[PATCH v3] cpufreq: scmi: Account for malformed DT in scmi_dev_used_by_cpus()

[Florian Fainelli <florian.fainelli@broadcom.com>]

Broadcom STB platforms were early adopters (2017) of the SCMI framework and as
a result, not all deployed systems have a Device Tree entry where SCMI
protocol 0x13 (PERFORMANCE) is declared as a clock provider, nor are the
CPU Device Tree node(s) referencing protocol 0x13 as their clock
provider. This was clarified in commit e11c480b6df1 ("dt-bindings:
firmware: arm,scmi: Extend bindings for protocol@13") in 2023.

For those platforms, we allow the checks done by scmi_dev_used_by_cpus()
to continue, and in the event of not having done an early return, we key
off the documented compatible string and give them a pass to continue to
use scmi-cpufreq.

Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Fixes: 6c9bb8692272 ("cpufreq: scmi: Skip SCMI devices that aren't used by the CPUs")
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
---
Changes in v3:

- corrected multi-line comment
- added details about when STB platforms used SCMI and which commit
  clarifid the binding requirements

Changes in v2:

- do not leverage the SCMI quirks framework which in spirit is for
  dealing with SCMI firmware implementations, this right here,
  specifically pertains to an older Device Tree. This also makes it
  easier to backport that change into stable trees where the commit in
  the Fixes tag is already present. There is no dependency upon the
  presence of the SCMI quirks framework

 drivers/cpufreq/scmi-cpufreq.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/cpufreq/scmi-cpufreq.c b/drivers/cpufreq/scmi-cpufreq.c
index ef078426bfd5..38c165d526d1 100644
--- a/drivers/cpufreq/scmi-cpufreq.c
+++ b/drivers/cpufreq/scmi-cpufreq.c
@@ -15,6 +15,7 @@
 #include <linux/energy_model.h>
 #include <linux/export.h>
 #include <linux/module.h>
+#include <linux/of.h>
 #include <linux/pm_opp.h>
 #include <linux/pm_qos.h>
 #include <linux/slab.h>
@@ -424,6 +425,15 @@ static bool scmi_dev_used_by_cpus(struct device *scmi_dev)
 			return true;
 	}
 
+	/*
+	 * Older Broadcom STB chips had a "clocks" property for CPU node(s)
+	 * that did not match the SCMI performance protocol node, if we got
+	 * there, it means we had such an older Device Tree, therefore return
+	 * true to preserve backwards compatibility.
+	 */
+	if (of_machine_is_compatible("brcm,brcmstb"))
+		return true;
+
 	return false;
 }
 
-- 
2.34.1