modules/zstd/memory: Add README describing the AXI interface

Co-authored-by: Pawel Czarnecki <pczarnecki@antmicro.com>
Signed-off-by: Robert Winkler <rwinkler@antmicro.com>

xls/modules/zstd/memory/README.md

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+# Memory
+
+This directory contains procs responsible for issuing read and write
+transactions compatible with AXI subordinates. The AXI bus was selected
+because it consists of five streams, which closely resemble the streams
+to which XLS channels are translated.
+
+Although XLS channels do not fully conform to the AXI standard, with proper
+I/O configuration, DSLX procs using XLS channels can successfully interface
+and communicate with AXI4 peripherals.
+
+The signals used to form AXI channels are represented as individual fields
+in dedicated structures. However, the XLS toolchain generates these fields
+as a flattened bit array. As a result, the Verilog code produced from these
+structures will not match the expected AXI bus signature. To interface with
+other AXI peripherals, additional Verilog wrappers may be needed to split
+the flattened bit vector into individual signals.
+
+# Data Structures
+
+As noted, the procs in this directory use channels with dedicated
+structures to represent AXI bus signals in DSLX. For instance, the AXI read
+interface comprises the AR channel, which is used to provide a read address,
+and the length of data to read, as well as the R channel, which is used
+to receive the read data. These channels are represented by the `AxiAr` and
+`AxiR` structures in the `axi.x` file, which contains AXI4 bus definitions.
+The structures used to represent AXI4 Stream interface can be found in
+the `axi_st.x` file.
+
+# Main components
+
+The primary components of this directory are `MemReader` and `MemWriter`,
+which facilitate issuing read and write transactions on the AXI bus.
+They provide a convenient interface for the DSLX side of the design,
+managing the complexities of AXI transactions.
+
+The `MemReader` includes several procs that can be used individually:
+
+- `AxiReader`: Handles the creation of AXI transactions, managing unaligned
+  addresses and issuing additional transactions when crossing the 4KB boundary
+  or when the read request is longer than maximum possible burst size on the AXI bus
+
+- `AxiStreamDownscaler`: An optional proc available in `MemReaderAdv`,
+  enabling DSLX designs to connect to a wider AXI bus.
+
+- `AxiStreamRemoveEmpty`: Removes empty data bits, identified by zeroed bits in
+  the `tkeep` and `tstr` signals of the AXI Stream.
+
+The `MemWriter` proc is organised in a similar manner, it consists of:
+
+- `AxiWriter`: Handles the creation of AXI write transactions, managing unaligned
+  addresses and issuing additional transactions when crossing the 4KB boundary,
+  or when the write request is longer than maximum possible burst size on the AXI bus
+
+- `AxiStreamAddEmpty`: Adds empty data bits in the stream of data to write.
+  It is used to shift the data in the stream to facilitate writes to unaligned
+  addresses.
+
+# Usage
+
+The list below shows the usage of the `MemReader` proc:
+
+1. Send a `MemReaderReq` to the `req_in_r` channel, providing the information
+   about the absolute address from which to read the data, and the length
+   of data to read.
+
+2. Wait for the response on the `resp_s` channel. The received packet
+   consists of:
+
+   - the `status` of the operation indicating if the read was successful
+   - `data` read from the bus
+   - `length` of the valid `data` in bytes
+   - `last` flag used for marking the last `packet` with data
+
+The list below shows the usage of the `MemWriter` proc:
+
+1. Send a `MemWriterReq` to the `req_in_r` channel, providing the information
+   about the absolute address to which data should be sent, and length of the
+   transaction in bytes.
+
+2. Provide the data to write using the `MemWriterDataPacket` structure, which
+   should be send to `data_in_r` channel. The structure consists of
+
+   - the actual `data`
+   - `length` of the `data` in bytes
+   - `last` flag used for marking the last `packet` with data.
+
+3. Wait for the response submitted on the `resp_s` channel, which indicates
+   if the write operation was successful or an error occurred.