Re: [PATCH v4 02/15] firmware/psci: Add psci_early_test_conduit()

[Will Deacon <will@kernel.org>]

On Mon, Jul 01, 2024 at 10:54:52AM +0100, Steven Price wrote:
> From: Jean-Philippe Brucker <jean-philippe@linaro.org>
> 
> Add a function to test early if PSCI is present and what conduit it
> uses. Because the PSCI conduit corresponds to the SMCCC one, this will
> let the kernel know whether it can use SMC instructions to discuss with
> the Realm Management Monitor (RMM), early enough to enable RAM and
> serial access when running in a Realm.
> 
> Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> Signed-off-by: Steven Price <steven.price@arm.com>
> ---
> v4: New patch
> ---
>  drivers/firmware/psci/psci.c | 25 +++++++++++++++++++++++++
>  include/linux/psci.h         |  5 +++++
>  2 files changed, 30 insertions(+)
> 
> diff --git a/drivers/firmware/psci/psci.c b/drivers/firmware/psci/psci.c
> index d9629ff87861..a40dcaf17822 100644
> --- a/drivers/firmware/psci/psci.c
> +++ b/drivers/firmware/psci/psci.c
> @@ -13,6 +13,7 @@
>  #include <linux/errno.h>
>  #include <linux/linkage.h>
>  #include <linux/of.h>
> +#include <linux/of_fdt.h>
>  #include <linux/pm.h>
>  #include <linux/printk.h>
>  #include <linux/psci.h>
> @@ -767,6 +768,30 @@ int __init psci_dt_init(void)
>  	return ret;
>  }
>  
> +/*
> + * Test early if PSCI is supported, and if its conduit matches @conduit
> + */
> +bool __init psci_early_test_conduit(enum arm_smccc_conduit conduit)
> +{
> +	int len;
> +	int psci_node;
> +	const char *method;
> +	unsigned long dt_root;
> +
> +	/* DT hasn't been unflattened yet, we have to work with the flat blob */
> +	dt_root = of_get_flat_dt_root();
> +	psci_node = of_get_flat_dt_subnode_by_name(dt_root, "psci");
> +	if (psci_node <= 0)
> +		return false;
> +
> +	method = of_get_flat_dt_prop(psci_node, "method", &len);
> +	if (!method)
> +		return false;
> +
> +	return  (conduit == SMCCC_CONDUIT_SMC && strncmp(method, "smc", len) == 0) ||
> +		(conduit == SMCCC_CONDUIT_HVC && strncmp(method, "hvc", len) == 0);
> +}

Hmm, I don't think this is sufficient to check for SMCCC reliably.
Instead, I think you need to do something more involved:

1. Check for PSCI in the DT
2. Check that the PSCI major version is >= 1
3. Use PSCI_FEATURES to check that you have SMCCC
4. Use SMCCC_VERSION to find out which version of SMCCC you have

That's roughly what the PSCI driver does, so we should avoid duplicating
that logic.

Will