* Carsten Emde | 2013-06-14 22:17:01 [+0200]: >Hi Sebastian, Hi Carsten, >> - a "fix" for i915 leads to high latencies due to wbinvd(). Not sure >> what is the best thing to do here. >Even if we do not yet know what is the best final solution to this >problem, there is no doubt that, for the time being, the patch in >question has to be reverted - the earlier the better. Please >understand that this RT kernel release it totally unusable on a >system that is equipped with one of several widely used Intel >graphics boards. It simply is a waste of time. Yes but reverting causes problems for others. Anyway, in order to make progress here and not break anything I include the patch below. So everyone who does not want the expensive sync can specify i915.do_wbinvd=no after they ensured that the same CPU is used for GPU related operations. Subject: [PATCH] gpu: i915: allow the user not to do the wbinvd The wbinvd() renders the system with i915 unusable on RT. Using this expensive instruction avoids GPU trouble according to https://bugs.freedesktop.org/show_bug.cgi?id=62191 As a workaround for RT it is recommended to pin each GPU related process to the same CPU and then disable this instruction via the module paramter. Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> --- drivers/gpu/drm/i915/i915_gem.c | 17 +++++++++++++++-- 1 file changed, 15 insertions(+), 2 deletions(-) diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c index 339540d..dce41f4 100644 --- a/drivers/gpu/drm/i915/i915_gem.c +++ b/drivers/gpu/drm/i915/i915_gem.c @@ -35,6 +35,7 @@ #include <linux/swap.h> #include <linux/pci.h> #include <linux/dma-buf.h> +#include <linux/module.h> static void i915_gem_object_flush_gtt_write_domain(struct drm_i915_gem_object *obj); static void i915_gem_object_flush_cpu_write_domain(struct drm_i915_gem_object *obj); @@ -2656,6 +2657,10 @@ static inline int fence_number(struct drm_i915_private *dev_priv, return fence - dev_priv->fence_regs; } +static bool do_wbinvd = true; +module_param(do_wbinvd, bool, 0644); +MODULE_PARM_DESC(do_wbinvd, "Do expensive synchronization. Say no after you pin each GPU process to the same CPU in order to lower the latency."); + static void i915_gem_write_fence__ipi(void *data) { wbinvd(); @@ -2679,8 +2684,16 @@ static void i915_gem_object_update_fence(struct drm_i915_gem_object *obj, * on each processor in order to manually flush all memory * transactions before updating the fence register. */ - if (HAS_LLC(obj->base.dev)) - on_each_cpu(i915_gem_write_fence__ipi, NULL, 1); + if (HAS_LLC(obj->base.dev)) { + if (do_wbinvd) { +#ifdef CONFIG_PREEMPT_RT_FULL + pr_err_once("WARNING! The i915 invalidates all caches which increases the latency."); + pr_err_once("As a workaround use 'i915.do_wbinvd=no' and PIN each process doing "); + pr_err_once("any kind of GPU activity to the same CPU to avoid problems."); +#endif + on_each_cpu(i915_gem_write_fence__ipi, NULL, 1); + } + } i915_gem_write_fence(dev, fence_reg, enable ? obj : NULL); if (enable) { -- 1.8.3.1 > > -Carsten. Sebastian -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
