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bus_dma(9) BUS_DMA_COHERENT is a hint (or not)

bus_dma(9) specifies that for bus_dmamem_map() the flag BUS_DMA_COHERENT is a 
hint; and that a device driver must not rely on this flag for correct 
operation.  All calls to bus_dmamap_sync() must still be made.

But for frame buffers this seems impractical to me and it appears in practice, 
that frame buffers that use DMA do indeed depend on this flag and do not call 
bus_dmamap_sync() functions.  An example of this is arch/arm/xcale/pxa2xo_lcd.c 

Does anyone have advice on how one should proceed when writing a driver for a 
new graphics device?

Frank Zerangue

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