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

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Date: Thu, 20 Jun 2024 16:37:09 -0700
From: Elliot Berman <quic_eberman@...cinc.com>
To: Sudeep Holla <sudeep.holla@....com>, Sebastian Reichel <sre@...nel.org>
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

Hi Sudeep and Sebastian,

On Wed, Jun 19, 2024 at 08:28:06AM -0700, Elliot Berman wrote:
> On Wed, Jun 19, 2024 at 02:51:43PM +0100, Sudeep Holla wrote:
> > 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.
> > >  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.
> > >  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.
> > >
> > 
> > I need to think through it but when you first introduced the generic
> > Documentation/devicetree/bindings/power/reset/reboot-mode.yaml bindings
> > I also looked at drivers/power/reset/reboot-mode.c
> > 
> > I assumed this extension to that binding would reuse the same and
> > PSCI would just do reboot_mode_register(). I didn't expect to see these
> > changes. I might have missing something but since the bindings is still
> > quite generic with additional cells that act as additional cookie for
> > reboot call, I still think that should be possible.
> > 
> > What am I missing here then ?
> > 
> 
> Right, if that was only thing to "solve" to make it easy to use
> reboot-mode framework, I agree we should update reboot mode framework to
> work with the additional cells. There are a few other issues I mention
> above which, when combined, make me feel that PSCI is different enough
> from how reboot mode framework works that we shouldn't try to make PSCI
> work with the framework. Issues #1 and #2 are pretty easy to solve
> (whether they should be solved is different); I'm not sure a good
> approach to issue #3.
> 

Does the reasoning I mention in the commit text make sense why PSCI should
avoid using the reboot-mode.c framework?

Thanks,
Elliot