4f54741e07
The used space in a ring is given by the cyclic distance from the consumer (HEAD) to the producer (TAIL), i.e. ((tail-head) MOD size); conversely, the available space in a ring is the cyclic distance from the producer to the consumer, MINUS the amount reserved for a "gap" that is supposed to guarantee that the producer never catches up with or overruns the consumer. Note that some GEN h/w requires that TAIL never approach to within one cacheline of HEAD, so the gap is usually set to twice the cacheline size to ensure this. While the existing code gives the correct answer for correct inputs, if the producer HAS overrun into the reserved space, the result can be a value larger than the maximum valid value (size-reserved). We can improve this by reorganising the calculation, so that in the event of overrun the result will be negative rather than over-large. This means that the commonly-used test (available >= required) will then reject further writes into the ring after an overrun, giving some chance that we can recover from or at least diagnose the original problem; whereas allowing more writes would likely both confuse the h/w and destroy the evidence of what went wrong. Signed-off-by: Dave Gordon <david.s.gordon@intel.com> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch> |
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