linux-kernel - Re: [PATCH 5.4 055/191] mfd: mfd-core: Honour Device Trees request to disable a child-device

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Message-ID: <20200104123106.GB1296856@kroah.com>
Date:   Sat, 4 Jan 2020 13:31:06 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Stephan Gerhold <stephan@...hold.net>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Barry Song <Baohua.Song@....com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Mark Brown <broonie@...nel.org>,
        Lee Jones <lee.jones@...aro.org>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 5.4 055/191] mfd: mfd-core: Honour Device Trees request
 to disable a child-device

On Fri, Jan 03, 2020 at 11:23:47AM +0100, Stephan Gerhold wrote:
> On Thu, Jan 02, 2020 at 11:05:37PM +0100, Greg Kroah-Hartman wrote:
> > From: Lee Jones <lee.jones@...aro.org>
> > 
> > [ Upstream commit 6b5c350648b857047b47acf74a57087ad27d6183 ]
> > 
> > Until now, MFD has assumed all child devices passed to it (via
> > mfd_cells) are to be registered. It does not take into account
> > requests from Device Tree and the like to disable child devices
> > on a per-platform basis.
> > 
> > Well now it does.
> > 
> > Link: https://www.spinics.net/lists/arm-kernel/msg366309.html
> > Link: https://lkml.org/lkml/2019/8/22/1350
> 
> As far as I understand it, this commit is not suitable for backporting.
> The link above explains the problem for a similar patch:
> 
> 	But I believe this would introduce a rather ugly bug in
> 	mfd_remove_devices() if the first sub-device is set to disabled:
> 	It iterates over the children devices to find the base address
> 	of the allocated "usage count" array, which is then used to free it.
> 	If the first sub-device is missing, it would free the wrong address.
> 
> This problem does not exist in mainline because the MFD usage counting
> was removed entirely as part of the larger "Simplify MFD Core" series [1].
> 
> None of the device trees in the stable kernels should depend on
> disabling MFD devices from the device tree. (They were written at a time
> when the MFD core ignored that request entirely...)
> Therefore, I would suggest to drop this patch entirely.

Thanks for catching this, now dropped from all stable queues.

greg k-h