

It's just that the bug in b8b2c7d845d5 made a wrong assumption of the
tpm_tis driver obvious, and now both need fixing.

If this were my patch, I'd not talk about tpm_tis at all. b8b2c7d845d5
broke binding for platform drivers with no probe function in certain
situations. This is corrected here.

> This patch restores the previous (4.3.0 and earlier) behavior of
> platform_drv_probe() in the case when the associated platform driver has
> no "probe" function.
> 
> v2: fixed style issues, rephrased commit message.

This must go after the triple-dash below to not be included in the
commit.

> Fixes: b8b2c7d845d5 ("base/platform: assert that dev_pm_domain callbacks are called unconditionally")
> Signed-off-by: Martin Wilck <Martin.Wilck@...fujitsu.com>
> ---

Best regards
Uwe
-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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