[PATCH v9 2/9] doc: Add documentation for Coresight CPU debug

Thu May 11 10:19:40 PDT 2017

On Tue, May 09, 2017 at 10:49:55AM +0800, Leo Yan wrote:
> Add detailed documentation for Coresight CPU debug driver, which
> contains the info for driver implementation, Mike Leach excellent
> summary for "clock and power domain". At the end some examples on how
> to enable the debugging functionality are provided.
> 
> Suggested-by: Mike Leach <mike.leach at linaro.org>
> Signed-off-by: Leo Yan <leo.yan at linaro.org>

Reviewed-by: Mathieu Poirier <mathieu.poirier at linaro.org>

> ---
>  Documentation/trace/coresight-cpu-debug.txt | 174 ++++++++++++++++++++++++++++
>  1 file changed, 174 insertions(+)
>  create mode 100644 Documentation/trace/coresight-cpu-debug.txt
> 
> diff --git a/Documentation/trace/coresight-cpu-debug.txt b/Documentation/trace/coresight-cpu-debug.txt
> new file mode 100644
> index 0000000..0426d50
> --- /dev/null
> +++ b/Documentation/trace/coresight-cpu-debug.txt
> @@ -0,0 +1,174 @@
> +		Coresight CPU Debug Module
> +		==========================
> +
> +   Author:   Leo Yan <leo.yan at linaro.org>
> +   Date:     April 5th, 2017
> +
> +Introduction
> +------------
> +
> +Coresight CPU debug module is defined in ARMv8-a architecture reference manual
> +(ARM DDI 0487A.k) Chapter 'Part H: External debug', the CPU can integrate
> +debug module and it is mainly used for two modes: self-hosted debug and
> +external debug. Usually the external debug mode is well known as the external
> +debugger connects with SoC from JTAG port; on the other hand the program can
> +explore debugging method which rely on self-hosted debug mode, this document
> +is to focus on this part.
> +
> +The debug module provides sample-based profiling extension, which can be used
> +to sample CPU program counter, secure state and exception level, etc; usually
> +every CPU has one dedicated debug module to be connected. Based on self-hosted
> +debug mechanism, Linux kernel can access these related registers from mmio
> +region when the kernel panic happens. The callback notifier for kernel panic
> +will dump related registers for every CPU; finally this is good for assistant
> +analysis for panic.
> +
> +
> +Implementation
> +--------------
> +
> +- During driver registration, use EDDEVID and EDDEVID1 two device ID
> +  registers to decide if sample-based profiling is implemented or not. On some
> +  platforms this hardware feature is fully or partialy implemented; and if
> +  this feature is not supported then registration will fail.
> +
> +- When write this doc, the debug driver mainly relies on three sampling
> +  registers. The kernel panic callback notifier gathers info from EDPCSR
> +  EDVIDSR and EDCIDSR; from EDPCSR we can get program counter, EDVIDSR has
> +  information for secure state, exception level, bit width, etc; EDCIDSR is
> +  context ID value which contains the sampled value of CONTEXTIDR_EL1.
> +
> +- The driver supports CPU running mode with either AArch64 or AArch32. The
> +  registers naming convention is a bit different between them, AArch64 uses
> +  'ED' for register prefix (ARM DDI 0487A.k, chapter H9.1) and AArch32 uses
> +  'DBG' as prefix (ARM DDI 0487A.k, chapter G5.1). The driver is unified to
> +  use AArch64 naming convention.
> +
> +- ARMv8-a (ARM DDI 0487A.k) and ARMv7-a (ARM DDI 0406C.b) have different
> +  register bits definition. So the driver consolidates two difference:
> +
> +  If PCSROffset=0b0000, on ARMv8-a the feature of EDPCSR is not implemented;
> +  but ARMv7-a defines "PCSR samples are offset by a value that depends on the
> +  instruction set state". For ARMv7-a, the driver checks furthermore if CPU
> +  runs with ARM or thumb instruction set and calibrate PCSR value, the
> +  detailed description for offset is in ARMv7-a ARM (ARM DDI 0406C.b) chapter
> +  C11.11.34 "DBGPCSR, Program Counter Sampling Register".
> +
> +  If PCSROffset=0b0010, ARMv8-a defines "EDPCSR implemented, and samples have
> +  no offset applied and do not sample the instruction set state in AArch32
> +  state". So on ARMv8 if EDDEVID1.PCSROffset is 0b0010 and the CPU operates
> +  in AArch32 state, EDPCSR is not sampled; when the CPU operates in AArch64
> +  state EDPCSR is sampled and no offset are applied.
> +
> +
> +Clock and power domain
> +----------------------
> +
> +Before accessing debug registers, we should ensure the clock and power domain
> +have been enabled properly. In ARMv8-a ARM (ARM DDI 0487A.k) chapter 'H9.1
> +Debug registers', the debug registers are spread into two domains: the debug
> +domain and the CPU domain.
> +
> +                                +---------------+
