stm class: Support devices that override software assigned masters

Some STM devices adjust software assigned master numbers depending on
the trace source and its runtime state and whatnot. This patch adds
a sysfs attribute to inform the trace-side software that master numbers
assigned to software sources will not match those in the STP stream,
so that, for example, master/channel allocation policy can be adjusted
accordingly.

Signed-off-by: Alexander Shishkin <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/ABI/testing/sysfs-class-stm

···
 Contact:	Alexander Shishkin <[e-mail]>
 Description:
 		Shows the number of channels per master on this STM device.
+
+What:		/sys/class/stm/<stm>/hw_override
+Date:		March 2016
+KernelVersion:	4.7
+Contact:	Alexander Shishkin <[e-mail]>
+Description:
+		Reads as 0 if master numbers in the STP stream produced by
+		this stm device will match the master numbers assigned by
+		the software or 1 if the stm hardware overrides software
+		assigned masters.

drivers/hwtracing/stm/core.c

···
 
 static DEVICE_ATTR_RO(channels);
 
+static ssize_t hw_override_show(struct device *dev,
+				struct device_attribute *attr,
+				char *buf)
+{
+	struct stm_device *stm = to_stm_device(dev);
+	int ret;
+
+	ret = sprintf(buf, "%u\n", stm->data->hw_override);
+
+	return ret;
+}
+
+static DEVICE_ATTR_RO(hw_override);
+
 static struct attribute *stm_attrs[] = {
 	&dev_attr_masters.attr,
 	&dev_attr_channels.attr,
+	&dev_attr_hw_override.attr,
 	NULL,
 };
 

include/linux/stm.h

···
  * @sw_end:		last STP master available to software
  * @sw_nchannels:	number of STP channels per master
  * @sw_mmiosz:		size of one channel's IO space, for mmap, optional
+ * @hw_override:	masters in the STP stream will not match the ones
+ *			assigned by software, but are up to the STM hardware
  * @packet:		callback that sends an STP packet
  * @mmio_addr:		mmap callback, optional
  * @link:		called when a new stm_source gets linked to us, optional
···
 	unsigned int		sw_end;
 	unsigned int		sw_nchannels;
 	unsigned int		sw_mmiosz;
+	unsigned int		hw_override;
 	ssize_t			(*packet)(struct stm_data *, unsigned int,
 					  unsigned int, unsigned int,
 					  unsigned int, unsigned int,