From mboxrd@z Thu Jan 1 00:00:00 1970 From: Suzuki.Poulose@arm.com (Suzuki K Poulose) Date: Tue, 21 Mar 2017 10:16:45 +0000 Subject: [v3 3/5] coresight: add support for debug module In-Reply-To: <20170321025924.GA7429@leoy-linaro> References: <20170309175915.GA964@leoy-linaro> <3f27efee-3b63-81fd-eb96-73fd7e6f5e92@arm.com> <20170313165600.GB32431@linaro.org> <516f8989-4dde-2686-d549-0761feb14d1b@arm.com> <20170317101328.GA19877@leoy-linaro> <20170317155007.GA20435@linaro.org> <20170317162836.GD2020@leoy-linaro> <5995f6a7-08b1-c868-fc88-a145d1748a33@arm.com> <20170321025924.GA7429@leoy-linaro> Message-ID: <7226bc83-24f5-f609-2f87-f0afc7657488@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 21/03/17 02:59, Leo Yan wrote: > On Mon, Mar 20, 2017 at 10:40:00AM -0600, Mathieu Poirier wrote: > > [...] > >>>>> If we don't check for "nohlt" some platform may freeze, others may work. >>>>> If we >>>>> mandate that "nohlt" be present on the kernel cmd line it works in all >>>>> cases. >>>>> As such mandating that "nohlt" be present is a better way to go. >>>> >>>> >>>> Sure, so I will add below checking code in the probe function, please >>>> let me know if you have alter better way to implement this: >>>> >>>> + if (IS_ENABLED(CONFIG_CPU_IDLE) && >>>> + !strstr(boot_command_line, "nohlt")) { >>>> + dev_err(dev, "May not be accessible in CPU power >>>> domain.\n"); >>>> + return -EPERM; >>>> + } >>>> >>> >>> There is an API which kind of achieves what "nohlt" does at runtime : >>> >>> cpu_idle_poll_ctrl(true) >>> >>> So may be we could use that instead of depending on "nohlt". The other side >>> of the issues is "when do we decide to use the API". May be we could add >>> something >>> like : enable_debug, which could then trigger the panic notifier >>> registrations >>> and the above. That would still leave us with a case where the system >>> crashes >>> even before the user gets a terminal. May be the following is the best >>> option : >>> >>> 1) Dedicated kernel command line parameter for enabling the CPU debug at >>> boot/probe. >>> >>> and >>> >>> 2) Runtime enable method via sysfs. >>> >>> What do you think ? >> >> In my opinion booting with "nohlt" on the cmd line is sufficient to >> determine if we should use the driver or not. That way we also avoid >> declaring yet another sysfs flag, something I really want to avoid. > > Agree. > > I did spend some time to implement coresight core framework to support > debug module, you could see it on:http://termbin.com/k2fj; this also > gives me more sense which is better choice. If declaring another sysfs > flag to support debug module in coresight framework, this lets the > codes and interfaces more complex. E.g. for best fit into coresight > framework, finally we can get 8 sysfs nodes for 8 CPUs in system; so > that means we need enable every CPU one by one. Having a node for each debug area indeed doesn't look good. We could as will stick a single node under /sys/kernel/debug/ which would enable/disable the debug component. I am OK with it being tied to nohlt. In that case we will have to add a Kconfig dependency on GENERIC_IDLE_POLL_SETUP (though it is selected by default on ARM/ARM64). Parsing the boot command line for nohlt doesn't look like a good idea. We may have to figure out a way to do that. Also, please could you add support for building this as a module ? Since it doesn't depend on the coresight bus anyway, it should be pretty straight forward. Suzuki