Re: [PATCH V25 0/3] misc: Add driver support for Data Capture and Compare unit(DCC)

From: Souradeep Chowdhury
Date: Thu Aug 03 2023 - 03:08:21 EST

On 6/28/2023 3:53 PM, Souradeep Chowdhury wrote:
DCC(Data Capture and Compare) is a DMA engine designed for debugging purposes.
In case of a system crash or manual software triggers by the user the DCC hardware
stores the value at the register addresses which can be used for debugging purposes.
The DCC driver provides the user with debugfs interface to configure the register
addresses. The options that the DCC hardware provides include reading from registers,
writing to registers, first reading and then writing to registers and looping
through the values of the same register.

In certain cases a register write needs to be executed for accessing the rest of the
registers, also the user might want to record the changing values of a register with
time for which he has the option to use the loop feature.

The options mentioned above are exposed to the user by debugfs files once the driver
is probed. The details and usage of this debugfs files are documented in

As an example let us consider a couple of debug scenarios where DCC has been proved to be
effective for debugging purposes:-

i)TimeStamp Related Issue

On SC7180, there was a coresight timestamp issue where it would occasionally be all 0
instead of proper timestamp values.

Proper timestamp:
Idx:3373; ID:10; I_TIMESTAMP : Timestamp.; Updated val = 0x13004d8f5b7aa; CC=0x9e

Zero timestamp:
Idx:3387; ID:10; I_TIMESTAMP : Timestamp.; Updated val = 0x0; CC=0xa2

Now this is a non-fatal issue and doesn't need a system reset, but still needs
to be rootcaused and fixed for those who do care about coresight etm traces.
Since this is a timestamp issue, we would be looking for any timestamp related
clocks and such.

We get all the clk register details from IP documentation and configure it
via DCC config_read debugfs node. Before that we set the current linked list.

/* Program the linked list with the addresses */
echo R 0x10c004 > /sys/kernel/debug/qcom-dcc/../3/config
echo R 0x10c008 > /sys/kernel/debug/qcom-dcc/../3/config
echo R 0x10c00c > /sys/kernel/debug/qcom-dcc/../3/config
echo R 0x10c010 > /sys/kernel/debug/qcom-dcc/../3/config
..... and so on for other timestamp related clk registers

/* Other way of specifying is in "addr len" pair, in below case it
specifies to capture 4 words starting 0x10C004 */

echo R 0x10C004 4 > /sys/kernel/debug/qcom-dcc/../3/config_read

/* Enable DCC */
echo 1 > /sys/kernel/debug/qcom-dcc/../3/enable

/* Run the timestamp test for working case */

/* Send SW trigger */
echo 1 > /sys/kernel/debug/qcom-dcc/../trigger

/* Read SRAM */
cat /dev/dcc_sram > dcc_sram1.bin

/* Run the timestamp test for non-working case */

/* Send SW trigger */
echo 1 > /sys/kernel/debug/qcom-dcc/../trigger

/* Read SRAM */
cat /dev/dcc_sram > dcc_sram2.bin

Get the parser from [1] and checkout the latest branch.

/* Parse the SRAM bin */
python -s dcc_sram1.bin --v2 -o output/
python -s dcc_sram2.bin --v2 -o output/

Sample parsed output of dcc_sram1.bin:

<hwioDump version="1">
<generator>Linux DCC Parser</generator>
<chip name="None" version="None">
<register address="0x0010c004" value="0x80000000" />
<register address="0x0010c008" value="0x00000008" />
<register address="0x0010c00c" value="0x80004220" />
<register address="0x0010c010" value="0x80000000" />
<next_ll_offset>next_ll_offset : 0x1c </next_ll_offset>

ii)NOC register errors

A particular class of registers called NOC which are functional registers was reporting
errors while logging the values.To trace these errors the DCC has been used effectively.
The steps followed were similar to the ones mentioned above.
In addition to NOC registers a few other dependent registers were configured in DCC to
monitor it's values during a crash. A look at the dependent register values revealed that
the crash was happening due to a secured access to one of these dependent registers.
All these debugging activity and finding the root cause was achieved using DCC.

DCC parser is available at the following open source location

Changes in v25

* Updated the documentation of the structure dcc_config_entry as per the comments in V23
* Updated the documentation of the dcc Kconfig definition as per comment in V24
* Used u64 where applicable
* Removed the mutex locks where it is not needed
* Removed the use of unlikely keyword
* Renamed "nr_link_list" to "max_link_list"

Souradeep Chowdhury (3):
dt-bindings: misc: qcom,dcc: Add the dtschema
misc: dcc: Add driver support for Data Capture and Compare unit(DCC)
MAINTAINERS: Add the entry for DCC(Data Capture and Compare) driver

Documentation/ABI/testing/debugfs-driver-dcc | 10 +-
.../devicetree/bindings/misc/qcom,dcc.yaml | 44 +
drivers/misc/Kconfig | 8 +
drivers/misc/Makefile | 1 +
drivers/misc/qcom-dcc.c | 1312 +++++++++++++++++
6 files changed, 1378 insertions(+), 5 deletions(-)
create mode 100644 Documentation/devicetree/bindings/misc/qcom,dcc.yaml
create mode 100644 drivers/misc/qcom-dcc.c

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