firmware: qcom: scm: Expose download-mode control

In order to aid post-mortem debugging the Qualcomm platforms provide a
"memory download mode", where the boot loader will provide an interface
for custom tools to "download" the content of RAM to a host machine.

The mode is triggered by writing a magic value somewhere in RAM, that is
read in the boot code path after a warm-restart. Two mechanism for
setting this magic value are supported in modern platforms; a direct SCM
call to enable the mode or through a secure io write of a magic value.

In order for a normal reboot not to trigger "download mode" the magic
must be cleared during a clean reboot.

Download mode has to be enabled by including qcom_scm.download_mode=1 on
the command line.

Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>

drivers/firmware/Kconfig

@@ -215,6 +215,17 @@ config QCOM_SCM_64
 	def_bool y
 	depends on QCOM_SCM && ARM64
 
+config QCOM_SCM_DOWNLOAD_MODE_DEFAULT
+	bool "Qualcomm download mode enabled by default"
+	depends on QCOM_SCM
+	help
+	  A device with "download mode" enabled will upon an unexpected
+	  warm-restart enter a special debug mode that allows the user to
+	  "download" memory content over USB for offline postmortem analysis.
+	  The feature can be enabled/disabled on the kernel command line.
+
+	  Say Y here to enable "download mode" by default.
+
 config TI_SCI_PROTOCOL
 	tristate "TI System Control Interface (TISCI) Message Protocol"
 	depends on TI_MESSAGE_MANAGER