can: m_can: add support for optional reset

This patch has been split from the original series [1].

In some SoCs (observed on the STM32MP15) the M_CAN IP core keeps the CAN
state and CAN error counters over an internal reset cycle. The STM32MP15
SoC provides an external reset, which is shared between both M_CAN cores.

Add support for an optional external reset. Take care of shared resets,
de-assert reset during the probe phase in m_can_class_register() and while
the interface is up, assert the reset otherwise.

[1] https://lore.kernel.org/all/[e-mail]

Reviewed-by: Philipp Zabel <[e-mail]>
Reviewed-by: Markus Schneider-Pargmann <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Marc Kleine-Budde <[e-mail]>

drivers/net/can/m_can/m_can.c

···
 #include <linux/pinctrl/consumer.h>
 #include <linux/platform_device.h>
 #include <linux/pm_runtime.h>
+#include <linux/reset.h>
 
 #include "m_can.h"
 
···
 
 	close_candev(dev);
 
+	reset_control_assert(cdev->rst);
 	m_can_clk_stop(cdev);
 	phy_power_off(cdev->transceiver);
 
···
 	if (err)
 		goto out_phy_power_off;
 
+	err = reset_control_deassert(cdev->rst);
+	if (err)
+		goto exit_disable_clks;
+
 	/* open the can device */
 	err = open_candev(dev);
 	if (err) {
 		netdev_err(dev, "failed to open can device\n");
-		goto exit_disable_clks;
+		goto out_reset_control_assert;
 	}
 
 	if (cdev->is_peripheral)
···
 	else
 		napi_disable(&cdev->napi);
 	close_candev(dev);
+out_reset_control_assert:
+	reset_control_assert(cdev->rst);
 exit_disable_clks:
 	m_can_clk_stop(cdev);
 out_phy_power_off:
···
 		}
 	}
 
+	cdev->rst = devm_reset_control_get_optional_shared(cdev->dev, NULL);
+	if (IS_ERR(cdev->rst))
+		return dev_err_probe(cdev->dev, PTR_ERR(cdev->rst),
+				     "Failed to get reset line\n");
+
 	ret = m_can_clk_start(cdev);
 	if (ret)
 		return ret;
+
+	ret = reset_control_deassert(cdev->rst);
+	if (ret)
+		goto clk_disable;
 
 	if (cdev->is_peripheral) {
 		ret = can_rx_offload_add_manual(cdev->net, &cdev->offload,
 						NAPI_POLL_WEIGHT);
 		if (ret)
-			goto clk_disable;
+			goto out_reset_control_assert;
 	}
 
 	if (!cdev->net->irq) {
···
 		 KBUILD_MODNAME, cdev->net->irq, cdev->version);
 
 	/* Probe finished
-	 * Stop clocks. They will be reactivated once the M_CAN device is opened
+	 * Assert reset and stop clocks.
+	 * They will be reactivated once the M_CAN device is opened
 	 */
+	reset_control_assert(cdev->rst);
 	m_can_clk_stop(cdev);
 
 	return 0;
···
 rx_offload_del:
 	if (cdev->is_peripheral)
 		can_rx_offload_del(&cdev->offload);
+out_reset_control_assert:
+	reset_control_assert(cdev->rst);
 clk_disable:
 	m_can_clk_stop(cdev);
 

drivers/net/can/m_can/m_can.h

···
 	struct device *dev;
 	struct clk *hclk;
 	struct clk *cclk;
+	struct reset_control *rst;
 
 	struct workqueue_struct *tx_wq;
 	struct phy *transceiver;