docs: fusa: Add Assumption of Use (AOU)

AoU are the assumptions that Xen relies on other components (eg platform
platform, domains) to fulfill its requirements. In our case, platform means
a combination of hardware, firmware and bootloader.

We have defined AoU in the intro.rst and added AoU for the generic
timer.

Also, fixed a requirement to denote that Xen shall **not** expose the
system counter frequency via the "clock-frequency" device tree property.
The reason being the device tree documentation strongly discourages the
use of this peoperty. Further if the "clock-frequency" is exposed, then
it overrides the value programmed in the CNTFRQ_EL0 register.

So, the frequency shall be exposed via the CNTFRQ_EL0 register only and
consequently there is an assumption on the platform to program the
register correctly.

Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@amd.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>

docs/fusa/reqs/design-reqs/arm64/generic-timer.rst

@@ -30,7 +30,7 @@ Read system counter frequency
 
 Description:
 Xen shall expose the frequency of the system counter to the domains in
-CNTFRQ_EL0 register and/or domain device tree's "clock-frequency" property.
+CNTFRQ_EL0 register.
 
 Rationale:
 
@@ -116,6 +116,28 @@ Rationale:
 
 Comments:
 
+Covers:
+ - `XenProd~emulated_timer~1`
+
+Assumption of Use on the Platform
+=================================
+
+Expose system timer frequency via register
+------------------------------------------
+
+`XenSwdgn~arm64_generic_timer_plat_program_cntfrq_el0~1`
+
+Description:
+CNTFRQ_EL0 register shall be programmed with the value of the system timer
+frequency.
+
+Rationale:
+Xen reads the CNTFRQ_EL0 register to get the value of system timer frequency.
+
+Comments:
+While there is a provision to get this value by reading the "clock-frequency"
+dt property [2], the use of this property is strongly discouraged.
+
 Covers:
  - `XenProd~emulated_timer~1`
 

docs/fusa/reqs/intro.rst

@@ -38,6 +38,16 @@ The requirements are linked using OpenFastTrace
 OpenFastTrace parses through the requirements and generates a traceability
 report.
 
+Assumption of Use
+=================
+
+Xen is making several assumptions on the status of the platform or on some
+functions being present and operational. For example, Xen might assume that
+some registers are set.
+Anybody who wants to use Xen must validate that the platform it is used on
+(meaning the hardware and any software running before Xen like the firmware)
+fulfils all the AoU described by Xen.
+
 The following is the skeleton for a requirement.
 
 Title of the requirement