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minor API comment update

This commit is contained in:
Lephe 2022-09-07 23:25:43 +02:00
parent 3e5c45c5ad
commit e95a91d5ab
Signed by: Lephenixnoir
GPG Key ID: 1BBA026E13FC0495
2 changed files with 14 additions and 18 deletions
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30
include/gint/display-cg.h
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@ -70,31 +70,27 @@ typedef image_t bopti_image_t;
// Video RAM management
//---
/* dsetvram() - Control video RAM address and triple buffering
/* dsetvram(): Control video RAM address and triple buffering
Normal rendering under gint uses double-buffering: there is one image
displayed on the screen and one in memory, in a region called the video RAM
(VRAM). The application draws frames in the VRAM then sends them to the
screen only when they are finished, using dupdate().
On fxcg50, the performance bottleneck is almost always the graphical
rendering (especially in games) because the high amount of data, 173 kB per
frame in full-resolution, makes graphics manipulation computationally
expensive. The transfer also takes about 10 ms in itself.
On fx-CG, sending frames with dupdate() is a common bottleneck because it
takes about 11 ms. Fortunately, while the DMA is sending the frame to the
display, the CPU is available to do work in parallel. This function sets up
triple buffering (ie. a second VRAM) so that the CPU can start working on
the next frame while the DMA is sending the current one.
Since gint transfers data to the screen using the DMA, it is possible to run
the application while the finished frame is being transferred. However,
writing to the VRAM during this period will cause display artifacts since
the VRAM it is still being read by the DMA.
However, experience shows minimal performance improvements, because writing
to main RAM does not parallelize with DMA transfers. Since gint 2.8, this
is no longer the default, and the memory for the extra VRAM is instead
available via malloc().
The solution to this is to use triple-buffering with the display and two
VRAMs that are alternately being written to while the other is being
transferred. The VRAM switching is handled by dupdate() and is activated
whenever two VRAMs are configured.
By default gint uses triple buffering with two VRAMs in the system stack.
VRAMs must be contiguous, 32-aligned, (2*396*224)-byte buffers.
VRAMs must be contiguous, 32-aligned, (2*396*224)-byte buffers with 32 bytes
of extra data on each side (ie. 32 bytes into a 32-aligned buffer of size
177472).
@main Main VRAM area, used alone if [secondary] is NULL
@secondary Additional VRAM area, enables triple buffering if non-NULL */

2
src/dma/dma.c
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@ -178,7 +178,7 @@ static void dma_channel_wait(int channel, bool foreign)
if(!ch) return;
/* If interrupts are disabled or we don't own the device, spin-wait by
checking either for TE to be set (Transfere Ended) or DE to be gone
checking either for TE to be set (Transfer Ended) or DE to be gone
(channel disabled).
There are definitely race conditions if the DMA is restarted between