[PATCH] mach-snapdragon: handle platforms without PSCI support

[Sam Day]

Hi Mark,

On Monday, 27 January 2025 at 17:51, Sam Day <me at samcday.com> wrote:

> 
> 
> Hello Mark,
> 
> On Monday, 27 January 2025 at 17:36, Mark Kettenis mark.kettenis at xs4all.nl wrote:
> 
> > > Date: Mon, 27 Jan 2025 14:48:55 +0000
> > 
> > > From: Sam Day me at samcday.com
> > 
> > Hi Sam,
> > 
> > > Most MSM8916 devices shipped without PSCI support. The history is quite
> > > nuanced (a good overview can be found in 1), but the end result is
> > > that the upstream DTs for this SoC pretend that PSCI exists, and it's
> > > expected that the bootloader handles the case where it doesn't. This is
> > > codified by the de-facto bootloader for MSM8916 devices, lk2nd 2.
> > > 
> > > So we handle it here by deleting the /psci node if we detect the absence
> > > of PSCI. We need to do this early to ensure sysreset works correctly,
> > > since the PSCI firmware driver is PRE_RELOC and binds the PSCI sysreset
> > > driver.
> > 
> > But deleting the /psci node isn't enough is it? At least on systems
> > with more than a single core the OS will need a way to spin up the
> > additonal cores. So you'll need to modify the CPU nodes as well and
> > change the "enable-method" property to "spin-table" and add the
> > necessary infrastructure for that.
> 
> 
> Yes, you're absolutely right. I'm preparing to submit a patch series that
> does exactly that. The spin-table setup bits already exists in U-Boot, the
> MSM8916 specific bits (qcom-scm SMC call + poking APCS registers) will be
> the bulk of the work I submit later tonight.
> 
> I opted to submit this patch alone without that work for a few reasons:
> 
> * I've already submitted some patches to enable sysreset support for
> MSM8916 [1] and with this patch, sysreset will then work properly on
> this platform.
> * As it currently stands, the secondary cores can't be booted on this
> platform, regardless of whether the /psci node exists or not. Deleting
> the psci node doesn't make matters any worse when booting on devices
> that have no PSCI implementation.
> * The SMP bring-up code is a lot meatier (I had to pull in a bunch of qcom
> SCM calling code from the kernel) and might need a few rounds before it
> can land.
> * U-Boot can be experimentally chained from lk2nd (which itself is chained
> from aboot, which itself is chained from ... well, you get the idea!),
> and so in that case the SMP spin-table setup + DT patching has already
> happened in that previous bootloader. But that BL only disables the
> /psci node, which doesn't prevent the U-Boot PSCI firmware driver from
> binding and causing the (non-functional) PSCI sysreset driver to also be
> bound.
> * And so considering the previous points, my hope is that this modest
> patch will land in time for v2025.04 release and make U-Boot a bit more
> functional on (non-PSCI) MSM8916 devices.
> 
> [1]: https://patchwork.ozlabs.org/project/uboot/list/?series=441769

Uh, disregard bullet point 4, I don't know why I thought that a disabled
DT node would still be bound. The rest of the points still hopefully make
sense, though :)

Cheers,
-Sam

> 
> Warm Regards,
> -Sam
> 
> > > Additionally, show_psci_version is updated to check that PSCI exists.
> > > Currently this banner outputs "PSCI: 65535.65535" on devices without
> > > PSCI support, which isn't very useful :)
> > > 
> > > Signed-off-by: Sam Day me at samcday.com
> > > ---
> > > arch/arm/mach-snapdragon/board.c | 43 ++++++++++++++++++++++++++++++++++++----
> > > 1 file changed, 39 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/arch/arm/mach-snapdragon/board.c b/arch/arm/mach-snapdragon/board.c
> > > index e87551784b8d7afb4186533d43b8bddacba9faec..75b9cf1a8a1b3383665eb2d4e6725efe59718de0 100644
> > > --- a/arch/arm/mach-snapdragon/board.c
> > > +++ b/arch/arm/mach-snapdragon/board.c
> > > @@ -162,11 +162,42 @@ static void show_psci_version(void)
> > > 
> > > arm_smccc_smc(ARM_PSCI_0_2_FN_PSCI_VERSION, 0, 0, 0, 0, 0, 0, 0, &res);
> > > 
> > > + /* Some older SoCs like MSM8916 don't always support PSCI */
> > > + if ((int)res.a0 == PSCI_RET_NOT_SUPPORTED)
> > > + return;
> > > +
> > > debug("PSCI: v%ld.%ld\n",
> > > PSCI_VERSION_MAJOR(res.a0),
> > > PSCI_VERSION_MINOR(res.a0));
> > > }
> > > 
> > > +/**
> > > + * Most MSM8916 devices in the wild shipped without PSCI support, but the
> > > + * upstream DTs pretend that PSCI exists. If that situation is detected here,
> > > + * the /psci node is deleted. This is done very early to ensure the PSCI
> > > + * firmware driver doesn't bind (which then binds a sysreset driver that won't
> > > + * work).
> > > + */
> > > +static void qcom_psci_fixup(void fdt)
> > > +{
> > > + int offset, ret;
> > > + struct arm_smccc_res res;
> > > +
> > > + arm_smccc_smc(ARM_PSCI_0_2_FN_PSCI_VERSION, 0, 0, 0, 0, 0, 0, 0, &res);
> > > +
> > > + if ((int)res.a0 != PSCI_RET_NOT_SUPPORTED)
> > > + return;
> > > +
> > > + offset = fdt_path_offset(fdt, "/psci");
> > > + if (offset < 0)
> > > + return;
> > > +
> > > + debug("Found /psci DT node on device with no PSCI. Deleting.\n");
> > > + ret = fdt_del_node(fdt, offset);
> > > + if (ret)
> > > + log_err("Failed to delete /psci node: %d\n", ret);
> > > +}
> > > +
> > > / We support booting U-Boot with an internal DT when running as a first-stage bootloader
> > > * or for supporting quirky devices where it's easier to leave the downstream DT in place
> > > * to improve ABL compatibility. Otherwise, we use the DT provided by ABL.
> > > @@ -212,12 +243,16 @@ int board_fdt_blob_setup(void **fdtp)
> > > 
> > > if (internal_valid) {
> > > debug("Using built in FDT\n");
> > > - return -EEXIST;
> > > + ret = -EEXIST;
> > > + } else {
> > > + debug("Using external FDT\n");
> > > + *fdtp = external_fdt;
> > > + ret = 0;
> > > }
> > > 
> > > - debug("Using external FDT\n");
> > > - *fdtp = external_fdt;
> > > - return 0;
> > > + qcom_psci_fixup(*fdtp);
> > > +
> > > + return ret;
> > > }
> > > 
> > > void reset_cpu(void)
> > > 
> > > ---
> > > base-commit: 2eed5a1ff36217372e19f7513bd07077fc76718a
> > > change-id: 20250127-qcom-handle-absent-psci-6e8e8af7bcd7
> > > prerequisite-change-id: 20250120-qcom-parse-memory-updates-96ffe248cdf1:v3
> > > prerequisite-patch-id: 65448b5a93120d2117c1361035afc9bd911d005f
> > > 
> > > Best regards,
> > > --
> > > Sam Day me at samcday.com