Thomas Hellström wrote:
As some of you might have read on another thread, there is a discussion how to handle the X server (and possibly other) 2d contexts in the via / unichrome family of drivers that expects certain 2d engine register values to stay the same even when other contexts or DMA commands have played.
Looking at the now obsolete gamma driver and ffb_context.c, some register values are saved at context switches whereas in other drivers this does not seem to happen.
What is the common practice? That all clients always reinitialize the engines every time they are used or that the drm saves the registers that are commonly used by different clients as part of the context switch?
The common practice is to define a set of register "groups", and define a bit mask for with 1 bit per group (usually called "dirty bits"). When a context gets the hardware lock, it checks to see if any other context has held the lock since the last time it held it. If another context has, it looks at the bit mask (stored in the SAREA). Any set dirty bits mean the some other context modified some register in that group. The dirty register(s) is then reset to the value expect by the context now holding the lock.
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