docs(esp_psram): document MALLOC_CAP_SPIRAM_NO_ENC carve-out

Extends the External RAM encryption section to describe
CONFIG_SPIRAM_ENC_EXEMPT and the MALLOC_CAP_SPIRAM_NO_ENC heap
capability, including a security warning and a typical DMA-alignment
use case. Mirrors the change in zh_CN.

components/esp_psram/Kconfig.spiram.common

@@ -160,7 +160,7 @@ config SPIRAM_ALLOW_NOINIT_SEG_EXTERNAL_MEMORY
 config SPIRAM_ENC_EXEMPT
     bool "Reserve a PSRAM region exempt from encryption"
     default n
-    depends on SPIRAM && SECURE_FLASH_ENC_ENABLED && SOC_PSRAM_ENCRYPTION_PAGE_CONFIGURABLE
+    depends on SPIRAM && SOC_PSRAM_ENCRYPTION_PAGE_CONFIGURABLE
     help
         !!! SECURITY WARNING !!!
 

docs/en/api-guides/external-ram.rst

@@ -249,6 +249,28 @@ By default, failure to initialize external RAM will cause the ESP-IDF startup to
 
         On {IDF_TARGET_NAME}, PSRAM encryption can be controlled on a per-MMU-page basis, allowing individual PSRAM pages to be selectively encrypted or left unencrypted. However, in the default configuration, all PSRAM pages are encrypted when flash encryption is enabled.
 
+        Reserving an Unencrypted PSRAM Region
+        -------------------------------------
+
+        Enabling :ref:`CONFIG_SPIRAM_ENC_EXEMPT` reserves a region at the top of PSRAM (sized by :ref:`CONFIG_SPIRAM_ENC_EXEMPT_SIZE`, in KB, rounded up to the MMU page size) that is mapped without encryption. This region is registered as a separate heap pool reachable only via the ``MALLOC_CAP_SPIRAM_NO_ENC`` capability. The rest of PSRAM (and flash) remains encrypted.
+
+        .. warning::
+
+            Memory allocated with ``MALLOC_CAP_SPIRAM_NO_ENC`` is stored as plaintext in PSRAM and can be observed by an attacker with physical access to the PSRAM interface. Never place TLS state, keys, or other secrets in this region.
+
+        Typical use case: PSRAM encryption imposes alignment constraints on buffers that some DMA engines (for example, 2D-DMA) cannot satisfy. Buffers that need to be DMA-accessed from such engines can be allocated from this unencrypted region:
+
+        .. code-block:: c
+
+            #if CONFIG_SPIRAM_ENC_EXEMPT
+            uint32_t caps = MALLOC_CAP_SPIRAM_NO_ENC;
+            #else
+            uint32_t caps = MALLOC_CAP_SPIRAM;
+            #endif
+            uint8_t *buf = heap_caps_malloc(buf_size, caps);
+
+        ``MALLOC_CAP_SPIRAM_NO_ENC`` must be requested explicitly. It is intentionally not combined with ``MALLOC_CAP_SPIRAM`` or ``MALLOC_CAP_DEFAULT``, so ordinary SPIRAM/heap allocations cannot accidentally land in the unencrypted region.
+
     .. only:: SOC_PSRAM_ENCRYPTION_SEPARATE_KEY
 
         On {IDF_TARGET_NAME}, PSRAM encryption can use an independent encryption key. If the PSRAM encryption key is not programmed, the flash encryption key will be used as the PSRAM encryption key.

docs/zh_CN/api-guides/external-ram.rst

@@ -249,6 +249,28 @@ ESP-IDF 启动过程中，片外 RAM 被映射到数据虚拟地址空间，该
 
         在 {IDF_TARGET_NAME} 上，PSRAM 加密可以按 MMU 页面粒度进行控制，允许对单个 PSRAM 页面选择性地加密或不加密。但在默认配置下，启用 flash 加密时所有 PSRAM 页面都会被加密。
 
+        预留未加密的 PSRAM 区域
+        -----------------------
+
+        启用 :ref:`CONFIG_SPIRAM_ENC_EXEMPT` 会在 PSRAM 顶部预留一段区域（大小由 :ref:`CONFIG_SPIRAM_ENC_EXEMPT_SIZE` 指定，单位为 KB，向上取整到 MMU 页面大小），该区域在映射时不启用加密。此区域被注册为一个独立的堆池，仅可通过 ``MALLOC_CAP_SPIRAM_NO_ENC`` 能力位访问。其余 PSRAM（以及 flash）仍然保持加密。
+
+        .. warning::
+
+            通过 ``MALLOC_CAP_SPIRAM_NO_ENC`` 分配的内存以明文形式存储在 PSRAM 中，攻击者若能物理接触 PSRAM 接口即可读取其内容。切勿将 TLS 状态、密钥或其他敏感数据放入该区域。
+
+        典型使用场景：PSRAM 加密会对缓冲区施加对齐约束，部分 DMA 引擎（如 2D-DMA）无法满足这些约束。需要被此类引擎进行 DMA 访问的缓冲区可以从该未加密区域分配：
+
+        .. code-block:: c
+
+            #if CONFIG_SPIRAM_ENC_EXEMPT
+            uint32_t caps = MALLOC_CAP_SPIRAM_NO_ENC;
+            #else
+            uint32_t caps = MALLOC_CAP_SPIRAM;
+            #endif
+            uint8_t *buf = heap_caps_malloc(buf_size, caps);
+
+        必须显式请求 ``MALLOC_CAP_SPIRAM_NO_ENC``。该能力位有意未与 ``MALLOC_CAP_SPIRAM`` 或 ``MALLOC_CAP_DEFAULT`` 组合，因此普通 SPIRAM/堆分配不会意外落入该未加密区域。
+
     .. only:: SOC_PSRAM_ENCRYPTION_SEPARATE_KEY
 
         在 {IDF_TARGET_NAME} 上，PSRAM 加密可以使用独立的加密密钥。如果未烧录 PSRAM 加密密钥，则会使用 flash 加密密钥作为 PSRAM 加密密钥。