(Having trouble reporting this - so forwarding manually on behalf of vb) > > > From: Volker Barthelmann <vb@xxxxxxxxxxxx> > > > > Hello, > > > > I recently bought a raspberry pi and I want to use the GPU as a > > test-architecture for some vector-extensions for my C compiler. As a > > first step, I want to get scalar code to work reasonably well on the GPU. > > > > So far I have backends for the compiler and assembler that can compile, > > assemble and link small programs (not all features are done yet) to be > > run with a slightly modified version of the mailbox.c from > > https://github.com/hermanhermitage/videocoreiv/wiki/VideoCore-IV-Kernels-under-Linux. > > > > During my work so far I have encountered a few questions that maybe > > someone here can answer. > > > > 1. Arithmetic operations like add/sub seem to do some saturating or so. > > Is this true? If yes, can this be turned off or are there instruction > > variants that do simple two's complement arithmetic? > > > > 2. What restrictions are known regarding the registers r16-r24? It > > seems, for example, that they can not be used in ld/st instructions, > > although some instruction encodings should have 5 bits for the register. > > > > 3. Is there any more detailed documentation on the mailbox interface? > > E.g., are there any registers that may not be used, how much stack is > > allocated, etc.? > > > > 4. (Maybe the most interesting question at the moment) Is it possible to > > call back ARM code from the GPU, i.e. the other direction of the mailbox > > interface? I would like to port a C library and for that I would need > > access to a few Linux syscalls. So I would need a mechanism to pass the > > information to the ARM, have ARM code execute the syscall and pass the > > information back to the GPU. I assume it could be done by a thread > > polling some shared memory, but I hope there is a better solution. > > > > If anybody can answer any of the questions that would be great. Also, if > > you can point me to more sources of information (my starting point was > > https://github.com/hermanhermitage/videocoreiv), that would be much > > appreciated. > > > > Thanks, > > > > Volker