dma debug: account for cachelines and read-only mappings in overlap tracking
While debug_dma_assert_idle() checks if a given *page* is actively undergoing dma the valid granularity of a dma mapping is a *cacheline*. Sander's testing shows that the warning message "DMA-API: exceeded 7 overlapping mappings of pfn..." is falsely triggering. The test is simply mapping multiple cachelines in a given page. Ultimately we want overlap tracking to be valid as it is a real api violation, so we need to track active mappings by cachelines. Update the active dma tracking to use the page-frame-relative cacheline of the mapping as the key, and update debug_dma_assert_idle() to check for all possible mapped cachelines for a given page. However, the need to track active mappings is only relevant when the dma-mapping is writable by the device. In fact it is fairly standard for read-only mappings to have hundreds or thousands of overlapping mappings at once. Limiting the overlap tracking to writable (!DMA_TO_DEVICE) e...
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