Re: [PATCH v3 1/3] x86: baytrail/cherrytrail: Rework and move P-Unit PMIC bus semaphore code

From: Jarkko Nikula <jarkko.nikula@linux.intel.com>
To: Wolfram Sang <wsa@the-dreams.de>, Hans de Goede <hdegoede@redhat.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Mika Westerberg <mika.westerberg@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H . Peter Anvin" <hpa@zytor.com>,
	linux-i2c@vger.kernel.org, linux-acpi@vger.kernel.org,
	x86@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 1/3] x86: baytrail/cherrytrail: Rework and move P-Unit PMIC bus semaphore code
Date: Mon, 15 Oct 2018 17:15:40 +0300	[thread overview]
Message-ID: <f040787b-3f64-e338-7a9c-0d44f968f496@linux.intel.com> (raw)
In-Reply-To: <20181014131724.GA863@kunai>

On 10/14/2018 04:17 PM, Wolfram Sang wrote:
> On Thu, Oct 11, 2018 at 04:29:09PM +0200, Hans de Goede wrote:
>> On some BYT/CHT systems the SoC's P-Unit shares the I2C bus with the
>> kernel. The P-Unit has a semaphore for the PMIC bus which we can take to
>> block it from accessing the shared bus while the kernel wants to access it.
>>
>> Currently we have the I2C-controller driver acquiring and releasing the
>> semaphore around each I2C transfer. There are 2 problems with this:
>>
>> 1) PMIC accesses often come in the form of a read-modify-write on one of
>> the PMIC registers, we currently release the P-Unit's PMIC bus semaphore
>> between the read and the write. If the P-Unit modifies the register during
>> this window?, then we end up overwriting the P-Unit's changes.
>> I believe that this is mostly an academic problem, but I'm not sure.
>>
>> 2) To safely access the shared I2C bus, we need to do 3 things:
>> a) Notify the GPU driver that we are starting a window in which it may not
>> access the P-Unit, since the P-Unit seems to ignore the semaphore for
>> explicit power-level requests made by the GPU driver
>> b) Make a pm_qos request to force all CPU cores out of C6/C7 since entering
>> C6/C7 while we hold the semaphore hangs the SoC
>> c) Finally take the P-Unit's PMIC bus semaphore
>> All 3 these steps together are somewhat expensive, so ideally if we have
>> a bunch of i2c transfers grouped together we only do this once for the
>> entire group.
>>
>> Taking the read-modify-write on a PMIC register as example then ideally we
>> would only do all 3 steps once at the beginning and undo all 3 steps once
>> at the end.
>>
>> For this we need to be able to take the semaphore from within e.g. the PMIC
>> opregion driver, yet we do not want to remove the taking of the semaphore
>> from the I2C-controller driver, as that is still necessary to protect many
>> other code-paths leading to accessing the shared I2C bus.
>>
>> This means that we first have the PMIC driver acquire the semaphore and
>> then have the I2C controller driver trying to acquire it again.
>>
>> To make this possible this commit does the following:
>>
>> 1) Move the semaphore code from being private to the I2C controller driver
>> into the generic iosf_mbi code, which already has other code to deal with
>> the shared bus so that it can be accessed outside of the I2C bus driver.
>>
>> 2) Rework the code so that it can be called multiple times nested, while
>> still blocking I2C accesses while e.g. the GPU driver has indicated the
>> P-Unit needs the bus through a iosf_mbi_punit_acquire() call.
>>
>> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
> 
> For the record: once the designware maintainers are okay with this
> change, I am also okay with it going via the x86 platform tree.
> 
Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Tested-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>