From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934115AbeAXO7T (ORCPT ); Wed, 24 Jan 2018 09:59:19 -0500 Received: from mail-wm0-f65.google.com ([74.125.82.65]:39082 "EHLO mail-wm0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933688AbeAXO7Q (ORCPT ); Wed, 24 Jan 2018 09:59:16 -0500 X-Google-Smtp-Source: AH8x226j1Yv5CuuawkDneEzUYTrbK5pF/JGbysa91lrdiwZW8tntbzmqI2EvIJc+nTrfLHxZrQpvnQ== Subject: Re: [PATCH v2] i2c: Allow ACPI_I2C_OPREGION if I2C is built as a module To: Andy Shevchenko , Sinan Kaya Cc: Mika Westerberg , Wolfram Sang , Eric Auger , eric.auger.pro@gmail.com, linux-i2c , Linux Kernel Mailing List , "linux-acpi@vger.kernel.org" References: <1514988151-12248-1-git-send-email-eric.auger@redhat.com> <20180124055615.c3qwhwnj7spmcr3w@ninjato> <20180124062725.GK27654@lahna.fi.intel.com> From: Hans de Goede Message-ID: <54ef10bd-6209-6983-1d3a-ddf7d7fea252@redhat.com> Date: Wed, 24 Jan 2018 15:59:13 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On 24-01-18 15:44, Andy Shevchenko wrote: > On Wed, Jan 24, 2018 at 3:29 PM, Sinan Kaya wrote: >> +linux-acpi > > +Cc: Hans Thank you. >> On 1/24/2018 1:27 AM, Mika Westerberg wrote: >>> On Wed, Jan 24, 2018 at 06:56:16AM +0100, Wolfram Sang wrote: >>>> On Wed, Jan 03, 2018 at 03:02:31PM +0100, Eric Auger wrote: >>>>> If I2C is built as a module, ACPI_I2C_OPREGION cannot be set >>>>> and any ACPI opregion calls targeting I2C fail with no opregion found. >>>>> >>>>> This patch allows ACPI_I2C_OPREGION to be enabled both if I2C is >>>>> built into the kernel or built as a module. >>>>> >>>>> Signed-off-by: Eric Auger >>>> >>>> I recall that we had some discussion until ending up with the current >>>> solution. And I finally found it again: >>>> >>>> http://www.serverphorums.com/read.php?12,1001402 >>>> >>>> In any case, I surely want Mika's ack on any change to ACPI related >>>> Kconfig symbols. Adding him to CC... >>> >>> So the problem is/was that what happens if you are in a middle of BIOS >>> AML code touching the opregion and someone unloads the opregion handler? >>> If you can quarantee nothing bad happens, then I'm fine with the patch :) >>> >> >> Rafael to correct me if I got this right. >> >> The behavior of the operating system is well defined in the ACPI specification. >> >> Here is what I tested recently: >> >> ACPI defines _REG method to inform firmware of presence/removal of an operating >> region. >> >> When driver gets loaded, ACPI calls the _REG method with 1 argument. When driver >> gets unloaded, ACPI call the _REG method with 0 argument. >> >> Firmware can use this notification to its advantage to determine when an I2C >> related functionality should be accessed or not. >> >> If firmware doesn't use the _REG method, ACPI defines that AML statements >> accessing the operating region are ignored. >> >> You'll also see a warning from ACPICA saying the OperatingRegion 9 is no longer >> accessible and AML code execution failed. >> There is the ACPI specification, and then there is reality. In reality many DSTDs do not use the typically named AVB? counter-part of _REG to check before accessing OpRegions. Also OpRegion availability vs probe ordering is a nightmare. Lets pretend that all DSTDs are perfect and that some device described in ACPI has a _PS0 method which uses an opregion to turn on some regulator powering the device through i2c, but only if the _REG method for that opregion has been called. So now lets say that the driver for this device loads and tries to bind before the i2c-module has loaded. Before the driver's probe method gets called the driver-core will call _PS0 to power-up the device, which is a nop (*). Then the drivers probe function tries to talk to the device, but fails as the device is not powered, so it returns with -ENODEV. And then later the i2c-module loads, which is to late to get things working, unless the user rmmods and modprobes the driver for the device manually. TL;DR: I have to NAK this, I'm sorry but with the current state of ACPI we must simply have some stuff builtin to help with probe-ordering issues. Now if the ACPI code where ever to honor the _DEP method everywhere instead of only for battery devices this might change, but even then things will still be tricky. Regards, Hans *) Typically while logging errors about unavailable OpRegions, but as said lets pretend DSTDs actually check the flags set by _REG everywhere.