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From:   Jan Dabros <jsd@semihalf.com>
To:     linux-kernel@vger.kernel.org, linux-i2c@vger.kernel.org,
        jarkko.nikula@linux.intel.com, andriy.shevchenko@linux.intel.com
Cc:     mika.westerberg@linux.intel.com, rrangel@chromium.org,
        mw@semihalf.com, upstream@semihalf.com, jsd@semihalf.com
Subject: [PATCH v2] i2c: designware: Introduce semaphore reservation timer to AMDPSP driver
Date:   Fri, 12 Aug 2022 09:15:26 +0200
Message-Id: <20220812071526.414285-1-jsd@semihalf.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

In order to optimize performance, limit amount of back and forth
transactions between x86 and PSP. This is done by introduction of
semaphore reservation period - that is window in which x86 isn't
releasing the bus immediately after each I2C transaction.

In order to protect PSP from being starved while waiting for
arbitration, after a programmed time bus is automatically released by a
deferred function.

Signed-off-by: Jan Dabros <jsd@semihalf.com>
---
 drivers/i2c/busses/i2c-designware-amdpsp.c | 68 +++++++++++++++++-----
 1 file changed, 53 insertions(+), 15 deletions(-)

diff --git a/drivers/i2c/busses/i2c-designware-amdpsp.c b/drivers/i2c/busses/i2c-designware-amdpsp.c
index b624356c945f..b9bdc7db89df 100644
--- a/drivers/i2c/busses/i2c-designware-amdpsp.c
+++ b/drivers/i2c/busses/i2c-designware-amdpsp.c
@@ -6,6 +6,7 @@
 #include <linux/io-64-nonatomic-lo-hi.h>
 #include <linux/psp-sev.h>
 #include <linux/types.h>
+#include <linux/workqueue.h>
 
 #include <asm/msr.h>
 
@@ -15,6 +16,8 @@
 #define PSP_MBOX_OFFSET		0x10570
 #define PSP_CMD_TIMEOUT_US	(500 * USEC_PER_MSEC)
 
+#define PSP_I2C_RESERVATION_TIME_MS 100
+
 #define PSP_I2C_REQ_BUS_CMD		0x64
 #define PSP_I2C_REQ_RETRY_CNT		400
 #define PSP_I2C_REQ_RETRY_DELAY_US	(25 * USEC_PER_MSEC)
@@ -240,6 +243,42 @@ static int psp_send_i2c_req(enum psp_i2c_req_type i2c_req_type)
 	return ret;
 }
 
+static void release_bus(void)
+{
+	int status;
+
+	if (!psp_i2c_sem_acquired)
+		return;
+
+	status = psp_send_i2c_req(PSP_I2C_REQ_RELEASE);
+	if (status)
+		return;
+
+	dev_dbg(psp_i2c_dev, "PSP semaphore held for %ums\n",
+		jiffies_to_msecs(jiffies - psp_i2c_sem_acquired));
+
+	psp_i2c_sem_acquired = 0;
+}
+
+static void psp_release_i2c_bus_deferred(struct work_struct *work)
+{
+
+	mutex_lock(&psp_i2c_access_mutex);
+
+	/*
+	 * If there is any pending transaction, cannot release the bus here.
+	 * psp_release_i2c_bus will take care of this later.
+	 */
+	if (psp_i2c_access_count)
+		goto cleanup;
+
+	release_bus();
+
+cleanup:
+	mutex_unlock(&psp_i2c_access_mutex);
+}
+static DECLARE_DELAYED_WORK(release_queue, psp_release_i2c_bus_deferred);
+
 static int psp_acquire_i2c_bus(void)
 {
 	int status;
@@ -250,21 +289,23 @@ static int psp_acquire_i2c_bus(void)
 	if (psp_i2c_mbox_fail)
 		goto cleanup;
 
+	psp_i2c_access_count++;
+
 	/*
-	 * Simply increment usage counter and return if PSP semaphore was
-	 * already taken by kernel.
+	 * No need to request bus arbitration once we are inside semaphore
+	 * reservation period.
 	 */
-	if (psp_i2c_access_count) {
-		psp_i2c_access_count++;
+	if (psp_i2c_sem_acquired)
 		goto cleanup;
-	}
 
 	status = psp_send_i2c_req(PSP_I2C_REQ_ACQUIRE);
 	if (status)
 		goto cleanup;
 
 	psp_i2c_sem_acquired = jiffies;
-	psp_i2c_access_count++;
+
+	schedule_delayed_work(&release_queue,
+			      msecs_to_jiffies(PSP_I2C_RESERVATION_TIME_MS));
 
 	/*
 	 * In case of errors with PSP arbitrator psp_i2c_mbox_fail variable is
@@ -279,8 +320,6 @@ static int psp_acquire_i2c_bus(void)
 
 static void psp_release_i2c_bus(void)
 {
-	int status;
-
 	mutex_lock(&psp_i2c_access_mutex);
 
 	/* Return early if mailbox was malfunctional */
@@ -295,13 +334,12 @@ static void psp_release_i2c_bus(void)
 	if (psp_i2c_access_count)
 		goto cleanup;
 
-	/* Send a release command to PSP */
-	status = psp_send_i2c_req(PSP_I2C_REQ_RELEASE);
-	if (status)
-		goto cleanup;
-
-	dev_dbg(psp_i2c_dev, "PSP semaphore held for %ums\n",
-		jiffies_to_msecs(jiffies - psp_i2c_sem_acquired));
+	/*
+	 * Send a release command to PSP if the semaphore reservation timeout
+	 * elapsed but x86 still owns the controller.
+	 */
+	if (!delayed_work_pending(&release_queue))
+		release_bus();
 
 cleanup:
 	mutex_unlock(&psp_i2c_access_mutex);
-- 
2.37.1.595.g718a3a8f04-goog