2016-11-21, 00:31
(2016-11-20, 23:23)Milhouse Wrote: [*]New firmware - new bootcode.bin makes an extra 16MB RAM available on Pi2/Pi3
A bit of background for what has changed.
ARM accesses GPU memory through a simple MMU which has 64 x 16M pages.
Because arm needs to use GPU peripherals one of those pages is dedicated to peripheral access resulting in only 1008M being accessible from arm.
In theory the gpu could use that top 16M, but so far there has been no guarantee that the arm won't also want to access this memory.
There were 3 places where there was a problem. vcbdg, touchscreen driver (which uses a shared buffer allocated by GPU) and virtual gpio driver (which uses a shared buffer allocated by GPU).
Recent kernel changes allow virtual gpio and touchscreen drivers to allocate the shared buffer on arm side.
And we have provided an API to read memory through DMA (which can see the top 16M) which can be used by vcdbg.
One advantage of using DMA is it is L2 coherent with GPU (but not L1 coherent), so you are less likely to get corruption with "vcdbg reloc" type commands.
So, today we enable the top 16M of ram, and you should find (e.g. from "vcgencmd get_mem arm") an extra 16M is available. Enjoy!
