ti6446 codec engine内存分配的一个重要实例

A Real World Example

This section describes an actual case in which a particular customer tried to resize their memory map.

The application is a four-channel CIF MPEG4 Simple Profile (or H.264 Baseline Profile) Digital Video Recorder (DVR) based on DM6446 with 64MB DDR2. 4 channels of CIF video are encoded and 1 channel of CIF video is decoded. The MPEG4 (or H.264) & Audio encoder & decoder conform to xDM. In this discussion, we will focus on the video codecs. So, 4 CIF encoding instances and 1 CIF decoding instance will be created by calling the VISA API:s. This example is based on Codec Engine 1.02 & DSPLINK 1.30.08.02. Below is the system block diagram:

The final 64MB memory map looks like:

0x80000000 .. 0x83200000-1 (0-50MB; size 50MB): Linux: booted with MEM = 50M 
0x83200000 .. 0x83A00000-1 (50-58MB; size 8MB): CMEM: shared ARM/DSP I/O buffers 
0x83A00000 .. 0x83C00000-1 (58-60MB; size 2MB): DDRALGHEAP: codec dynamic memory 
0x83C00000 .. 0x83E00000-1 (60-62MB; size 2MB): DDR: code, stack, system data 
0x83E00000 .. 0x83F00000-1 (62-63MB; size 1MB): DSPLINKMEM: memory for DSPLINK
0x83F00000 .. 0x83F00080-1 (63-63MB; size 128B): RESET_VECTOR: reset vectors  
0x83F00080 .. 0x84000000-1 (63-64MB; size 1MB): Unused memory

How did we arrive at this memory map? First of all, 1MB DSPLINKMEM is the default size of DSPLINK 1.30.08.02. It is important to correctly allocate the right memory size for CMEM, DDRALGHEAP & DDR. Then we will have enough space for DSP S/W & Linux OS.

Below diagram illustrates the system application data flow. VPFE puts the video input data (to be encoded data) in CMEM. The encoder running on DSP outputs the encoded data to CMEM. The encoded data is stored on hard disk finally. As for the decoding data flow, to-be-decoded data is copied from hard disk to CMEM first. Then, the decoder decompresses the data and outputs the result into decoded data buffers. The VPBE output resolution is CIF. Sometimes, we can use the resizer of the VPFE peripheral on DM6446 to get D1 resolution. So we need to allocate a buffer for resizer results in CMEM too.

Allocating CMEM Memory Space

As for to-be-encoded data buffers, the size is ((352 * 288) * 4 * 2B) * 3 = 2433024B. 352*288 is CIF resolution, 4 means 4 channels, one pixel in YUV4:2:2 needs 2 bytes and three (352 * 288) * 4 * 2B buffers are allocated for encoder algorithm. The size of decoded data buffers is same: 2433024B. Because we encode 4 channels CIF, you can calculate the size of encoded data buffers by 50% D1 ((720 * 576 * 3 / 2 ) / 2 = 303.75KB, YUV4:2:0) or standard MPEG4 compression ratio. Here we allocate 256KB (262144B) for encoded data buffers less than 303.75KB. This is chosen based on experience. So, we configure three 256KB buffers (786432B) for to-be-decoded data buffer accordingly. As for the buffer of resizer result, we need 720 * 576 * 2B = 829440B in YUV4:2:2. So, the insmod cmemk command looks like:

insmod cmemk.ko phys_start=0x83200000 phys_end=0x83A00000 pools=1x262144, 2x2433024, 1x829440,1x786432

Allocating DDRALGHEAP Memory Space

DDRALGHEAP is the memory allocated for codec dynamic memory requests. Both encoder and decoder will process and accept data with YUV4:2:0. One channel CIF data in YUV4:2:0 is 352 * 288 * 3 / 2 B (one pixel with YUV4:2:0 format needs 3/2 byte). Encoder and decoder algorithms need the current frame and previous frame data. To compress or decompress one channel CIF, we need to allocate 352 * 288 * 3 / 2 * 2 B memory for encoder and decoder respectively. Because 4 channels CIF will be encoded and 1 channel CIF will be decoded. So, the encoder needs 352 * 288 * 3 / 2 * 2 * 4B (about 1.16MB) of dynamic memory and the decoder needs 352 * 288 * 3 / 2 * 2 * 1B (about 297KB) of dynamic memory. The total of them is about 1.45MB. 2MB DDRALGHEAP is allocated in this example.

Allocating DDR Memory Space

DDR is the DSP-side segment including all the system code, data, stack, heaps and code and static data for the codecs. The code size for the most complex video codecs is less than several hundred KBs. We can use the script sectti.pl to determine DDR section size:

ofd6x -x codec_server.x64P | perl c:\temp\cg_xml\ofd\sectti.pl > codec_server.x64P.sectti.csv

The script generated a report file, we can get about 416 KB of the totals of data and code. So 2MB DDR of this application is enough.

REPORT FOR FILE: codec_server.x64P               
          Name :         Size (dec) Size (hex)  Type    Load Addr   Run Addr            
          MPEG4ENC :     23840	    0x00005d20  CODE   	0x83c71000  0x83c71000            
          MPEG4DEC :     10784      0x00002a20  CODE   	0x83c82000  0x83c82000                 
          .bss :         910        0x0000038e  UDATA   0x83c88000  0x83c88000            
          .hwi_vec :     512        0x00000200  CODE    0x83c70c00  0x83c70c00                  
          .far :         204920     0x00032078  UDATA   0x83c00000  0x83c00000                
          .bios :        22912 	    0x00005980  CODE    0x83c76d20  0x83c76d20                 
          .text :        123136     0x0001e100  CODE    0x83c52080  0x83c52080                
          .cinit :       8196  	    0x00002004  DATA    0x83c84a20  0x83c84a20              
          .sysinit :     1792  	    0x00000700  CODE    0x83c70180  0x83c70180               
          .const :       21288 	    0x00005328  DATA    0x83c7c6a0  0x83c7c6a0               
          .stack :       4096  	    0x00001000  UDATA   0x83c86a28  0x83c86a28

Totals by section type (about 416KB)  
Uninitialized Data: 212958  0x00033fde    
 Initialized Data : 30080  0x00007580             
             Code : 182976  0x0002cac0

Allocating Linux OS Memory space

We computed it by calculating the DSP needs first and subtracting that from the total amount of memory available. We know our production system has only 64MB of memory. Given we need 1MB for "DSPLINKMEM", 2MB for "DDR", 2MB for "DDRALGHEAP", 1MB for "RESET_VECTOR" & unused memory and 8MB for CMEM, that gives a total of 14MB for DSP and shared buffers, leaving 50MB for Linux.

Conclusion

Memory map configuration for Davinci-based system can be systematically performed after the user has designed the memory map to suit the amount of memory available. In order for the procedure to go smoothly, a reminder is to:

1. Know your system. Plan the memory map based on how many and which codec instances will need to be available at the same time in different modes of execution in the application. Calculate or measure the size for each segment and write down the desired memory map.
2. Be thorough. Apply the mechanical steps to adapt the DSP server, ARM application, DSP Link, CMEM and boot loader to match the desired memory map. Always double-check the changes to ensure all numbers agree with each other.

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