On 09/05/17 03:50, Leo Yan wrote:
Coresight includes debug module and usually the module connects with CPU debug logic. ARMv8 architecture reference manual (ARM DDI 0487A.k) has description for related info in "Part H: External Debug". Chapter H7 "The Sample-based Profiling Extension" introduces several sampling registers, e.g. we can check program counter value with combined CPU exception level, secure state, etc. So this is helpful for analysis CPU lockup scenarios, e.g. if one CPU has run into infinite loop with IRQ disabled. In this case the CPU cannot switch context and handle any interrupt (including IPIs), as the result it cannot handle SMP call for stack dump. This patch is to enable coresight debug module, so firstly this driver is to bind apb clock for debug module and this is to ensure the debug module can be accessed from program or external debugger. And the driver uses sample-based registers for debug purpose, e.g. when system triggers panic, the driver will dump program counter and combined context registers (EDCIDSR, EDVIDSR); by parsing context registers so can quickly get to know CPU secure state, exception level, etc. Some of the debug module registers are located in CPU power domain, so this requires the CPU power domain stays on when access related debug registers, but the power management for CPU power domain is quite dependent on SoC integration for power management. For the platforms which with sane power controller implementations, this driver follows the method to set EDPRCR to try to pull the CPU out of low power state and then set 'no power down request' bit so the CPU has no chance to lose power. If the SoC has not followed up this design well for power management controller, the user should use the command line parameter or sysfs to constrain all or partial idle states to ensure the CPU power domain is enabled and access coresight CPU debug component safely. Signed-off-by: Leo Yan <leo.yan@xxxxxxxxxx>
With comments from Mathieu addressed, Reviewed-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx> -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
