From 2e70b8c6bf0003bf1691ac5cab5aff128747d494 Mon Sep 17 00:00:00 2001 From: Chris Wilson Date: Thu, 23 Mar 2017 13:48:03 +0000 Subject: [PATCH] drm/i915/execlists: Relax the locked clear_bit(IRQ_EXECLIST) We only need to care about the ordering of the clearing of the bit with the uncached CSB read in order to correctly detect a new interrupt before the read completes. The uncached read itself acts as a full memory barrier, so we do not need to enforce another in the form of a locked clear_bit. v2: Clarify why the split and unlocked test/clear is harmless. Signed-off-by: Chris Wilson Cc: Tvrtko Ursulin Link: http://patchwork.freedesktop.org/patch/msgid/20170323134803.10418-1-chris@chris-wilson.co.uk Reviewed-by: Tvrtko Ursulin --- drivers/gpu/drm/i915/intel_lrc.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c index eec1e714f531..dd0e9d587852 100644 --- a/drivers/gpu/drm/i915/intel_lrc.c +++ b/drivers/gpu/drm/i915/intel_lrc.c @@ -540,7 +540,17 @@ static void intel_lrc_irq_handler(unsigned long data) dev_priv->regs + i915_mmio_reg_offset(RING_CONTEXT_STATUS_BUF_LO(engine, 0)); unsigned int csb, head, tail; - clear_bit(ENGINE_IRQ_EXECLIST, &engine->irq_posted); + /* The write will be ordered by the uncached read (itself + * a memory barrier), so we do not need another in the form + * of a locked instruction. The race between the interrupt + * handler and the split test/clear is harmless as we order + * our clear before the CSB read. If the interrupt arrived + * first between the test and the clear, we read the updated + * CSB and clear the bit. If the interrupt arrives as we read + * the CSB or later (i.e. after we had cleared the bit) the bit + * is set and we do a new loop. + */ + __clear_bit(ENGINE_IRQ_EXECLIST, &engine->irq_posted); csb = readl(csb_mmio); head = GEN8_CSB_READ_PTR(csb); tail = GEN8_CSB_WRITE_PTR(csb);