wip: flashimg/cpu_hdmi: Initial version of the TMDS decoder/downscalerlukeshu/vid-hdmi

7 files changed, 1251 insertions, 3 deletions

diff --git a/.editorconfig b/.editorconfig
index d281cf0..6d6ef60 100644
--- a/.editorconfig
+++ b/.editorconfig
@@ -20,6 +20,9 @@ trim_trailing_whitespace = true
 [*.{c,h}]
 _mode = c
 
+[*.S]
+_mode = asm
+
 [GNUmakefile]
 _mode = make
 
@@ -51,7 +54,7 @@ _mode = sh
 [{build-aux/lint-{src,bin},build-aux/gcov-prune,libmisc/error_generated.c.gen,libusb/include/libusb/tusb_helpers.h.gen}]
 _mode = bash
 
-[{build-aux/stack.c.gen,build-aux/tent-graph,libmisc/wrap-cc,lib9p_util/nut.h.gen}]
+[{build-aux/stack.c.gen,build-aux/tent-graph,libmisc/wrap-cc,lib9p_util/nut.h.gen,flashimg/cpu_hdmi/tmds_decode_table.h.gen}]
 _mode = python3
 indent_style = space
 indent_size = 4
diff --git a/GNUmakefile b/GNUmakefile
index d4422d4..a4dfb56 100644
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -69,6 +69,10 @@ generate/files += lib9p_util/nut.h
 lib9p_util/nut.h: %: %.gen lib9p_util/nut_gen lib9p_util/nut_gen/*.py
 	$< $@
 
+generate/files += flashimg/cpu_hdmi/tmds_decode_table.h
+flashimg/cpu_hdmi/tmds_decode_table.h: %: %.gen
+	$^ >$@
+
 generate/files += build-aux/sources.mk
 ifeq ($(INNER),)
 nonsource/files = $(generate/files)
@@ -183,5 +187,5 @@ format/sh format/bash: format/%:
 format/python3: ./build-aux/venv
 	./build-aux/venv/bin/black $(sources_$(@F))
 	./build-aux/venv/bin/isort $(sources_$(@F))
-format/make format/cmake format/gitignore format/ini format/9p-idl format/9p-log format/markdown format/pip format/man-cat:
+format/asm format/make format/cmake format/gitignore format/ini format/9p-idl format/9p-log format/markdown format/pip format/man-cat:
 	@: TODO: Write/adopt formatters for these file types
diff --git a/build-aux/lint-src b/build-aux/lint-src
index d536631..15327f4 100755
--- a/build-aux/lint-src
+++ b/build-aux/lint-src
@@ -148,7 +148,7 @@ get-dscname() {
 			done
 			./build-aux/venv/bin/pytest "${testfiles[@]}" || exit $?
 			;;
-		make | cmake | gitignore | ini | 9p-idl | 9p-log | markdown | pip | man-cat)
+		asm | make | cmake | gitignore | ini | 9p-idl | 9p-log | markdown | pip | man-cat)
 			# TODO: Write/adopt linters for these file types
 			:
 			;;
diff --git a/flashimg/cpu_hdmi/tmds_decode-interp.S b/flashimg/cpu_hdmi/tmds_decode-interp.S
new file mode 100644
index 0000000..de1605a
--- /dev/null
+++ b/flashimg/cpu_hdmi/tmds_decode-interp.S
@@ -0,0 +1,95 @@
+// flashimg/cpu_hdmi/tmds_decode-interp.S - Variant of tmds_decode.S that does linear interpolation
+//
+// Copyright (C) 2025  Luke T. Shumaker <lukeshu@lukeshu.com>
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+.syntax unified
+.cpu cortex-m0plus
+.thumb
+
+// .tmds_table is a LUT from 10-bit TMDS-encoded values to the 8-bit
+// decoded values.
