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| -rw-r--r-- | lib/842/842.h | 130 |
1 files changed, 130 insertions, 0 deletions
diff --git a/lib/842/842.h b/lib/842/842.h new file mode 100644 index 000000000..7b1f581a2 --- /dev/null +++ b/lib/842/842.h @@ -0,0 +1,130 @@ +/* SPDX-License-Identifier: GPL-2.0 */ + +#ifndef __842_H__ +#define __842_H__ + +/* The 842 compressed format is made up of multiple blocks, each of + * which have the format: + * + * <template>[arg1][arg2][arg3][arg4] + * + * where there are between 0 and 4 template args, depending on the specific + * template operation. For normal operations, each arg is either a specific + * number of data bytes to add to the output buffer, or an index pointing + * to a previously-written number of data bytes to copy to the output buffer. + * + * The template code is a 5-bit value. This code indicates what to do with + * the following data. Template codes from 0 to 0x19 should use the template + * table, the static "decomp_ops" table used in decompress. For each template + * (table row), there are between 1 and 4 actions; each action corresponds to + * an arg following the template code bits. Each action is either a "data" + * type action, or a "index" type action, and each action results in 2, 4, or 8 + * bytes being written to the output buffer. Each template (i.e. all actions + * in the table row) will add up to 8 bytes being written to the output buffer. + * Any row with less than 4 actions is padded with noop actions, indicated by + * N0 (for which there is no corresponding arg in the compressed data buffer). + * + * "Data" actions, indicated in the table by D2, D4, and D8, mean that the + * corresponding arg is 2, 4, or 8 bytes, respectively, in the compressed data + * buffer should be copied directly to the output buffer. + * + * "Index" actions, indicated in the table by I2, I4, and I8, mean the + * corresponding arg is an index parameter that points to, respectively, a 2, + * 4, or 8 byte value already in the output buffer, that should be copied to + * the end of the output buffer. Essentially, the index points to a position + * in a ring buffer that contains the last N bytes of output buffer data. + * The number of bits for each index's arg are: 8 bits for I2, 9 bits for I4, + * and 8 bits for I8. Since each index points to a 2, 4, or 8 byte section, + * this means that I2 can reference 512 bytes ((2^8 bits = 256) * 2 bytes), I4 + * can reference 2048 bytes ((2^9 = 512) * 4 bytes), and I8 can reference 2048 + * bytes ((2^8 = 256) * 8 bytes). Think of it as a kind-of ring buffer for + * each of I2, I4, and I8 that are updated for each byte written to the output + * buffer. In this implementation, the output buffer is directly used for each + * index; there is no additional memory required. Note that the index is into + * a ring buffer, not a sliding window; for example, if there have been 260 + * bytes written to the output buffer, an I2 index of 0 would index to byte 256 + * in the output buffer, while an I2 index of 16 would index to byte 16 in the + * output buffer. + * + * There are also 3 special template codes; 0x1b for "repeat", 0x1c for + * "zeros", and 0x1e for "end". The "repeat" operation is followed by a 6 bit + * arg N indicating how many times to repeat. The last 8 bytes written to the + * output buffer are written again to the output buffer, N + 1 times. The + * "zeros" operation, which has no arg bits, writes 8 zeros to the output + * buffer. The "end" operation, which also has no arg bits, signals the end + * of the compressed data. There may be some number of padding (don't care, + * but usually 0) bits after the "end" operation bits, to fill the buffer + * length to a specific byte multiple (usually a multiple of 8, 16, or 32 + * bytes). + * + * This software implementation also uses one of the undefined template values, + * 0x1d as a special "short data" template code, to represent less than 8 bytes + * of uncompressed data. It is followed by a 3 bit arg N indicating how many + * data bytes will follow, and then N bytes of data, which should be copied to + * the output buffer. This allows the software 842 compressor to accept input + * buffers that are not an exact multiple of 8 bytes long. However, those + * compressed buffers containing this sw-only template will be rejected by + * the 842 hardware decompressor, and must be decompressed with this software + * library. The 842 software compression module includes a parameter to + * disable using this sw-only "short data" template, and instead simply + * reject any input buffer that is not a multiple of 8 bytes long. + * + * After all actions for each operation code are processed, another template + * code is in the next 5 bits. The decompression ends once the "end" template + * code is detected. + */ + +#include <linux/module.h> +#include <linux/kernel.h> +#include <linux/bitops.h> +#include <linux/crc32.h> +#include <asm/unaligned.h> + +#include <linux/sw842.h> + +/* special templates */ +#define OP_REPEAT (0x1B) +#define OP_ZEROS (0x1C) +#define OP_END (0x1E) + +/* sw only template - this is not in the hw design; it's used only by this + * software compressor and decompressor, to allow input buffers that aren't + * a multiple of 8. + */ +#define OP_SHORT_DATA (0x1D) + +/* additional bits of each op param */ +#define OP_BITS (5) +#define REPEAT_BITS (6) +#define SHORT_DATA_BITS (3) +#define I2_BITS (8) +#define I4_BITS (9) +#define I8_BITS (8) +#define CRC_BITS (32) + +#define REPEAT_BITS_MAX (0x3f) +#define SHORT_DATA_BITS_MAX (0x7) + +/* Arbitrary values used to indicate action */ +#define OP_ACTION (0x70) +#define OP_ACTION_INDEX (0x10) +#define OP_ACTION_DATA (0x20) +#define OP_ACTION_NOOP (0x40) +#define OP_AMOUNT (0x0f) +#define OP_AMOUNT_0 (0x00) +#define OP_AMOUNT_2 (0x02) +#define OP_AMOUNT_4 (0x04) +#define OP_AMOUNT_8 (0x08) + +#define D2 (OP_ACTION_DATA | OP_AMOUNT_2) +#define D4 (OP_ACTION_DATA | OP_AMOUNT_4) +#define D8 (OP_ACTION_DATA | OP_AMOUNT_8) +#define I2 (OP_ACTION_INDEX | OP_AMOUNT_2) +#define I4 (OP_ACTION_INDEX | OP_AMOUNT_4) +#define I8 (OP_ACTION_INDEX | OP_AMOUNT_8) +#define N0 (OP_ACTION_NOOP | OP_AMOUNT_0) + +/* the max of the regular templates - not including the special templates */ +#define OPS_MAX (0x1a) + +#endif |