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Subject: Some comments on the iFP port

Some comments on the iFP port

From: Tomasz Malesinski <tmal_at_mimuw.edu.pl>
Date: 2006-04-10

I haven't had much time for Rockbox recently and I will not have much
of it in the following weeks. I just wanted to show that I'm alive and
discuss some things that are important for the iFP port.

1. Adjusting what is in IRAM and what is not.

I started implementing the idea discussed some time ago on IRC. I
introduced macors like IBSS_ATTR_MPC_SAMPLE_BUF. The macros may be
defined in config-<model>.h to be empty or not. The default values are
defined in config.h or, as I think is better, in files like
config-tremor.h, which are included by all files which may use a
specific macro.

2. Decreasing the audio buffer size.

There is no memory for 5s audio buffer. Currently I use a 0.5s
(~80kB) long buffer. I guess we should create a SMALL_AUDIO_BUFFER or
similar config macro. Crossfeeding should be probably disabled when
using a small audio buffer.

3. Reusing codecs iramcopy areas.

I tried putting codec bss over iramcopy area. It lets me save some
precious kB. It worked (I just had to move ci = api or rb = api
assingments after IRAM initialization). I have some problems with
codecs (sometimes they fail to initialize after playing one song, the
player shows "codec not found"), but I haven't checked if it is
related to this change.

4. Reducing memory usage of Unicode support.

I haven't tried it, but I suppose that we could use much less than 64
kB for the codepage table. Either only part of the codepage could be
loaded (on a flash player) or the codepage could be stored in
memory in a compressed form (run length encoding could be efficient
both in terms of compression ratio and decompression speed).

5. Noninterleaved output from dsp.

The iFP hardware uses noninterleaved samples or at least I don't know
how to make it accept interleaved ones. An option to make audio buffer
noninterleaved could save some unnecessary processing on iFP. I
haven't tried to implement this.

6. Codecs optimization.

Although I believe I switched the CPU to 60 MHz, I have performance
problems. After some optimizations I managed to play 80 kbps Ogg
Vorbis file in realtime. A 192 kbps Ogg and all mp3s still skip.

I will try to prepare clean patches in a week or two. Meanwhile, if
you are _really_ interested you can see some not up-to-date and quite
messy patches at:

http://www.mimuw.edu.pl/~tmal/rockbox/0410_patches/

The Tremor codec contains optimizations of vector operations (merged
with the Coldfire code), MDCT (block load and store instructions) and
Huffman decoding. The MP3 codec contains only one optimization in
synth.c, with a small impact on decoding speed. More is needed.

--
Tomek Malesinski
Received on Mon Apr 10 22:51:10 2006

Page was last modified "Jan 10 2012" The Rockbox Crew
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