linux-kernel - Re: [PATCH v2 0/4] Implement vendor resets for PSCI SYSTEM

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Date: Thu, 18 Apr 2024 10:52:51 -0700
From: Elliot Berman <quic_eberman@...cinc.com>
To: Florian Fainelli <florian.fainelli@...adcom.com>
CC: Sudeep Holla <sudeep.holla@....com>,
        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>, <linux-pm@...r.kernel.org>,
        <linux-arm-msm@...r.kernel.org>
Subject: Re: [PATCH v2 0/4] Implement vendor resets for PSCI SYSTEM_RESET2

On Wed, Apr 17, 2024 at 03:01:00PM -0700, Florian Fainelli wrote:
> On 4/17/24 14:54, Elliot Berman wrote:
> > On Tue, Apr 16, 2024 at 10:35:22AM +0100, Sudeep Holla wrote:
> > > On Sun, Apr 14, 2024 at 12:30:23PM -0700, Elliot Berman wrote:
> > > > The PSCI SYSTEM_RESET2 call allows vendor firmware to define additional
> > > > reset types which could be mapped to the reboot argument.
> > > > 
> > > > Setting up reboot on Qualcomm devices can be inconsistent from chipset
> > > > to chipset.
> > > 
> > > That doesn't sound good. Do you mean PSCI SYSTEM_RESET doesn't work as
> > > expected ? Does it mean it is not conformant to the specification ?
> > > 
> > 
> > I was motivating the reason for using SYSTEM_RESET2. How to set the PMIC
> > register and IMEM cookie can change between chipsets. Using
> > SYSTEM_RESET2 alows us to abstract how to perform the reset.
> > 
> > > > Generally, there is a PMIC register that gets written to
> > > > decide the reboot type. There is also sometimes a cookie that can be
> > > > written to indicate that the bootloader should behave differently than a
> > > > regular boot. These knobs evolve over product generations and require
> > > > more drivers. Qualcomm firmwares are beginning to expose vendor
> > > > SYSTEM_RESET2 types to simplify driver requirements from Linux.
> > > > 
> > > 
> > > Why can't this be fully userspace driven ? What is the need to keep the
> > > cookie in the DT ?
> > 
> > As Dmitry pointed out, this information isn't discoverable. I suppose
> > we could technically use bootconfig or kernel command-line to convey the
> > map although I think devicetree is the right spot for this mapping.
> > 
> > - Other vendor-specific bits for PSCI are described in the devicetree.
> >    One example is the suspend param (e.g. the StateID) for cpu idle
> >    states.
> > - Describing firmware bits in the DT isn't unprecedented, and putting
> >    this information outside the DT means that other OSes (besides Linux)
> >    need their own way to convey this information.
> > - PSCI would be the odd one out that reboot mode map is not described in
> >    DT. Other reboot-mode drivers specify the mapping in the DT. Userspace
> >    that runs with firmware that support vendor reset2 need to make sure
> >    they can configure the mapping early enough.
> 
> FWIW, I read Sudeep's response as being specifically inquiring about the
> 'cookie' parameter, do you see a need for that to be in described in the DT
> or could that just be an user-space parameter that is conveyed through the
> reboot system call?

Ah, I had thought the ask was for the reboot type as well as the cookie.
We don't do this for hardware-based reboot mode cookies and I didn't see
why we should ask userspace to do something different for PSCI.
It seems to me that SYSTEM_RESET2 fits nicely with the existing design
for reboot-mode bindings.