UM0851 datasheet(158/245 Pages) STMICROELECTRONICS

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Accelerator engine device drivers

UM0851

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Doc ID 16604 Rev 2

On success, mmap returns a pointer to the mapped area. On error, the value MAP_FAILED

(that is, (void *) -1) is returned, and errno is set appropriately.

The mmap function asks to map length bytes starting at offset "offset" from the file specified

by the file descriptor "fd" into memory, preferably at address "start". This "start" address is a

hint only, and is usually specified as 0.

You can use following parameters to call this function for rfd and wfd.

mmap(0, size, PROT_READ | PROT_WRITE, MAP_SHARED, file_desc, 0)

After completion of one codec operation (encoding/decoding), if you need to encode/decode

again without closing nodes of jpegread/jpegwrite, then user is not required to allocate/map

this memory again. Still if user needs to map it again with a different size, then you must first

unmap any memory that has been already mapped.

Write source JPEG data

After mapping memory for the write buffer, you can write input data to write buffers (data

length less than equal to size of one write buffer). This can be done by writing or copying

data directly to the mapped virtual memory addresses of write buffers.

Start encoding or decoding

Once input data is written to write buffer, encoding/decoding can be started/resumed. Use

the following system call to start and resume JPEG encoding/decoding with new input data:

ioctl(wfd, JPEGIOC_START, size);

wfd: file descriptor of write node.

JPEGIOC_START: command to start/resume jpeg encoding/decoding

size: amount of data written on jpeg write node, <= memory mapped for each jpeg write

buffer.

This is a blocking system call which unblocks or returns only when encoding/decoding with

data supplied from current write buffer is started or an error has occurred. On success, zero

is returned otherwise, -1 is returned on error and errno is set appropriately. For example:

total size of input data is 15 K, size of each write buffer is 4 K, then you need to follow these

steps to pass data to JPEG:

Set current write buffer to write_buffer0

Write 4K data in current write buffer

Call JPEGIOC_START ioctl with size equal to 4 K

Toggle current write buffer to buffer1 if current buffer is buffer0, else to buffer0 if current

buffer is buffer1

Follow steps 2, 3, 4 two more times with size 4 K and one time with size 3 K.

Get encoded/decoded data

Once encoding/decoding is started, then output data must be copied from read buffer. Use

the following system call used to get the output data:

ioctl(rfd, JPEGIOC_GET_OUT_DATA_SIZE, &size);

rfd: read node file descriptor.

JPEGIOC_GET_OUT_DATA_SIZE: command to get output data.

UM0851 Datasheet(PDF) 158 Page - STMicroelectronics

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