+.section .scratch_x.tmds_table, "a"
+tmds_table:
+#define X(dec) .byte dec
+#include "tmds_decode_table.h"
+#undef X
+
+// void tmds_decode_line(uint16_t in_r[width],
+//                       uint16_t in_g[width],
+//                       uint16_t in_b[width],
+//                       uint8_t  out[width/2],
+//                       size_t width);
+//
+// Read in `width` values from 10-bit-encoded TMDS streams of R/G/B
+// channels of 24-bit RGB pixels; and write that out as RGB 2:3:2,
+// scaled down horizontally by half.
+//
+// `width` must be either 640 or 720.
+.section .scratch_x.tmds_decode_line, "ax"
+.global tmds_decode_line
+.type tmds_decode_line,%function
+.thumb_func
+tmds_decode_line:
+	// Of our supported screen formats, the one with the tightest
+	// cycles/pixel requirement is 720x480p@60Hz, which has
+	// H_active=720, H_total=858, meaning that we need to decode
+	// 720 pixels in under 858*10 cycles, giving us a budget of
+	// ~11.9 cycles per pixel.
+#define iR      r0
+#define iG      r1
+#define iB      r2
+#define o       r3
+#define w       r4
+#define vTab    r4
+#define vAcc    r5
+#define vTmp1   r6
+#define vTmp2   r7
+#define vOEnd   r12
+	push    {r4-r7, lr}             // ONCE += 1+5 cyc
+	lsrs    w, w, #1                // ONCE += 1 cyc
+	add     w, o, w                 // ONCE += 1 cyc
+	mov     vOEnd, w                // ONCE += 1 cyc
+#undef w
+	ldr     vTab, =tmds_table       // ONCE += 2 cyc
+loop:
+.rept 4
+	// red
+	ldmia   iR!, {vTmp2}            // REPT += 1+1 cyc
+	lsrs    vTmp1, vTmp2, #16       // REPT += 1 cyc
+	uxth    vTmp2, vTmp2            // REPT += 1 cyc
+	ldsb    vTmp1, [vTab, vTmp1]    // REPT += 2 cyc
+	ldsb    vTmp2, [vTab, vTmp2]    // REPT += 2 cyc
+	add     vTmp1, vTmp1, vTmp2     // REPT += 1 cyc // add 2 8-bit values, producing a 9-bit value...
+	lsrs    vTmp1, vTmp1, #7        // REPT += 1 cyc // ...shrink it from 9-bits to 2-bits
+	lsls    vAcc, vTmp1, #5         // REPT += 1 cyc
+	// green
+	ldmia   iG!, {vTmp2}            // REPT += 1+1 cyc
+	lsrs    vTmp1, vTmp2, #16       // REPT += 1 cyc
+	uxth    vTmp2, vTmp2            // REPT += 1 cyc
+	ldsb    vTmp1, [vTab, vTmp1]    // REPT += 2 cyc
+	ldsb    vTmp2, [vTab, vTmp2]    // REPT += 2 cyc
+	add     vTmp1, vTmp1, vTmp2     // REPT += 1 cyc // add 2 8-bit values, producing a 9-bit value...
+	lsrs    vTmp1, vTmp1, #6        // REPT += 1 cyc // ...shrink it from 9-bits to 3-bits
+	lsls    vTmp1, vTmp1, #2        // REPT += 1 cyc
+	orrs    vAcc, vAcc, vTmp1       // REPT += 1 cyc
+	// blue
+	ldmia   iB!, {vTmp2}            // REPT += 1+1 cyc
+	lsrs    vTmp1, vTmp2, #16       // REPT += 1 cyc
+	uxth    vTmp2, vTmp2            // REPT += 1 cyc
+	ldsb    vTmp1, [vTab, vTmp1]    // REPT += 2 cyc
+	ldsb    vTmp2, [vTab, vTmp2]    // REPT += 2 cyc
+	add     vTmp1, vTmp1, vTmp2     // REPT += 1 cyc // add 2 8-bit values, producing a 9-bit value...
