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[doc] Add some basic info on using ffi-cdecl (#5808)

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Thomas Spurden
2020-02-12 16:40:38 +00:00
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doc/Porting.md
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@@ -25,6 +25,19 @@ Otherwise, when there'd be signficiant gain in doing so (i.e., when displaying m
The actual framebuffer content is then refreshed (i.e., displayed) via device-specific ioctls, making the best effort in using device-specific capabilities, whether that be [optimized waveform modes](https://github.com/koreader/koreader-base/blob/a1fc4e43b7cce7a76b13224e145f9bada343d8ea/ffi/framebuffer_mxcfb.lua#L643-L655), hardware dithering or [hardware inversion](https://github.com/koreader/koreader-base/blob/a1fc4e43b7cce7a76b13224e145f9bada343d8ea/ffi/framebuffer_mxcfb.lua#L253-L256).
The platform-specific Lua FFI modules for using the mxcfb driver are generated from the [kernel header files](https://github.com/koreader/koreader-base/ffi-cdecl/include) using [ffi-cdecl](https://github.com/koreader/ffi-cdecl). `ffi-cdecl` operates on [C files](https://github.com/koreader/koreader-base/ffi-cdecl/) which specify what to export.
To use `ffi-cdecl` first you need to build the gcc plugin for the platform toolchain, e.g.:
```
cd ffi-cdecl
make clean
PATH=$HOME/x-tools/arm-remarkable-linux-gnueabihf/bin/:$PATH make CHOST=arm-remarkable-linux-gnueabihf CROSS_DIR=$HOME/x-tools/arm-remarkable-linux-gnueabihf/
```
Then you can (re)generate the [ffi lua files](https://github.com/koreader/koreader-base/ffi), e.g.
```
PATH=$HOME/x-tools/arm-remarkable-linux-gnueabihf/bin/:$PATH ./ffi-cdecl arm-remarkable-linux-gnueabihf-gcc ../koreader/base/ffi-cdecl/mxcfb_remarkable_decl.c ../koreader/base/ffi/mxcfb_remarkable_h.lua
```
### Legacy einkfb eInk devices
KOReader uses the Linux framebuffer to control eInk devices, so the output module for legacy einkfb devices is [`base/ffi/framebuffer_einkfb.lua`](https://github.com/koreader/koreader-base/blob/master/ffi/framebuffer_einkfb.lua).