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

[Sudeep Holla <sudeep.holla@arm.com>]

On Thu, Jun 20, 2024 at 04:37:09PM -0700, Elliot Berman wrote:
> 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?

Sorry, I completely missed to see that you had already answered those
in your commit message. As mentioned earlier I haven't looked at the
reboot mode framework completely yet, so I can't comment on it yet.

I don't want to be blocker though if others are happy with this.

--
Regards,
Sudeep