Re: PSCI version 1.1 and SYSTEM_RESET2

From: Sodagudi Prasad
Date: Wed May 01 2019 - 14:43:19 EST


On 2019-05-01 02:49, Sudeep Holla wrote:
On Tue, Apr 30, 2019 at 05:07:31PM -0700, Sodagudi Prasad wrote:
On 2019-04-30 14:44, Sodagudi Prasad wrote:
+Sudeep

> Hi Mark/Will,
>
> I would like to understand whether ARM linux community have plans to
> support PSCI version 1.1 or not.
> PSCI_1_1 specification introduced support for SYSTEM_RESET2 command
> and this new command helps mobile devices to SYSTEM_WARM_RESET
> support. Rebooting devices with warm reboot helps to capture the
> snapshot of the ram contents for post-mortem analysis.

I think, there is a recent discussion from Sudeep for the SYSTEM_RESET2
support.
https://patchwork.kernel.org/patch/10884345/


This has landed in -next, and hopefully must appear in v5.2


Hi Sudeep,

I was going through your discussion in the below list -
https://lore.kernel.org/lkml/d73d3580-4ec1-a281-4585-5c776fc08c79@xxxxxxxxxx/

There is no provision to set up reboot mode dynamically instead kernel
command line parameter.
Looking for options to reboot device with warm reboot option when kernel
crashed.

panic() --> emergency_restart() --> machine_emergency_restart() -->
machine_restart(NULL);

It would nice if there is a config option to reboot the device either in
warm or cold in the case of kernel panic.

I presume you prefer to do warm boot in case of panic to get a dump of
the memory to inspect ? If so, is kexec/kdump not the mechanism to
achieve that ?

Hi Sudeep,

Thanks for your response and sharing details about your patch.
<Sudeep> If so, is kexec/kdump not the mechanism to achieve that?
Qualcomm is having vendor specific solution to capture ram contents and for offline analysis.


I am just trying to understand the use case. Xilinx asked for the same
but never got to understand their use case.

Here is the background -
Usually, power off drivers are overriding arm_pm_restart and pm_power_off callbacks and registering with reboot notifier with some priority for the reboot operations. Here is the Qualcomm poweroff driver for reference.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/power/reset/msm-poweroff.c

Before vendor chip set specific power off driver is probed, arm_pm_restart functions pointer holds the psci_sys_reset function. Once vendor power off driver is probed, vendor drivers can override the arm_pm_restart function pointer.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/firmware/psci.c#n562

Once vendor driver is probed, drivers can take care of devices warm or hard reset configuration part properly. But there is a window from start_kernel() to vendor specific driver probed, devices are getting cold resets even if kernel crashed. This is due to arm_pm_restart points to psci_sys_reset function by default. Is this problem clear now?

Qualcomm downstream kernel has a lot of use cases with respect device reset sequence and the downstream driver is much different from upstream drivers. I think, the above-mentioned problem is common for all the chipset vendors and it is not specific Qualcomm use cases. I have one downstream solution to this problem but thought to bring up this problem to the upstream community for a common solution, so that all the vendors can use it.

I have modified below flow to avoid cold restart in the case of early kernel panic.
panic() --> emergency_restart() --> machine_emergency_restart() --> machine_restart(NULL);

-Thanks, Prasad


--
Regards,
Sudeep

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