[PATCH RFC 0/4] coresight: support dump ETB RAM

[Leo Yan]

### Introduction ###

Embedded Trace Buffer (ETB) provides on-chip storage of trace data,
usually has buffer size from 2KB to 8KB. These data has been used for
profiling and this has been well implemented in coresight driver.

This patch is to explore ETB RAM data for postmortem debugging. Due ETB
RAM buffer has small size, so the real trace data caused error is
easily to be overwritten by other PEs; but we could consider ETB RAM
data is quite useful for postmortem debugging with below scenarios:

Case 1: if system is bus lockup and CPU pipeline stalls for bus
accessing, CPUs have no more chance to fill enough data into ETB RAM
so after analyze ETB RAM we can quickly get to know the culprit if bus
lock is caused by improper programs, one often example is wrongly to
access the module without enable the module's clock. For this case,
we can rely on watchdog to trigger SoC reset and if lucky the ETB RAM
can survive after reset. So for this case, after system reboot we can
save ETB RAM before any new data input into it.

Case 2: There also has another hardware design with local ETB buffer
(ARM DDI 0461B) chapter 1.2.7. Local ETF, with this kind design every
CPU may has one dedicated ETB RAM. So it's quite handy that we can use
alive CPU to help dump the hang CPU ETB RAM. Then we can quickly get
to know what's the last point the CPU has executed before its hang.


### Implementation ###

Based on current Coresight ETB driver, we only needs some minor
enhancement so can support dump ETB RAM with two methods.

Patches 0001/0002 are minor fixes so can support more scenarios for ETB
RAM dumping.

Patch 0003 is to dump ETB RAM after system reboot, this is for the
platforms which use watchdog reset and ETB RAM can survive.

Patch 0004 is to dump ETB RAM when panic happens, so we can save ETB RAM
into memory. If we connect this with Kdump, then we can easily extract
the ETB RAM from vmcore.


### Usage ###

To dump ETB RAM after reboot, simply use below command:
# dd if=/dev/f6402000.etf of=cstrace.bin

To dump ETB RAM for kernel panic, we need add "crash_kexec_post_notifiers"
into kernel command line so let kernel call panic notifiers before launch
dump kernel. After dump kernel has booted up, we need use below methods
to ETB RAM offline analysis:

On the target:
# cp /proc/vmcore ./vmcore
# scp ./vmcore your@hostpc

On the host PC:
# ./crash vmcore vmlinux

crash> log
[...]
[  112.600051] coresight-tmc f6402000.etf: Flush ETB buffer 0x2000@0xffff800038300080
[  112.614743] Starting crashdump kernel...
[  112.618681] Bye!
crash> rd 0xffff800038300080 0x2000 -r /tmp/cstrace.bin
8192 bytes copied from 0xffff800038300080 to /tmp/cstrace.bin

After we get cstrace.bin data, we can use OpenCSD snapshot method to parse
ETB trace data. These two methods have been verified on Hikey, For Hikey
snapshot config files you can refer [1]. For total kernel patches for
integration Kdump and Coresight, you can refer [2].

[1] http://people.linaro.org/~leo.yan/opencsd_hikey/hikey_snapshot.tgz
[2] https://git.linaro.org/people/leo.yan/linux-debug-workshop.git/log/?h=coresight_etb_dump


### TODO ###

Need work for ETB1.0 driver, this is based on review and comments
for this patch set.


Leo Yan (4):
  coresight: tmc: check dump buffer is overflow
  coresight: tmc: set read pointer before dump RAM
  coresight: tmc: dump RAM when device is disabled
  coresight: tmc: dump RAM for panic

 drivers/hwtracing/coresight/coresight-tmc-etf.c | 86 ++++++++++++++++++++++++-
 drivers/hwtracing/coresight/coresight-tmc.h     |  2 +
 2 files changed, 85 insertions(+), 3 deletions(-)

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2.7.4