coresight: tmc: Read TMC mode only when TMC hw is enabled

Reading TMC mode register in tmc_read_prepare_etb without
enabling the TMC hardware leads to async exceptions like
the one in the call trace below. This can happen if the
user tries to read the TMC etf data via device node without
setting up source and the sink first which enables the TMC
hardware in the path. So make sure that the TMC is enabled
before we try to read TMC data.

 Kernel panic - not syncing: Asynchronous SError Interrupt
 CPU: 7 PID: 2605 Comm: hexdump Tainted: G S                5.4.30 torvalds#122
 Call trace:
  dump_backtrace+0x0/0x188
  show_stack+0x20/0x2c
  dump_stack+0xdc/0x144
  panic+0x168/0x36c
  panic+0x0/0x36c
  arm64_serror_panic+0x78/0x84
  do_serror+0x130/0x138
  el1_error+0x84/0xf8
  tmc_read_prepare_etb+0x88/0xb8
  tmc_open+0x40/0xd8
  misc_open+0x120/0x158
  chrdev_open+0xb8/0x1a4
  do_dentry_open+0x268/0x3a0
  vfs_open+0x34/0x40
  path_openat+0x39c/0xdf4
  do_filp_open+0x90/0x10c
  do_sys_open+0x150/0x3e8
  __arm64_compat_sys_openat+0x28/0x34
  el0_svc_common+0xa8/0x160
  el0_svc_compat_handler+0x2c/0x38
  el0_svc_compat+0x8/0x10

Fixes: 4525412 ("coresight: tmc: making prepare/unprepare functions generic")
Reported-by: Stephen Boyd <[email protected]>
Signed-off-by: Sai Prakash Ranjan <[email protected]>

Author: Sai Prakash Ranjan

drivers/hwtracing/coresight/coresight-tmc.c

@@ -62,11 +62,13 @@ void tmc_flush_and_stop(struct tmc_drvdata *drvdata)
 
 void tmc_enable_hw(struct tmc_drvdata *drvdata)
 {
+	drvdata->enable = true;
 	writel_relaxed(TMC_CTL_CAPT_EN, drvdata->base + TMC_CTL);
 }
 
 void tmc_disable_hw(struct tmc_drvdata *drvdata)
 {
+	drvdata->enable = false;
 	writel_relaxed(0x0, drvdata->base + TMC_CTL);
 }
 
@@ -102,6 +104,9 @@ static int tmc_read_prepare(struct tmc_drvdata *drvdata)
 {
 	int ret = 0;
 
+	if (!drvdata->enable)
+		return -EINVAL;
+
 	switch (drvdata->config_type) {
 	case TMC_CONFIG_TYPE_ETB:
 	case TMC_CONFIG_TYPE_ETF:

drivers/hwtracing/coresight/coresight-tmc.h

@@ -191,6 +191,7 @@ struct tmc_drvdata {
 	struct miscdevice	miscdev;
 	spinlock_t		spinlock;
 	pid_t			pid;
+	bool			enable;
 	bool			reading;
 	union {
 		char		*buf;		/* TMC ETB */