linux-kernel - Re: [PATCH v2 00/10] Split i2c_lock_adapter into i2c_lock_root and i2c_lock

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Date:   Tue, 26 Jun 2018 11:37:36 +0900
From:   Wolfram Sang <wsa@...-dreams.de>
To:     Peter Rosin <peda@...ntia.se>
Cc:     linux-kernel@...r.kernel.org, Peter Huewe <peterhuewe@....de>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Jason Gunthorpe <jgg@...pe.ca>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Brian Norris <computersforpeace@...il.com>,
        Gregory Fong <gregory.0xf0@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        bcm-kernel-feedback-list@...adcom.com,
        Sekhar Nori <nsekhar@...com>,
        Kevin Hilman <khilman@...nel.org>,
        Haavard Skinnemoen <hskinnemoen@...il.com>,
        Kukjin Kim <kgene@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Orson Zhai <orsonzhai@...il.com>,
        Baolin Wang <baolin.wang@...aro.org>,
        Chunyan Zhang <zhang.lyra@...il.com>,
        Guenter Roeck <linux@...ck-us.net>, Crt Mori <cmo@...exis.com>,
        Jonathan Cameron <jic23@...nel.org>,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Antti Palosaari <crope@....fi>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Michael Krufky <mkrufky@...uxtv.org>,
        Lee Jones <lee.jones@...aro.org>,
        linux-integrity@...r.kernel.org, linux-i2c@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, linux-iio@...r.kernel.org,
        linux-input@...r.kernel.org, linux-media@...r.kernel.org
Subject: Re: [PATCH v2 00/10] Split i2c_lock_adapter into i2c_lock_root and
 i2c_lock_segment

On Wed, Jun 20, 2018 at 07:17:53AM +0200, Peter Rosin wrote:
> Hi!
> 
> With the introduction of mux-locked I2C muxes, the concept of
> locking only a segment of the I2C adapter tree was added. At the
> time, I did not want to cause a lot of extra churn, so left most
> users of i2c_lock_adapter alone and apparently didn't think enough
> about it; they simply continued to lock the whole adapter tree.
> However, i2c_lock_adapter is in fact wrong for almost every caller
> (there is naturally an exception) that is itself not a driver for
> a root adapter. What normal drivers generally want is to only
> lock the segment of the adapter tree that their device sits on.
> 
> In fact, if a device sits behind a mux-locked I2C mux, and its
> driver calls i2c_lock_adapter followed by an unlocked I2C transfer,
> things will deadlock (since even a mux-locked I2C adapter will lock
> its parent at some point). If the device is not sitting behind a
> mux-locked I2C mux (i.e. either directly on the root adapter or
> behind a (chain of) parent-locked I2C muxes) the root/segment
> distinction is of no consequence; the root adapter is locked either
> way.
> 
> Mux-locked I2C muxes are probably not that common, and putting any
> of the affected devices behind one is probably even rarer, which
> is why we have not seen any deadlocks. At least not that I know
> of...
> 
> Since silently changing the semantics of i2c_lock_adapter might
> be quite a surprise, especially for out-of-tree users, this series
> instead removes the function and forces all users to explicitly
> name I2C_LOCK_SEGMENT or I2C_LOCK_ROOT_ADAPTER in a call to
> i2c_lock_bus, as suggested by Wolfram. Yes, users will be a teensy
> bit more wordy, but open-coding I2C locking from random drivers
> should be avoided, so it's perhaps a good thing if it doesn't look
> too neat?
> 
> I suggest that Wolfram takes this series through the I2C tree and
> creates an immutable branch for the other subsystems. The series
> is based on v4.18-r1.

Applied to a seperate branch named "i2c/precise-locking-names" which I
will merge into for-next, so it will get proper testing already. Once we
get the missing acks from media, MFD, and IIO maintainers, I will merge
it into for-4.19.

Thank you, Peter!