Date:   Thu, 11 Oct 2018 12:14:26 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
        Wolfram Sang <wsa@...-dreams.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>, linux-acpi@...r.kernel.org,
        linux-i2c@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] x86: baytrail/cherrytrail: Rework and move P-Unit
 PMIC bus semaphore code

Hi,

On 24-09-18 11:48, Andy Shevchenko wrote:
> On Sun, Sep 23, 2018 at 04:45:08PM +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@...hat.com>
>> ---
>> Note this commit deliberately limits the i2c-designware changes to
>> only touch i2c-designware-baytrail.c, deliberately not doing some cleanups
>> which become possible after removing the semaphore code from the
>> i2c-designmware code. This is done so that this commit can be merged
>> through the x86 tree without causing conflicts in the i2c tree.
>>
>> The cleanups to the i2c-designware tree will be done in a follow up
>> patch which can be merged once this commit is in place.
> 
>> +static void iosf_mbi_reset_semaphore(void)
>> +{
>> +	if (iosf_mbi_modify(BT_MBI_UNIT_PMC, MBI_REG_READ,
>> +			    iosf_mbi_sem_address, 0, PUNIT_SEMAPHORE_BIT))
>> +		dev_err(&mbi_pdev->dev, "Error punit semaphore reset failed\n");
>> +
>> +	pm_qos_update_request(&iosf_mbi_pm_qos, PM_QOS_DEFAULT_VALUE);
>> +
>> +	blocking_notifier_call_chain(&iosf_mbi_pmic_bus_access_notifier,
>> +				     MBI_PMIC_BUS_ACCESS_END, NULL);
> 
>> +	mutex_unlock(&iosf_mbi_punit_mutex);
> 
> Can we actually move this to the callers?
> To me sounds slightly more logical to see lock in *block*() call and unlock in
> *unblock*() respectively.

Done for v2, which I will send out as soon as I've ran some tests with it.

>> +}
> 
>> +int iosf_mbi_block_punit_i2c_access(void)
>> +{
>> +	unsigned long start, end;
>> +	int ret = 0;
>> +	u32 sem;
>> +
>> +	if (WARN_ON(!mbi_pdev || !iosf_mbi_sem_address))
>> +		return -ENXIO;
>> +
>> +	mutex_lock(&iosf_mbi_block_punit_i2c_access_count_mutex);
>> +
>> +	if (iosf_mbi_block_punit_i2c_access_count > 0)
>> +		goto out;
>> +
>> +	mutex_lock(&iosf_mbi_punit_mutex);
>> +	blocking_notifier_call_chain(&iosf_mbi_pmic_bus_access_notifier,
>> +				     MBI_PMIC_BUS_ACCESS_BEGIN, NULL);
>> +
>> +	/*
>> +	 * Disallow the CPU to enter C6 or C7 state, entering these states
>> +	 * requires the punit to talk to the pmic and if this happens while
>> +	 * we're holding the semaphore, the SoC hangs.
>> +	 */
>> +	pm_qos_update_request(&iosf_mbi_pm_qos, 0);
>> +
>> +	/* host driver writes to side band semaphore register */
>> +	ret = iosf_mbi_write(BT_MBI_UNIT_PMC, MBI_REG_WRITE,
>> +			     iosf_mbi_sem_address, PUNIT_SEMAPHORE_ACQUIRE);
>> +	if (ret) {
>> +		dev_err(&mbi_pdev->dev, "Error punit semaphore request failed\n");
>> +		goto out;
>> +	}
>> +
>> +	/* host driver waits for bit 0 to be set in semaphore register */
>> +	start = jiffies;
>> +	end = start + msecs_to_jiffies(SEMAPHORE_TIMEOUT);
>> +	do {
>> +		ret = iosf_mbi_get_sem(&sem);
>> +		if (!ret && sem) {
>> +			iosf_mbi_sem_acquired = jiffies;
>> +			dev_dbg(&mbi_pdev->dev, "punit semaphore acquired after %ums\n",
>> +				jiffies_to_msecs(jiffies - start));
>> +			goto out; /* Success, done. */
>> +		}
>> +
>> +		usleep_range(1000, 2000);
>> +	} while (time_before(jiffies, end));
>> +
>> +	ret = -ETIMEDOUT;
>> +	dev_err(&mbi_pdev->dev, "Error punit semaphore timed out, resetting\n");
>> +	iosf_mbi_reset_semaphore();
>> +
>> +	if (!iosf_mbi_get_sem(&sem))
>> +		dev_err(&mbi_pdev->dev, "PUNIT SEM: %d\n", sem);
>> +out:
>> +	if (!WARN_ON(ret))
>> +		iosf_mbi_block_punit_i2c_access_count++;
>> +
>> +	mutex_unlock(&iosf_mbi_block_punit_i2c_access_count_mutex);
>> +
>> +	return ret;
>> +}
>> +EXPORT_SYMBOL(iosf_mbi_block_punit_i2c_access);
>> +
>> +void iosf_mbi_unblock_punit_i2c_access(void)
>> +{
>> +	mutex_lock(&iosf_mbi_block_punit_i2c_access_count_mutex);
>> +
>> +	iosf_mbi_block_punit_i2c_access_count--;
>> +	if (iosf_mbi_block_punit_i2c_access_count == 0) {
>> +		iosf_mbi_reset_semaphore();
>> +		dev_dbg(&mbi_pdev->dev, "punit semaphore held for %ums\n",
>> +			jiffies_to_msecs(jiffies - iosf_mbi_sem_acquired));
>> +	}
>> +
>> +	mutex_unlock(&iosf_mbi_block_punit_i2c_access_count_mutex);
>> +}
>> +EXPORT_SYMBOL(iosf_mbi_unblock_punit_i2c_access);
> 
>> +	{ PCI_DEVICE(PCI_VENDOR_ID_INTEL, PCI_DEVICE_ID_BAYTRAIL),
>> +	  .driver_data = PUNIT_SEMAPHORE_BYT },
>> +	{ PCI_DEVICE(PCI_VENDOR_ID_INTEL, PCI_DEVICE_ID_BRASWELL),
>> +	  .driver_data = PUNIT_SEMAPHORE_CHT },
>>   	{ PCI_DEVICE(PCI_VENDOR_ID_INTEL, PCI_DEVICE_ID_QUARK_X1000) },
>>   	{ PCI_DEVICE(PCI_VENDOR_ID_INTEL, PCI_DEVICE_ID_TANGIER) },
>>   	{ 0, },
> 
> Perhaps it can be converted to use PCI_DEVICE_DATA() macro.

Also done for v2.

Regards,

Hans

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