From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 828BA1E487; Fri, 5 Sep 2025 08:38:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757061494; cv=none; b=nE0xntPBt3UuFvmLwt5ijvgYoo95GhHE5fIK8hAxAcr+rljm53yW3rhtpxnUiyPgamCWmZqlu/9/RYIQ650Qpf3u3iZ80jBNbdXUWQ8P6MkqI/oB7FmLejZ8IC3jHD6DhdLc3C52XOOtjBGlQw/pKTx5cZDsuSjJu9nqEpEdILM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757061494; c=relaxed/simple; bh=kVJcCIgesGiuVScQFp+2Alk9JjzRYcuW99Y4IEbJ3Ds=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=EwYH6KxNePVTfrSTj/N6Zi4sFr4pp1pGiKDo3V/+Ohs/cUvLAfhuNv5g9YZztO7VRA3kwqzXe9FAbYQtxg2D8PSEvE1Jurgu43J+8sgPmAyqvvInJc9yYDvLxJVrpkdVnRDT9hxSqxVWIeTH14y8KYileDfV/PaKHp3RByPqVsw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=tRlxbGKL; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="tRlxbGKL" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 406ABC4CEF1; Fri, 5 Sep 2025 08:38:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1757061494; bh=kVJcCIgesGiuVScQFp+2Alk9JjzRYcuW99Y4IEbJ3Ds=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=tRlxbGKLRR6fntuRfcvXSrUC0dNEASjnzCz8iQ1snWVKU/45oWQ9qqPZDtHWZ8AQF SR75bKEezu50VZ5Tn2Rkpqj0wmWfq7J+KG/sYtNTscoyGSazfB3ef9G5XkZDWB2pWm OWnvUyr+LlYPJyjI2mUpRRMJr/O9gDsxXDocCUUJIoWxYC++x/cLXQpn/pNIHcCdJi ZOq/GwObb48NDs3g48qwr0TGROwPizkvggxMVOewdvHHjfv8CWVTWv3UIwafejba57 mLR6u4Fiq/k+papej0D16ubJhJt2DnigrMOxjzTpKBZWO0IjUz1tAdUrWcpuinxdJy IKlymmpk/Ip5g== Date: Fri, 5 Sep 2025 08:38:10 +0000 From: Tzung-Bi Shih To: Dmitry Torokhov Cc: Benson Leung , linux-input@vger.kernel.org, chrome-platform@lists.linux.dev Subject: Re: [PATCH 0/5] platform/chrome: Fix a race when probing drivers Message-ID: References: <20250828083601.856083-1-tzungbi@kernel.org> <4gtrvxpo6zqk54uvavrox7hszszdpvdubz4w6iaks72zq3jjsw@b6cfvi5ysj2u> Precedence: bulk X-Mailing-List: linux-input@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4gtrvxpo6zqk54uvavrox7hszszdpvdubz4w6iaks72zq3jjsw@b6cfvi5ysj2u> On Thu, Sep 04, 2025 at 07:06:23AM -0700, Dmitry Torokhov wrote: > On Tue, Sep 02, 2025 at 09:18:47PM +0800, Tzung-Bi Shih wrote: > > On Fri, Aug 29, 2025 at 08:50:01PM +0800, Tzung-Bi Shih wrote: > > > On Fri, Aug 29, 2025 at 11:28:55AM +0000, Dmitry Torokhov wrote: > > > > On Thu, Aug 28, 2025 at 08:35:56AM +0000, Tzung-Bi Shih wrote: > > > > > A race is observed when cros_ec_lpc and cros-ec-keyb are all built as > > > > > modules. cros_ec_lpc is cros-ec-keyb's parent. However, they can be > > > > > probed at the same time. > > > > > > > > > > Example: > > > > > > > > > > + -----------------------------------------------------------------+ > > > > > | Some init process (e.g. udevd) | deferred_probe_work_func worker | > > > > > + -----------------------------------------------------------------+ > > > > > | Probe cros-ec-keyb. | | > > > > > | - Decide to defer[1]. | | > > > > > | | A device bound to a driver[2]. | > > > > > | Probe cros_ec_lpc. | | > > > > > | - Init the struct[3]. | | > > > > > | | Retry cros-ec-keyb from the | > > > > > | | deferred list[4]. | > > > > > | | - Won't defer again as [3]. | > > > > > | | - Access uninitialized data in | > > > > > | | the struct. | > > > > > | - Register the device. | | > > > > > + -----------------------------------------------------------------+ > > > > > > > > > > [1] https://elixir.bootlin.com/linux/v6.16/source/drivers/input/keyboard/cros_ec_keyb.c#L707 > > > > > [2] https://elixir.bootlin.com/linux/v6.16/source/drivers/base/dd.c#L405 > > > > > [3] https://elixir.bootlin.com/linux/v6.16/source/drivers/platform/chrome/cros_ec_lpc.c#L644 > > > > > [4] https://elixir.bootlin.com/linux/v6.16/source/drivers/base/dd.c#L418 > > > > > > > > > > Note that the device link[5] can't help as in the observed environment, > > > > > the devices are already added via device_add()[6]. > > > > > > > > > > [5] https://www.kernel.org/doc/html/latest/driver-api/device_link.html#usage > > > > > [6] https://elixir.bootlin.com/linux/v6.16/source/drivers/acpi/acpi_platform.c#L177 > > > > > > > > > > The series fixes the issue by ensuring the struct is ready for accessing > > > > > before continuing to probe cros-ec-keyb. > > > > > > > > Why is the keyboard platform device instantiated before the transport > > > > (cros_ec_lpc) is done initializing? I think this is the root of the > > > > issue... > > > > > > I may misunderstand but it seems to me: > > > > > > - The ACPI bus enumerated and instantiated the platform devices[6] first. > > > > > > - The keyboard platform device was probed when `cros_ec_keyb_driver` > > > registered. It deferred as its parent's drvdata was NULL[1]. > > > > > > - The transport platform device was probed when `cros_ec_lpc_driver` > > > registered. It set the drvdata[3]. > > > > > > - The keyboard platform device was probed again from retrying the deferred > > > list, by another thread `deferred_probe_work_func`. The parent's drvdata > > > wasn't NULL and cros_ec_register() for the transport device weren't > > > finished. The race happened. > > > > Hi Dmitry, > > > > Does it make sense to you? > > I'll have to research how MFD mixes up statically described and > DT-described platform devices and makes sure that children are not > probed before the parent is ready - I think we need to make cros_ec > behave the same way. I may misunderstand but FWIW: I failed to find relevant code in MFD [7] that guarantees the probe order. Also, I'm curious about wouldn't code at [7] results in duplicate platform devices? E.g., 1 populated from OF; 1 created by MFD. Note: current cros_ec_dev.c doesn't use `of_compatible` in struct mfd_cell. If we're looking at how cros_ec_dev.c guarantees the order: - The transport platfrom device is probed first. It calls cros_ec_register(). - In cros_ec_register(), it registers the MFD device "cros-ec-dev". And the children devices are added via mfd_add_devices(). Back to the issue we observed: - The platform devices are created when it scans the tree in ACPI[6]. We probably have no way to prevent the devices from adding unless specifying `enumeration_by_parent`[8]. - When some of them are modules, the driver registrations are tied to the module insertion. They can be arrived by anytime unless we use something similar to soft dependency[9]. A Kconfig dependency will also be needed to prevent cros_ec_lpcs=m but cros_ec_keyb=y. However, cros_ec_keyb would need to specify 2 possible dependencies "cros_ec_lpcs" and "cros_ec_spi"[10]. I'm not sure what would be happening if a system has no cros_ec_spi module at all. [7] https://elixir.bootlin.com/linux/v6.16/source/drivers/mfd/mfd-core.c#L184 [8] https://elixir.bootlin.com/linux/v6.16/source/drivers/acpi/scan.c#L2204 [9] https://elixir.bootlin.com/linux/v6.16/source/include/linux/module.h#L176 [10] https://elixir.bootlin.com/linux/v6.16/source/drivers/input/keyboard/Kconfig#L743