> +                                |               |
> +                                |               |
> +                     +----------+--+            |
> +          dbg_clk -->|          |**|            |<-- cpu_clk
> +                     |    Debug |**|   CPU      |
> +           dbg_pd -->|          |**|            |<-- cpu_pd
> +                     +----------+--+            |
> +                                |               |
> +                                |               |
> +                                +---------------+
> +
> +For debug domain, the user uses DT binding "clocks" and "power-domains" to
> +specify the corresponding clock source and power supply for the debug logic.
> +The driver calls the pm_runtime_{put|get} operations as needed to handle the
> +debug power domain.
> +
> +For CPU domain, the different SoC designs have different power management
> +schemes and finally this heavily impacts external debug module. So we can
> +divide into below cases:
> +
> +- On systems with a sane power controller which can behave correctly with
> +  respect to CPU power domain, the CPU power domain can be controlled by
> +  register EDPRCR in driver. The driver firstly writes bit EDPRCR.COREPURQ
> +  to power up the CPU, and then writes bit EDPRCR.CORENPDRQ for emulation
> +  of CPU power down. As result, this can ensure the CPU power domain is
> +  powered on properly during the period when access debug related registers;
> +
> +- Some designs will power down an entire cluster if all CPUs on the cluster
> +  are powered down - including the parts of the debug registers that should
> +  remain powered in the debug power domain. The bits in EDPRCR are not
> +  respected in these cases, so these designs do not support debug over
> +  power down in the way that the CoreSight / Debug designers anticipated.
> +  This means that even checking EDPRSR has the potential to cause a bus hang
> +  if the target register is unpowered.
> +
> +  In this case, accessing to the debug registers while they are not powered
> +  is a recipe for disaster; so we need preventing CPU low power states at boot
> +  time or when user enable module at the run time. Please see chapter
> +  "How to use the module" for detailed usage info for this.
> +
> +
> +Device Tree Bindings
> +--------------------
> +
> +See Documentation/devicetree/bindings/arm/coresight-cpu-debug.txt for details.
> +
> +
> +How to use the module
> +---------------------
> +
> +If you want to enable debugging functionality at boot time, you can add
> +"coresight_cpu_debug.enable=1" to the kernel command line parameter.
> +
> +The driver also can work as module, so can enable the debugging when insmod
> +module:
> +# insmod coresight_cpu_debug.ko debug=1
> +
> +When boot time or insmod module you have not enabled the debugging, the driver
> +uses the debugfs file system to provide a knob to dynamically enable or disable
> +debugging:
> +
> +To enable it, write a '1' into /sys/kernel/debug/coresight_cpu_debug/enable:
> +# echo 1 > /sys/kernel/debug/coresight_cpu_debug/enable
> +
> +To disable it, write a '0' into /sys/kernel/debug/coresight_cpu_debug/enable:
> +# echo 0 > /sys/kernel/debug/coresight_cpu_debug/enable
> +
> +As explained in chapter "Clock and power domain", if you are working on one
> +platform which has idle states to power off debug logic and the power
> +controller cannot work well for the request from EDPRCR, then you should
> +firstly constraint CPU idle states before enable CPU debugging feature; so can
> +ensure the accessing to debug logic.
> +
> +If you want to limit idle states at boot time, you can use "nohlt" or
> +"cpuidle.off=1" in the kernel command line.
> +
> +At the runtime you can disable idle states with below methods:
> +
> +Set latency request to /dev/cpu_dma_latency to disable all CPUs specific idle
> +states (if latency = 0uS then disable all idle states):
> +# echo "what_ever_latency_you_need_in_uS" > /dev/cpu_dma_latency
> +
> +Disable specific CPU's specific idle state:
> +# echo 1 > /sys/devices/system/cpu/cpu$cpu/cpuidle/state$state/disable
> +
> +
> +Output format
> +-------------
> +
> +Here is an example of the debugging output format:
> +
> +ARM external debug module:
> +CPU[0]:
> + EDPRSR:  0000000b (Power:On DLK:Unlock)
> + EDPCSR:  [<ffff00000808eb54>] handle_IPI+0xe4/0x150
> + EDCIDSR: 00000000
> + EDVIDSR: 90000000 (State:Non-secure Mode:EL1/0 Width:64bits VMID:0)
> +CPU[1]:
> + EDPRSR:  0000000b (Power:On DLK:Unlock)
> + EDPCSR:  [<ffff0000087a64c0>] debug_notifier_call+0x108/0x288
> + EDCIDSR: 00000000
> + EDVIDSR: 90000000 (State:Non-secure Mode:EL1/0 Width:64bits VMID:0)
> -- 
> 2.7.4
>