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