Michael Zucchi

 B.E. (Comp. Sys. Eng.)

  also known as zed
  & handle of notzed


android (44)
beagle (63)
biographical (97)
blogz (9)
business (1)
code (73)
cooking (31)
dez (7)
dusk (30)
extensionz (1)
ffts (3)
forth (3)
free software (4)
games (32)
gloat (2)
globalisation (1)
gnu (4)
graphics (16)
gsoc (4)
hacking (451)
haiku (2)
horticulture (10)
house (23)
hsa (6)
humour (7)
imagez (28)
java (229)
java ee (3)
javafx (49)
jjmpeg (80)
junk (3)
kobo (15)
libeze (7)
linux (5)
mediaz (27)
ml (15)
nativez (9)
opencl (120)
os (17)
panamaz (3)
parallella (97)
pdfz (8)
philosophy (26)
picfx (2)
players (1)
playerz (2)
politics (7)
ps3 (12)
puppybits (17)
rants (137)
readerz (8)
rez (1)
socles (36)
termz (3)
videoz (6)
vulkan (3)
wanki (3)
workshop (3)
zcl (3)
zedzone (23)
Friday, 29 November 2013, 15:23

Sharing DRAM on parallella - the easier way.

Currently parallella reserves a block of memory at a fixed physical address which the epiphany accesses directly and can be mapped to a linux process via e_alloc(). But as e_alloc() just calls mmap(NULL, ...) the linux-side address is different, and infact it will create a new alias for every e_alloc() invocation with the same parameters.

This makes it a bit of a pain to use as memory locations must be calculated manually via offsets on at least one end.

With the current parallella-16 the shared block is accessible on the epiphany via 0x8e000000, and I was wondering if that could just be mapped to the same location on the ARM side. It's been a while since I looked at the memory map of linux and i wasn't sure if there was anything there - and it turns out there isn't. Using cat /proc/pid/maps I found that it's somewhere between and a long way away from both the heap and the stack.

So ... if instead of calling e_alloc() you can just use mmap directly to create a shared area that can be used to pass complex linked data structures without having to perform any address remapping by hand - which is error prone and simply a big pita.

e.g. to map 8K at the start of this block on the parallella-16 I have:

        // parallella-16: physical 3e000000 on arm == physical 8e000000 on epiphany
        int fd = open("/dev/mem", O_RDWR | O_SYNC);
        void *pmem = mmap((void *)0x8e000000, 8192, PROT_READ|PROT_WRITE, MAP_SHARED,
                          fd, 0x3e000000);

The specific numbers may vary on a given platform or configuration and can be determined at runtime.

This makes using the shared DRAM a lot easier including features like dynamic memory management (aka malloc).

Tagged parallella.
fpu mode, compiler options | microsoft's ex-box one more WTF?
Copyright (C) 2019 Michael Zucchi, All Rights Reserved. Powered by gcc & me!