+	lsrs    vTmp1, vTmp1, #7        // REPT += 1 cyc // ...shrink it from 9-bits to 2-bits
+	orrs    vAcc, vAcc, vTmp1       // REPT += 1 cyc
+	// store
+	strb    vAcc, [o, #0]           // REPT += 2 cyc
+	adds    o, o, #1                // REPT += 1 cyc
+.endr
+	cmp     o, vOEnd                // LOOP += 1 cyc
+	bne     loop                    // LOOP += 2 cyc ; ONCE -= 1 cyc
+	pop     {r4-r7, pc}             // ONCE += 3+5 cyc
+	// TOTAL  = ONCE+((720/2)/N)*LOOP+(720/2)*REPT cyc
+	//        =  18 +(  360  /4)*  3 +  360  * 37  cyc
+	//        = 13608 cyc
+	// BUDGET =  8580 cyc
diff --git a/flashimg/cpu_hdmi/tmds_decode.S b/flashimg/cpu_hdmi/tmds_decode.S
new file mode 100644
index 0000000..888b5b9
--- /dev/null
+++ b/flashimg/cpu_hdmi/tmds_decode.S
@@ -0,0 +1,82 @@
+// flashimg/cpu_hdmi/tmds_decode.S - Decode TMDS-encoded scanlines
+//
+// Copyright (C) 2025  Luke T. Shumaker <lukeshu@lukeshu.com>
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+.syntax unified
+.cpu cortex-m0plus
+.thumb
+
+// .tmds_table is a LUT from 10-bit TMDS-encoded values to the 8-bit
+// decoded values, shifted right 6 bits.
+.section .scratch_x.tmds_table, "a"
+tmds_table:
+#define X(dec) .byte dec>>6
+#include "tmds_decode_table.h"
+#undef X
+
+// void tmds_decode_line(uint16_t in_r[width],
+//                       uint16_t in_g[width],
+//                       uint16_t in_b[width],
+//                       uint8_t  out[width/2],
+//                       size_t width);
+//
+// Read in `width` values from 10-bit-encoded TMDS streams of R/G/B
+// channels of 24-bit RGB pixels; and write that out as RGB 2:3:2,
+// scaled down horizontally by half.
+//
+// `width` must be either 640 or 720.
+//
+// BUG: Scaling down happens by discarding every other pixel, not by
+// interpolation.
+.section .scratch_x.tmds_decode_line, "ax"
+.global tmds_decode_line
+.type tmds_decode_line,%function
+.thumb_func
+tmds_decode_line:
+	// Of our supported screen formats, the one with the tightest
+	// cycles/pixel requirement is 720x480p@60Hz, which has
+	// H_active=720, H_total=858, meaning that we need to decode
+	// 720 pixels in under 858*10 cycles, giving us a budget of
+	// ~11.9 cycles per pixel.
