bus: mhi: host: Add soc_reset sysfs

The MHI bus supports a standardized hardware reset, which is known as the
"SoC Reset".  This reset is similar to the reset sysfs for PCI devices -
a hardware mechanism to reset the state back to square one.

The MHI SoC Reset is described in the spec as a reset of last resort.  If
some unrecoverable error has occurred where other resets have failed, SoC
Reset is the "big hammer" that ungracefully resets the device.  This is
effectivly the same as yanking the power on the device, and reapplying it.
However, depending on the nature of the particular issue, the underlying
transport link may remain active and configured.  If the link remains up,
the device will flag a MHI system error early in the boot process after
the reset is executed, which allows the MHI bus to process a fatal error
event, and clean up appropiately.

While the SoC Reset is generally intended as a means of recovery when all
else has failed, it can be useful in non-error scenarios.  For example,
if the device loads firmware from the host filesystem, the device may need
to be fully rebooted inorder to pick up the new firmware.  In this
scenario, the system administrator may use the soc_reset sysfs to cause
the device to pick up the new firmware that the admin placed on the
filesystem.
Signed-off-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Reviewed-by: Bhaumik Bhatt <quic_bbhatt@quicinc.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/1650302327-30439-1-git-send-email-quic_jhugo@quicinc.comSigned-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>

Documentation/ABI/stable/sysfs-bus-mhi

@@ -19,3 +19,13 @@ Description:	The file holds the OEM PK Hash value of the endpoint device
 		read without having the device power on at least once, the file
 		will read all 0's.
 Users:		Any userspace application or clients interested in device info.
+
+What:           /sys/bus/mhi/devices/.../soc_reset
+Date:           April 2022
+KernelVersion:  5.19
+Contact:        mhi@lists.linux.dev
+Description:	Initiates a SoC reset on the MHI controller.  A SoC reset is
+                a reset of last resort, and will require a complete re-init.
+                This can be useful as a method of recovery if the device is
+                non-responsive, or as a means of loading new firmware as a
+                system administration task.

drivers/bus/mhi/host/init.c

@@ -108,9 +108,23 @@ static ssize_t oem_pk_hash_show(struct device *dev,
 }
 static DEVICE_ATTR_RO(oem_pk_hash);
 
+static ssize_t soc_reset_store(struct device *dev,
+			       struct device_attribute *attr,
+			       const char *buf,
+			       size_t count)
+{
+	struct mhi_device *mhi_dev = to_mhi_device(dev);
+	struct mhi_controller *mhi_cntrl = mhi_dev->mhi_cntrl;
+
+	mhi_soc_reset(mhi_cntrl);
+	return count;
+}
+static DEVICE_ATTR_WO(soc_reset);
+
 static struct attribute *mhi_dev_attrs[] = {
 	&dev_attr_serial_number.attr,
 	&dev_attr_oem_pk_hash.attr,
+	&dev_attr_soc_reset.attr,
 	NULL,
 };
 ATTRIBUTE_GROUPS(mhi_dev);