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intel_timeline_read_hwsp() is used to support semaphore waits between engines, that may themselves be deferred for arbitrary periods -- that is the read of the target request's HWSP is at an indeterminant point in the future. To support this, we need to prevent overwriting a HWSP that is being watched across a seqno wrap (otherwise the next request will write its value into the old HWSP preventing the watcher from making progress, ad infinitum.) To simulate the observer across a wrap, let's create a request that reads from the HWSP and dispatch it at different points around a wrap to see if the value is lost. Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20201021220411.5777-2-chris@chris-wilson.co.uk