fix(rt-linux): Improve RT-Linux Performance Guide

Add more details on RT-Linux latency spike observed while running
system benchmark tests due to OP-TEE TRNG. Add workarounds that can
be used to avoid it.

Signed-off-by: Shiva Tripathi <s-tripathi1@ti.com>

Author: shiva-ti

source/devices/AM62AX/linux/RT_Linux_Performance_Guide.rst

@@ -62,13 +62,17 @@ default SDK image:
 
 .. note::
 
-   A known issue in this SDK release is affecting this benchmark.
-   Using OP-TEEs PRNG drivers rather than the hardware accelerated
-   TRNG drivers restores the context switch latencies to the values you
-   see here.
-
-   More information on switching to the PRNG drivers can be found in the
-   Foundational Components section, here :ref:`building-optee-with-prng`
+   Using the OP-TEE TRNG driver can impact this benchmark's performance due to
+   frequent context switching between Normal World (Linux) and Secure World (OP-TEE),
+   that occurs when the kernel's hardware random number generator interface
+   (hwrng) requests entropy from the secure TRNG to replenish the Linux entropy
+   pool.
+
+   To avoid these latency spikes, the Linux TRNG driver can be used instead of 
+   the OP-TEE TRNG driver. This involves enabling the Pseudo RNG driver in OP-TEE 
+   as documented in the Foundational Components section: :ref:`building-optee-with-prng`, 
+   and enabling the RNG node in the Linux kernel device tree. This way the HW TRNG is 
+   accessed from the kernel itself.
 
 .. csv-table::
    :header: "Latencies","CPU 0","CPU 1","CPU 2","CPU 3"

source/devices/AM62LX/linux/RT_Linux_Performance_Guide.rst

@@ -61,13 +61,17 @@ default SDK image:
 
 .. note::
 
-   A known issue in this SDK release is affecting this benchmark.
-   Using OP-TEEs PRNG drivers rather than the hardware accelerated TRNG
-   drivers restores the context switch latencies to the values you see
-   here.
-
-   More information on switching to the PRNG drivers can be found in the
-   Foundational Components section, here :ref:`building-optee-with-prng`
+   Using the OP-TEE TRNG driver can impact this benchmark's performance due to
+   frequent context switching between Normal World (Linux) and Secure World (OP-TEE),
+   that occurs when the kernel's hardware random number generator interface
+   (hwrng) requests entropy from the secure TRNG to replenish the Linux entropy
+   pool.
+
+   To avoid these latency spikes, the Linux TRNG driver can be used instead of 
+   the OP-TEE TRNG driver. This involves enabling the Pseudo RNG driver in OP-TEE 
+   as documented in the Foundational Components section: :ref:`building-optee-with-prng`, 
+   and enabling the RNG node in the Linux kernel device tree. This way the HW TRNG is 
+   accessed from the kernel itself.
 
 .. csv-table::
    :header: "Latencies","CPU 0","CPU 1"

source/devices/AM62PX/linux/RT_Linux_Performance_Guide.rst

@@ -62,13 +62,17 @@ default SDK image
 
 .. note::
 
-   A known issue in this SDK release is affecting this benchmark.
-   Using OP-TEEs PRNG drivers rather than the hardware accelerated TRNG
-   drivers restores the context switch latencies to the values you see
-   here.
-
-   More information on switching to the PRNG drivers can be found in the
-   Foundational Components section, here :ref:`building-optee-with-prng`
+   Using the OP-TEE TRNG driver can impact this benchmark's performance due to
+   frequent context switching between Normal World (Linux) and Secure World (OP-TEE),
+   that occurs when the kernel's hardware random number generator interface
+   (hwrng) requests entropy from the secure TRNG to replenish the Linux entropy
+   pool.
+
+   To avoid these latency spikes, the Linux TRNG driver can be used instead of 
+   the OP-TEE TRNG driver. This involves enabling the Pseudo RNG driver in OP-TEE 
+   as documented in the Foundational Components section: :ref:`building-optee-with-prng`, 
+   and enabling the RNG node in the Linux kernel device tree. This way the HW TRNG is 
+   accessed from the kernel itself.
 
 .. csv-table::
    :header: "Latencies","CPU 0","CPU 1","CPU 2","CPU 3"

source/devices/AM62X/linux/RT_Linux_Performance_Guide.rst

@@ -68,13 +68,17 @@ default SDK image using the SK-AM62B-P1_ reference board:
 
 .. note::
 
-   A known issue in this SDK release is affecting this benchmark.
-   Using OP-TEE's PRNG drivers rather than the hardware accelerated TRNG
-   drivers restores the context switch latencies to the values you see
-   here.
-
-   More information on switching to the PRNG drivers can be found in the
-   Foundational Components section, here :ref:`building-optee-with-prng`
+   Using the OP-TEE TRNG driver can impact this benchmark's performance due to
+   frequent context switching between Normal World (Linux) and Secure World (OP-TEE),
+   that occurs when the kernel's hardware random number generator interface
+   (hwrng) requests entropy from the secure TRNG to replenish the Linux entropy
+   pool.
+
+   To avoid these latency spikes, the Linux TRNG driver can be used instead of 
+   the OP-TEE TRNG driver. This involves enabling the Pseudo RNG driver in OP-TEE 
+   as documented in the Foundational Components section: :ref:`building-optee-with-prng`, 
+   and enabling the RNG node in the Linux kernel device tree. This way the HW TRNG is 
+   accessed from the kernel itself.
 
 .. csv-table::
    :header: "Latencies","CPU 0","CPU 1","CPU 2","CPU 3"