+#define iR      r0
+#define iG      r1
+#define iB      r2
+#define o       r3
+#define w       r4
+#define vTab    r5
+#define vAcc    r6
+#define vTmp    r7
+	push    {r4-r7, lr}             // ONCE += 1+5 cyc
+	lsrs    w, w, #1                // ONCE += 1 cyc
+	add     w, o, w                 // ONCE += 1 cyc
+	ldr     vTab, =tmds_table       // ONCE += 2 cyc
+loop:
+.rept 4
+	// red
+	ldmia   iR!, {vTmp}             // REPT += 1+1 cyc
+	uxth    vTmp, vTmp              // REPT += 1 cyc
+	ldsb    vTmp, [vTab, vTmp]      // REPT += 2 cyc
+	lsls    vAcc, vTmp, #5          // REPT += 1 cyc
+	// green
+	ldmia   iG!, {vTmp}             // REPT += 1+1 cyc
+	uxth    vTmp, vTmp              // REPT += 1 cyc
+	ldsb    vTmp, [vTab, vTmp]      // REPT += 2 cyc
+	lsls    vTmp, vTmp, #3          // REPT += 1 cyc
+	orrs    vAcc, vAcc, vTmp        // REPT += 1 cyc
+	// blue
+	ldmia   iB!, {vTmp}             // REPT += 1+1 cyc
+	uxth    vTmp, vTmp              // REPT += 1 cyc
+	ldsb    vTmp, [vTab, vTmp]      // REPT += 2 cyc
+	orrs    vAcc, vAcc, vTmp        // REPT += 1 cyc
+	// store
+	strb    vAcc, [o, #0]           // REPT += 2 cyc
+	adds    o, o, #1                // REPT += 1 cyc
+.endr
+	cmp     o, w                    // LOOP += 1 cyc
+	bne     loop                    // LOOP += 2 cyc ; ONCE -= 1 cyc
+	pop     {r4-r7, pc}             // ONCE += 3+5 cyc
+	// TOTAL  = ONCE+((720/2)/N)*LOOP+(720/2)*REPT cyc
+	//        =  17 +(  360  /4)*  3 +  360  * 22  cyc
+	//        = 8207 cyc
+	// BUDGET = 8580 cyc
diff --git a/flashimg/cpu_hdmi/tmds_decode_table.h b/flashimg/cpu_hdmi/tmds_decode_table.h
new file mode 100644
index 0000000..4e1d6ea
--- /dev/null
+++ b/flashimg/cpu_hdmi/tmds_decode_table.h
@@ -0,0 +1,1025 @@
+// Generated by `flashimg/cpu_hdmi/tmds_decode_table.h.gen`.  DO NOT EDIT!
+X(0b01111111)
+X(0b11111111)
+X(0b00000000)
+X(0b10000000)
+X(0b01111110)
+X(0b11111110)
+X(0b00000001)
+X(0b10000001)
+X(0b01111100)
+X(0b11111100)
+X(0b00000011)
+X(0b10000011)
+X(0b01111101)
+X(0b11111101)
+X(0b00000010)
+X(0b10000010)
+X(0b01111001)
+X(0b11111001)
+X(0b00000110)
+X(0b10000110)
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+X(0b00010011)
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+X(0b11111111)
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+X(0b11111101)
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+X(0b01110101)
+X(0b11110101)
+X(0b00001000)
+X(0b10001000)
+X(0b01110111)
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diff --git a/flashimg/cpu_hdmi/tmds_decode_table.h.gen b/flashimg/cpu_hdmi/tmds_decode_table.h.gen
new file mode 100755
index 0000000..f3b330f
--- /dev/null
+++ b/flashimg/cpu_hdmi/tmds_decode_table.h.gen
@@ -0,0 +1,39 @@
+#!/usr/bin/env python3
+# flashimg/cpu_hdmi/tmds_decode_table.h.gen - Generate a table for TMDS decoding
+#
+# Copyright (C) 2025  Luke T. Shumaker <lukeshu@lukeshu.com>
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+import sys
+
+print(f"// Generated by `{' '.join(sys.argv)}`.  DO NOT EDIT!")
+for enc in range(1 << 10):
+    D = [bool((enc >> (9 - i)) & 1) for i in range(10)]
+    # Algorithm from DVI 1.0 Figure 3-6
+    if D[9]:
+        for i in range(8):
+            D[i] = not D[i]
+    if D[8]:
+        Q = [
+            D[0],
+            D[1] != D[0],
+            D[2] != D[1],
+            D[3] != D[2],
+            D[4] != D[3],
+            D[5] != D[4],
+            D[6] != D[5],
+            D[7] != D[6],
+        ]
+    else:
+        Q = [
+            D[0],
+            D[1] == D[0],
+            D[2] == D[1],
+            D[3] == D[2],
+            D[4] == D[3],
+            D[5] == D[4],
+            D[6] == D[5],
+            D[7] == D[6],
+        ]
+    s = "".join("1" if b else "0" for b in Q)
+    print(f"X(0b{s})")