From 279f5a00c9a9b39f4f6e9813e6d4da8c181d34c8 Mon Sep 17 00:00:00 2001 From: Chris Wilson Date: Thu, 5 Oct 2017 20:10:05 +0100 Subject: [PATCH] drm/i915/execlists: Add a comment for the extra MI_ARB_ENABLE MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Michel Thierry noticed that we were applying WaDisableCtxRestoreArbitration even to gen9, which does not require the w/a. The rationale is that we need to enable MI arbitration for execlists to work, and to be safe we do that before every batch (in addition to every context switch into the batch). Since this is not clear from the single line comment suggesting the MI_ARB_ENABLE is solely for the w/a, add a little more detail. Signed-off-by: Chris Wilson Cc: Michel Thierry Cc: Joonas Lahtinen Cc: Michał Winiarski Link: https://patchwork.freedesktop.org/patch/msgid/20171005191005.13462-1-chris@chris-wilson.co.uk Reviewed-by: Michel Thierry --- drivers/gpu/drm/i915/intel_lrc.c | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-) diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c index c5b76082d695..721432ddf403 100644 --- a/drivers/gpu/drm/i915/intel_lrc.c +++ b/drivers/gpu/drm/i915/intel_lrc.c @@ -1618,7 +1618,23 @@ static int gen8_emit_bb_start(struct drm_i915_gem_request *req, if (IS_ERR(cs)) return PTR_ERR(cs); - /* WaDisableCtxRestoreArbitration:bdw,chv */ + /* + * WaDisableCtxRestoreArbitration:bdw,chv + * + * We don't need to perform MI_ARB_ENABLE as often as we do (in + * particular all the gen that do not need the w/a at all!), if we + * took care to make sure that on every switch into this context + * (both ordinary and for preemption) that arbitrartion was enabled + * we would be fine. However, there doesn't seem to be a downside to + * being paranoid and making sure it is set before each batch and + * every context-switch. + * + * Note that if we fail to enable arbitration before the request + * is complete, then we do not see the context-switch interrupt and + * the engine hangs (with RING_HEAD == RING_TAIL). + * + * That satisfies both the GPGPU w/a and our heavy-handed paranoia. + */ *cs++ = MI_ARB_ON_OFF | MI_ARB_ENABLE; /* FIXME(BDW): Address space and security selectors. */ -- GitLab