Date:   Mon, 24 Sep 2018 12:48:55 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Hans de Goede <hdegoede@...hat.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

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.

> +}

> +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.

-- 
With Best Regards,
Andy Shevchenko

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