.db: add basic doc

Signed-off-by: John McMaster <johndmcmaster@gmail.com>

docs/format/db.rst

@@ -0,0 +1,58 @@
+Introduction
+================
+
+This section documents how prjxray represents the bitstream database
+
+These ".db" files come in two common flavors:
+ * segbits_*.db: encodes bitstream bits
+ * mask_*.db: which bits are used by a segment? Probably needs to be converted to tile
+
+Also note: .rdb (raw db) is a convention for a non-expanded .db file (see below)
+
+
+segbits
+================
+
+These are created by segmatch to describe bitstream IP encoding.
+
+Example lines:
+ * CLB.SLICE_X0.DFF.ZINI 31_58
+   * For feature CLB.SLICE_X0.DFF.ZINI
+   * Frame: 31
+   * Word: 58 // 32 = 1
+   * Mask: 1 << (58 % 32) = 0x04000000
+   * To set an actual bitstream location, you will need to adjust frame and word by their tile base addresses
+ * CLBLL_L.SLICEL_X0.AOUTMUX.A5Q !30_06 !30_08 !30_11 30_07
+   * A multi bit entry. Bit 30_06 should be cleared to use this feature
+ * INT_L.BYP_BOUNCE5.BYP_ALT5 always
+   * A pseudo pip: feaure always active => no bits required
+ * CLBLL_L.OH_NO.BAD.SOLVE <const0>
+   * Candidate bits exist, but they've only ever been set to 0
+   * Internal only
+ * CLBLL_L.OH_NO.BAD.SOLVE <const1>
+   * Candidate bits exist, but they've only ever been set to 1
+   * Internal only
+ * INT.BRAM_ADDRARDADDRL0.BRAM_IMUX_ADDRARDADDRL0 <m1 0> <const0>
+   * Internal only
+
+Related tools:
+ * segmatch: solves symbolic constraints on a bitstream to produce symbol bitmasks
+ * dbfixup.py: internal tool that expands multi-bit encodings (ex: one hot) into groups. For example:
+   * .rdb file with one hot: BRAM.RAMB18_Y1.WRITE_WIDTH_A_18 27_267
+   * .db: file expanded: BRAM.RAMB18_Y1.WRITE_WIDTH_A_18 !27_268 !27_269 27_267
+ * dbcheck.py: valides that a database is fully and consistently solved
+   * Ex: complains if const0 entries exist
+   * Ex: complains if symbols are duplicated (such as after a mergedb after rename)
+ * mergedb.sh: adds new bit entries to an existing db
+   * Ex: CLB is solved by first solving LUT bits, and then solving FF bits
+
+
+mask
+================
+
+These are just simple bit lists
+
+Example line: bit 01_256
+
+See previous section for number meaning
+