Initial stub for memory sectioning support

src/platform/sections.h

@@ -0,0 +1,42 @@
+/*
+ * section   | speed  | execute | shared | cached | buffered | DMA | ISR 
+ * ----------|--------|---------|--------|--------|----------|-----|----
+ * DTCM      | Fast   | No      | No     | No     | No       | No  | Yes
+ * ITCM      | Fast   | Yes     | No     | No     | No       | No  | Yes
+ * SHARED    | Normal | No      | Yes    | No     | No       | Yes | Yes
+ * <default> | Normal | Yes     | Yes    | Likely | Likely   | No  | No
+ * 
+ * Memory sections should be created based on each sections capabilities, 
+ * specifically, with attention to: 
+ *   - DMA R/W safety without any special cache coherence effort
+ *   - ISR R/W safety without any special cache coherence effort
+ * 
+ * If the platform needs no special considerations for any of these, 
+ * the names should still be defined but the content can be blank.
+ * 
+ * Applications and libraries should prepend these to appropriate 
+ * definitions when the following conditions are met, for each 
+ * individual variable / function : 
+ * 
+ *   - Variable R/W in ISR context + DMA context : SHARED
+ *   - Variable R/W in DMA context only : SHARED
+ *   - Variable R/W in ISR context only : DTCM
+ *   - Variable R/W in regular context only, high frequency : DTCM
+ *   - Variable R/W in regular context only, low frequency  : <default>
+ * 
+ *   - Function executes in ISR context : ITCM
+ *   - Function executes in regular context only, high frequency : ITCM
+ *   - Function executes in regular context only, low frequency : <default>
+ * 
+ * Note: The stack and heap presently exist in DTCM on the stm32 h7rsxx 
+ * linker file, so we might not be able to DMA to/from malloc defined 
+ * variables. Consider reconfiguring malloc to use SHARED or another 
+ * DMA-accessible region if possible, ideally optionally.
+ *
+ * TODO: 
+ * 
+ * HPDMA in CM7 might be able to write to DTCM. If that is the case 
+ * for the platform, then it may be ok to put SHARED memory into DTCM 
+ * as well, as long as we can ensure HPDMA-only usage. We presently 
+ * don't try to do this, and it's unclear what other constraints exist.
+ */