gareth evans <
headstone255@yahoo.com> wrote:
AIUI, the Binary Blob for driving the GPU is 32 bit code,
so when using 64 bit Raspbian, does that mean that one
CPU core is configured as 32 bit to provide the video?
If so, how is communication arranged between the 64 bit
and 32 bit CPU cores?
The amount of GPU memory is configured in config.txt with the
gpu_mem option. This memory region isn't allocated to virtual
address space by the CPU, so it's not used by any OS/user
software. Instead the GPU firmware allocates regions in this
memory itself, one key part of which is where it wants the CPU
to store the display frame buffer. The frame buffer memory address
is communicated back to the CPU via registers, which is also how
the CPU sends it commands.
None of this requires the CPU to run 32bit software, it just
reads/writes data in the size expected by the GPU, which may
be more/less than 32bits. The common unit for data in memory still
8 bit bytes anyway.
As for the issue of the 32bit address space potentially not
covering all the RAM on the better equipped Pis - this is probably
why unlike with the 32bit Pis, the Pi4's GPU doesn't have access
to the whole system memory. Instead it is said to require areas to
be allocated in advance with its MMU, before it can access them. So
it only sees the memory areas that it needs to see.
https://forums.raspberrypi.com/viewtopic.php?t=278081#p1694898
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