linux-kernel - Re: [PATCH v5 3/4] firmware: psci: Read and use vendor reset types

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Date: Mon, 24 Jun 2024 16:56:05 +0100
From: Sudeep Holla <sudeep.holla@....com>
To: Elliot Berman <quic_eberman@...cinc.com>
Cc: Bjorn Andersson <andersson@...nel.org>,
	Konrad Dybcio <konrad.dybcio@...aro.org>,
	Sebastian Reichel <sre@...nel.org>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
	Conor Dooley <conor+dt@...nel.org>, Vinod Koul <vkoul@...nel.org>,
	Andy Yan <andy.yan@...k-chips.com>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	"Mark Rutland" <mark.rutland@....com>,
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>,
	Satya Durga Srinivasu Prabhala <quic_satyap@...cinc.com>,
	Melody Olvera <quic_molvera@...cinc.com>,
	Shivendra Pratap <quic_spratap@...cinc.com>,
	<devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	Florian Fainelli <florian.fainelli@...adcom.com>,
	<linux-pm@...r.kernel.org>, <linux-arm-msm@...r.kernel.org>
Subject: Re: [PATCH v5 3/4] firmware: psci: Read and use vendor reset types

On Mon, Jun 17, 2024 at 10:18:09AM -0700, Elliot Berman wrote:
> SoC vendors have different types of resets and are controlled through
> various registers. For instance, Qualcomm chipsets can reboot to a
> "download mode" that allows a RAM dump to be collected. Another example
> is they also support writing a cookie that can be read by bootloader
> during next boot. PSCI offers a mechanism, SYSTEM_RESET2, for these
> vendor reset types to be implemented without requiring drivers for every
> register/cookie.
> 
> Add support in PSCI to statically map reboot mode commands from
> userspace to a vendor reset and cookie value using the device tree.
> 
> A separate initcall is needed to parse the devicetree, instead of using
> psci_dt_init because mm isn't sufficiently set up to allocate memory.
> 
> Reboot mode framework is close but doesn't quite fit with the
> design and requirements for PSCI SYSTEM_RESET2. Some of these issues can
> be solved but doesn't seem reasonable in sum:
>  1. reboot mode registers against the reboot_notifier_list, which is too
>     early to call SYSTEM_RESET2. PSCI would need to remember the reset
>     type from the reboot-mode framework callback and use it
>     psci_sys_reset.

Fair enough, good reason but as I mentioned I haven't looked at it in
details to provide any suggestion or to assert it can be solved.

>  2. reboot mode assumes only one cookie/parameter is described in the
>     device tree. SYSTEM_RESET2 uses 2: one for the type and one for
>     cookie.

I assumed it shouldn't be too difficult to extend it to support 2 parameters.

>  3. psci cpuidle driver already registers a driver against the
>     arm,psci-1.0 compatible. Refactoring would be needed to have both a
>     cpuidle and reboot-mode driver.

This one seems not a strong reason in my opinion. It was not designed for
cpuidle and it can't bind to that solely. If this becomes only reason, then
we need to fix that.

-- 
Regards,
Sudeep