coresight: tmc: Fix TMC mode read in tmc_read_prepare_etb()

coresight: tmc: Fix TMC mode read in tmc_read_prepare_etb()

On some QCOM platforms like SC7180, SDM845 and SM8150,
reading TMC mode register without proper coresight power
management can lead to async exceptions like the one in
the call trace below in tmc_read_prepare_etb(). This can
happen if the user tries to read the TMC etf data via
device node without setting up source and the sink first.
Fix this by having a check for coresight sysfs mode
before reading TMC mode management register.

 Kernel panic - not syncing: Asynchronous SError Interrupt
 CPU: 7 PID: 2605 Comm: hexdump Tainted: G S                5.4.30 #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: 4525412a5046 ("coresight: tmc: making prepare/unprepare functions generic")
Reported-by: Stephen Boyd <swboyd@chromium.org>
Suggested-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20200518180242.7916-14-mathieu.poirier@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>