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[73.88.245.53]) by smtp.gmail.com with ESMTPSA id c6sm3609080qkg.93.2020.06.11.21.07.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 11 Jun 2020 21:07:33 -0700 (PDT) Subject: Re: [RFC] MFD's relationship with Device Tree (OF) To: Lee Jones , Andy Shevchenko , Michael Walle , Rob Herring , Mark Brown , devicetree , Linux Kernel Mailing List , linux-arm Mailing List , Linus Walleij , Guenter Roeck , Andy Shevchenko , Robin Murphy , GregKroah-Hartmangregkh@linuxfoundation.org References: <20200609110136.GJ4106@dell> From: Frank Rowand Message-ID: <7cf94809-7346-31bc-877c-679ecc4d9710@gmail.com> Date: Thu, 11 Jun 2020 23:07:32 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: <20200609110136.GJ4106@dell> Content-Language: en-US X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200611_210737_480364_4610E6A9 X-CRM114-Status: GOOD ( 31.97 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org Hi Lee, Please add me to the distribution list for future versions of this. -Frank On 2020-06-09 06:01, Lee Jones wrote: > Good morning, > > After a number of reports/queries surrounding a known long-term issue > in the MFD core, including the submission of a couple of attempted > solutions, I've decided to finally tackle this one myself. > > Currently, when a child platform device (sometimes referred to as a > sub-device) is registered via the Multi-Functional Device (MFD) API, > the framework attempts to match the newly registered platform device > with its associated Device Tree (OF) node. Until now, the device has > been allocated the first node found with an identical OF compatible > string. Unfortunately, if there are, say for example '3' devices > which are to be handled by the same driver and therefore have the same > compatible string, each of them will be allocated a pointer to the > *first* node. > > Let me give you an example. > > I have knocked up an example 'parent' and 'child' device driver. The > parent utilises the MFD API to register 3 identical children, each > controlled by the same driver. This happens a lot. Fortunately, in > the majority of cases, the OF nodes are also totally identical, but > what if you wish to configure one of the child devices with different > attributes or resources supplied via Device Tree, like a clock? This > is currently impossible. > > Here is the Device Tree representation for the 1 parent and the 3 > child (sub) devices described above: > > parent { > compatible = "mfd,of-test-parent"; > > child@0 { > compatible = "mfd,of-test-child"; > clocks = <&clock 0>; > }; > > child@1 { > compatible = "mfd,of-test-child"; > clocks = <&clock 1>; > }; > > child@2 { > compatible = "mfd,of-test-child"; > clocks = <&clock 2>; > }; > }; > > This is how we register those devices from MFD: > > static const struct mfd_cell mfd_of_test_cell[] = { > OF_MFD_CELL("mfd_of_test_child", NULL, NULL, 0, 0, "mfd,of-test-child"), > OF_MFD_CELL("mfd_of_test_child", NULL, NULL, 0, 1, "mfd,of-test-child"), > OF_MFD_CELL("mfd_of_test_child", NULL, NULL, 0, 2, "mfd,of-test-child") > }; > > ... which we pass into mfd_add_devices() for processing. > > In an ideal world. The devices with the platform_id; 0, 1 and 2 would > be matched up to Device Tree nodes; child@0, child@1 and child@2 > respectively. Instead all 3 devices will be allocated a pointer to > child@0's OF node, which is obviously not correct. > > This is how it looks when each of the child devices are probed: > > [0.708287] mfd-of-test-parent mfd_of_test: Registering 3 devices > [...] > [0.712511] mfd-of-test-child mfd_of_test_child.0: Probing platform device: 0 > [0.712710] mfd-of-test-child mfd_of_test_child.0: Using OF node: child@0 > [0.713033] mfd-of-test-child mfd_of_test_child.1: Probing platform device: 1 > [0.713381] mfd-of-test-child mfd_of_test_child.1: Using OF node: child@0 > [0.713691] mfd-of-test-child mfd_of_test_child.2: Probing platform device: 2 > [0.713889] mfd-of-test-child mfd_of_test_child.2: Using OF node: child@0 > > "Why is it when I change child 2's clock rate, it also changes 0's?" > > Whoops! > > So in order to fix this, we need to make MFD more-cleverer! > > However, this is not so simple. There are some rules we should abide > by (I use "should" intentionally here, as something might just have to > give): > > a) Since Device Tree is designed to describe hardware, inserting > arbitrary properties into DT is forbidden. This precludes things > we would ordinarily be able to match on, like 'id' or 'name'. > b) As an extension to a) DTs should also be OS agnostic, so > properties like 'mfd-device', 'mfd-order' etc are also not > not suitable for inclusion. > c) The final solution should ideally be capable of supporting both > newly defined and current trees (without retroactive edits) > alike. > d) Existing properties could be used, but not abused. For example, > one of my suggestions (see below) is to use the 'reg' property. > This is fine in principle but loading 'reg' with arbitrary values > (such as; 0, 1, 2 ... x) which 1) clearly do not have anything to > do with registers and 2) would be meaningless in other OSes/ > implementations, just to serve our purpose, is to be interpreted > as an abuse. > > Proposal 1: > > As mentioned above, my initial thoughts were to use the 'reg' property > to match an MFD cell entry with the correct DT node. However, not > all Device Tree nodes have 'reg' properties. Particularly true in the > case of MFD, where memory resources are usually shared with the parent > via Regmap, or (as in the case of the ab8500) the MFD handles all > register transactions via its own API. > > Proposal 2: > > If we can't guarantee that all DT nodes will have at least one > property in common to be used for matching and we're prevented from > supplying additional, potentially bespoke properties, then we must > seek an alternative procedure. > > It should be possible to match based on order. However, the developer > would have to guarantee that the order in which the child devices are > presented to the MFD API are in exactly the same order as they are > represented in the Device Tree. The obvious draw-back to this > strategy is that it's potentially very fragile. > > Current Proposal: > > How about a collection of Proposal 1 and Proposal 2? First we could > attempt a match on the 'reg' property. Then, if that fails, we would > use the fragile-but-its-all-we-have Proposal 2 as the fall-back. > > Thoughts? > _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel