From patchwork Tue Jan 25 19:35:27 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Bob Beckett X-Patchwork-Id: 12724188 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id DDFD6C433EF for ; Tue, 25 Jan 2022 19:36:02 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 05F7E10E451; Tue, 25 Jan 2022 19:35:57 +0000 (UTC) Received: from bhuna.collabora.co.uk (bhuna.collabora.co.uk [46.235.227.227]) by gabe.freedesktop.org (Postfix) with ESMTPS id 7951210E3D6; Tue, 25 Jan 2022 19:35:54 +0000 (UTC) Received: from [127.0.0.1] (localhost [127.0.0.1]) (Authenticated sender: bbeckett) with ESMTPSA id 04CD61F44558 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=collabora.com; s=mail; t=1643139353; bh=uZe7NMt13FHSNWptdo6+nwfikNF+rS2wC9Rvf5n52VU=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=n/+m2YsjJEZ58YEXfIwdlDvDWIgw6+NRbI/9i9nxsip/ZTyN3W+Ii1H9aZd5EZDCV 7wv5ghsXfua407TSbeHNLUAexFIwT+gFmNnw9F+UaSwFlQQvqljdvtLOXuNWYgrVeZ BZBXX6v2qvJyYCy+PH+6ZTo1pb2nVa6DlsNiIBk+ij+NAu5JixO6F6z5pXg04pP79P xkNV5iO4h1JoT4bE7DXI5wMbNYuNoYZlWs2NrMmavZmfQWI0OuPb+vZwEBnJ22LwlB Dj8ZBYlRNzZ89su8PJy6ACY4FJOvUfup4GiFdU4fGRtkXQ1YXUE8EO87RDEZaUfNP/ kzEd6Yv5gWGqQ== From: Robert Beckett To: Jani Nikula , Joonas Lahtinen , Rodrigo Vivi , Tvrtko Ursulin , David Airlie , Daniel Vetter Date: Tue, 25 Jan 2022 19:35:27 +0000 Message-Id: <20220125193530.3272386-3-bob.beckett@collabora.com> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20220125193530.3272386-1-bob.beckett@collabora.com> References: <20220125193530.3272386-1-bob.beckett@collabora.com> MIME-Version: 1.0 Subject: [Intel-gfx] [PATCH v5 2/5] drm/i915: enforce min GTT alignment for discrete cards X-BeenThere: intel-gfx@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Intel graphics driver community testing & development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: intel-gfx@lists.freedesktop.org, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, Matthew Auld Errors-To: intel-gfx-bounces@lists.freedesktop.org Sender: "Intel-gfx" From: Matthew Auld For local-memory objects we need to align the GTT addresses to 64K, both for the ppgtt and ggtt. We need to support vm->min_alignment > 4K, depending on the vm itself and the type of object we are inserting. With this in mind update the GTT selftests to take this into account. For compact-pt we further align and pad lmem object GTT addresses to 2MB to ensure PDEs contain consistent page sizes as required by the HW. v3: * use needs_compact_pt flag to discriminate between 64K and 64K with compact-pt * add i915_vm_obj_min_alignment * use i915_vm_obj_min_alignment to round up vma reservation if compact-pt instead of hard coding v5: * fix i915_vm_obj_min_alignment for internal objects which have no memory region Signed-off-by: Matthew Auld Signed-off-by: Ramalingam C Signed-off-by: Robert Beckett Cc: Joonas Lahtinen Cc: Rodrigo Vivi --- .../i915/gem/selftests/i915_gem_client_blt.c | 23 +++-- drivers/gpu/drm/i915/gt/intel_gtt.c | 12 +++ drivers/gpu/drm/i915/gt/intel_gtt.h | 18 ++++ drivers/gpu/drm/i915/i915_vma.c | 9 ++ drivers/gpu/drm/i915/selftests/i915_gem_gtt.c | 96 ++++++++++++------- 5 files changed, 117 insertions(+), 41 deletions(-) diff --git a/drivers/gpu/drm/i915/gem/selftests/i915_gem_client_blt.c b/drivers/gpu/drm/i915/gem/selftests/i915_gem_client_blt.c index c8ff8bf0986d..f0bfce53258f 100644 --- a/drivers/gpu/drm/i915/gem/selftests/i915_gem_client_blt.c +++ b/drivers/gpu/drm/i915/gem/selftests/i915_gem_client_blt.c @@ -39,6 +39,7 @@ struct tiled_blits { struct blit_buffer scratch; struct i915_vma *batch; u64 hole; + u64 align; u32 width; u32 height; }; @@ -410,14 +411,21 @@ tiled_blits_create(struct intel_engine_cs *engine, struct rnd_state *prng) goto err_free; } - hole_size = 2 * PAGE_ALIGN(WIDTH * HEIGHT * 4); + t->align = I915_GTT_PAGE_SIZE_2M; /* XXX worst case, derive from vm! */ + t->align = max(t->align, + i915_vm_min_alignment(t->ce->vm, INTEL_MEMORY_LOCAL)); + t->align = max(t->align, + i915_vm_min_alignment(t->ce->vm, INTEL_MEMORY_SYSTEM)); + + hole_size = 2 * round_up(WIDTH * HEIGHT * 4, t->align); hole_size *= 2; /* room to maneuver */ - hole_size += 2 * I915_GTT_MIN_ALIGNMENT; + hole_size += 2 * t->align; /* padding on either side */ mutex_lock(&t->ce->vm->mutex); memset(&hole, 0, sizeof(hole)); err = drm_mm_insert_node_in_range(&t->ce->vm->mm, &hole, - hole_size, 0, I915_COLOR_UNEVICTABLE, + hole_size, t->align, + I915_COLOR_UNEVICTABLE, 0, U64_MAX, DRM_MM_INSERT_BEST); if (!err) @@ -428,7 +436,7 @@ tiled_blits_create(struct intel_engine_cs *engine, struct rnd_state *prng) goto err_put; } - t->hole = hole.start + I915_GTT_MIN_ALIGNMENT; + t->hole = hole.start + t->align; pr_info("Using hole at %llx\n", t->hole); err = tiled_blits_create_buffers(t, WIDTH, HEIGHT, prng); @@ -455,7 +463,7 @@ static void tiled_blits_destroy(struct tiled_blits *t) static int tiled_blits_prepare(struct tiled_blits *t, struct rnd_state *prng) { - u64 offset = PAGE_ALIGN(t->width * t->height * 4); + u64 offset = round_up(t->width * t->height * 4, t->align); u32 *map; int err; int i; @@ -486,8 +494,7 @@ static int tiled_blits_prepare(struct tiled_blits *t, static int tiled_blits_bounce(struct tiled_blits *t, struct rnd_state *prng) { - u64 offset = - round_up(t->width * t->height * 4, 2 * I915_GTT_MIN_ALIGNMENT); + u64 offset = round_up(t->width * t->height * 4, 2 * t->align); int err; /* We want to check position invariant tiling across GTT eviction */ @@ -500,7 +507,7 @@ static int tiled_blits_bounce(struct tiled_blits *t, struct rnd_state *prng) /* Reposition so that we overlap the old addresses, and slightly off */ err = tiled_blit(t, - &t->buffers[2], t->hole + I915_GTT_MIN_ALIGNMENT, + &t->buffers[2], t->hole + t->align, &t->buffers[1], t->hole + 3 * offset / 2); if (err) return err; diff --git a/drivers/gpu/drm/i915/gt/intel_gtt.c b/drivers/gpu/drm/i915/gt/intel_gtt.c index 46be4197b93f..df23ebdfc994 100644 --- a/drivers/gpu/drm/i915/gt/intel_gtt.c +++ b/drivers/gpu/drm/i915/gt/intel_gtt.c @@ -223,6 +223,18 @@ void i915_address_space_init(struct i915_address_space *vm, int subclass) GEM_BUG_ON(!vm->total); drm_mm_init(&vm->mm, 0, vm->total); + + memset64(vm->min_alignment, I915_GTT_MIN_ALIGNMENT, + ARRAY_SIZE(vm->min_alignment)); + + if (HAS_64K_PAGES(vm->i915) && NEEDS_COMPACT_PT(vm->i915)) { + vm->min_alignment[INTEL_MEMORY_LOCAL] = I915_GTT_PAGE_SIZE_2M; + vm->min_alignment[INTEL_MEMORY_STOLEN_LOCAL] = I915_GTT_PAGE_SIZE_2M; + } else if (HAS_64K_PAGES(vm->i915)) { + vm->min_alignment[INTEL_MEMORY_LOCAL] = I915_GTT_PAGE_SIZE_64K; + vm->min_alignment[INTEL_MEMORY_STOLEN_LOCAL] = I915_GTT_PAGE_SIZE_64K; + } + vm->mm.head_node.color = I915_COLOR_UNEVICTABLE; INIT_LIST_HEAD(&vm->bound_list); diff --git a/drivers/gpu/drm/i915/gt/intel_gtt.h b/drivers/gpu/drm/i915/gt/intel_gtt.h index 8073438b67c8..ba9f040f8606 100644 --- a/drivers/gpu/drm/i915/gt/intel_gtt.h +++ b/drivers/gpu/drm/i915/gt/intel_gtt.h @@ -29,6 +29,8 @@ #include "i915_selftest.h" #include "i915_vma_resource.h" #include "i915_vma_types.h" +#include "i915_params.h" +#include "intel_memory_region.h" #define I915_GFP_ALLOW_FAIL (GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOWARN) @@ -223,6 +225,7 @@ struct i915_address_space { struct device *dma; u64 total; /* size addr space maps (ex. 2GB for ggtt) */ u64 reserved; /* size addr space reserved */ + u64 min_alignment[INTEL_MEMORY_STOLEN_LOCAL + 1]; unsigned int bind_async_flags; @@ -384,6 +387,21 @@ i915_vm_has_scratch_64K(struct i915_address_space *vm) return vm->scratch_order == get_order(I915_GTT_PAGE_SIZE_64K); } +static inline u64 i915_vm_min_alignment(struct i915_address_space *vm, + enum intel_memory_type type) +{ + return vm->min_alignment[type]; +} + +static inline u64 i915_vm_obj_min_alignment(struct i915_address_space *vm, + struct drm_i915_gem_object *obj) +{ + struct intel_memory_region *mr = READ_ONCE(obj->mm.region); + enum intel_memory_type type = mr ? mr->type : INTEL_MEMORY_SYSTEM; + + return i915_vm_min_alignment(vm, type); +} + static inline bool i915_vm_has_cache_coloring(struct i915_address_space *vm) { diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c index b959e904c4d3..3b31d2ba6864 100644 --- a/drivers/gpu/drm/i915/i915_vma.c +++ b/drivers/gpu/drm/i915/i915_vma.c @@ -745,6 +745,14 @@ i915_vma_insert(struct i915_vma *vma, struct i915_gem_ww_ctx *ww, end = min_t(u64, end, (1ULL << 32) - I915_GTT_PAGE_SIZE); GEM_BUG_ON(!IS_ALIGNED(end, I915_GTT_PAGE_SIZE)); + alignment = max(alignment, i915_vm_obj_min_alignment(vma->vm, vma->obj)); + /* + * for compact-pt we round up the reservation to prevent + * any smaller pages being used within the same PDE + */ + if (NEEDS_COMPACT_PT(vma->vm->i915)) + size = round_up(size, alignment); + /* If binding the object/GGTT view requires more space than the entire * aperture has, reject it early before evicting everything in a vain * attempt to find space. @@ -757,6 +765,7 @@ i915_vma_insert(struct i915_vma *vma, struct i915_gem_ww_ctx *ww, } color = 0; + if (i915_vm_has_cache_coloring(vma->vm)) color = vma->obj->cache_level; diff --git a/drivers/gpu/drm/i915/selftests/i915_gem_gtt.c b/drivers/gpu/drm/i915/selftests/i915_gem_gtt.c index fba1c8be1649..b80788a2b7f9 100644 --- a/drivers/gpu/drm/i915/selftests/i915_gem_gtt.c +++ b/drivers/gpu/drm/i915/selftests/i915_gem_gtt.c @@ -238,6 +238,8 @@ static int lowlevel_hole(struct i915_address_space *vm, u64 hole_start, u64 hole_end, unsigned long end_time) { + const unsigned int min_alignment = + i915_vm_min_alignment(vm, INTEL_MEMORY_SYSTEM); I915_RND_STATE(seed_prng); struct i915_vma_resource *mock_vma_res; unsigned int size; @@ -251,9 +253,10 @@ static int lowlevel_hole(struct i915_address_space *vm, I915_RND_SUBSTATE(prng, seed_prng); struct drm_i915_gem_object *obj; unsigned int *order, count, n; - u64 hole_size; + u64 hole_size, aligned_size; - hole_size = (hole_end - hole_start) >> size; + aligned_size = max_t(u32, ilog2(min_alignment), size); + hole_size = (hole_end - hole_start) >> aligned_size; if (hole_size > KMALLOC_MAX_SIZE / sizeof(u32)) hole_size = KMALLOC_MAX_SIZE / sizeof(u32); count = hole_size >> 1; @@ -274,8 +277,8 @@ static int lowlevel_hole(struct i915_address_space *vm, } GEM_BUG_ON(!order); - GEM_BUG_ON(count * BIT_ULL(size) > vm->total); - GEM_BUG_ON(hole_start + count * BIT_ULL(size) > hole_end); + GEM_BUG_ON(count * BIT_ULL(aligned_size) > vm->total); + GEM_BUG_ON(hole_start + count * BIT_ULL(aligned_size) > hole_end); /* Ignore allocation failures (i.e. don't report them as * a test failure) as we are purposefully allocating very @@ -298,10 +301,10 @@ static int lowlevel_hole(struct i915_address_space *vm, } for (n = 0; n < count; n++) { - u64 addr = hole_start + order[n] * BIT_ULL(size); + u64 addr = hole_start + order[n] * BIT_ULL(aligned_size); intel_wakeref_t wakeref; - GEM_BUG_ON(addr + BIT_ULL(size) > vm->total); + GEM_BUG_ON(addr + BIT_ULL(aligned_size) > vm->total); if (igt_timeout(end_time, "%s timed out before %d/%d\n", @@ -344,7 +347,7 @@ static int lowlevel_hole(struct i915_address_space *vm, } mock_vma_res->bi.pages = obj->mm.pages; - mock_vma_res->node_size = BIT_ULL(size); + mock_vma_res->node_size = BIT_ULL(aligned_size); mock_vma_res->start = addr; with_intel_runtime_pm(vm->gt->uncore->rpm, wakeref) @@ -355,7 +358,7 @@ static int lowlevel_hole(struct i915_address_space *vm, i915_random_reorder(order, count, &prng); for (n = 0; n < count; n++) { - u64 addr = hole_start + order[n] * BIT_ULL(size); + u64 addr = hole_start + order[n] * BIT_ULL(aligned_size); intel_wakeref_t wakeref; GEM_BUG_ON(addr + BIT_ULL(size) > vm->total); @@ -399,8 +402,10 @@ static int fill_hole(struct i915_address_space *vm, { const u64 hole_size = hole_end - hole_start; struct drm_i915_gem_object *obj; + const unsigned int min_alignment = + i915_vm_min_alignment(vm, INTEL_MEMORY_SYSTEM); const unsigned long max_pages = - min_t(u64, ULONG_MAX - 1, hole_size/2 >> PAGE_SHIFT); + min_t(u64, ULONG_MAX - 1, (hole_size / 2) >> ilog2(min_alignment)); const unsigned long max_step = max(int_sqrt(max_pages), 2UL); unsigned long npages, prime, flags; struct i915_vma *vma; @@ -441,14 +446,17 @@ static int fill_hole(struct i915_address_space *vm, offset = p->offset; list_for_each_entry(obj, &objects, st_link) { + u64 aligned_size = round_up(obj->base.size, + min_alignment); + vma = i915_vma_instance(obj, vm, NULL); if (IS_ERR(vma)) continue; if (p->step < 0) { - if (offset < hole_start + obj->base.size) + if (offset < hole_start + aligned_size) break; - offset -= obj->base.size; + offset -= aligned_size; } err = i915_vma_pin(vma, 0, 0, offset | flags); @@ -470,22 +478,25 @@ static int fill_hole(struct i915_address_space *vm, i915_vma_unpin(vma); if (p->step > 0) { - if (offset + obj->base.size > hole_end) + if (offset + aligned_size > hole_end) break; - offset += obj->base.size; + offset += aligned_size; } } offset = p->offset; list_for_each_entry(obj, &objects, st_link) { + u64 aligned_size = round_up(obj->base.size, + min_alignment); + vma = i915_vma_instance(obj, vm, NULL); if (IS_ERR(vma)) continue; if (p->step < 0) { - if (offset < hole_start + obj->base.size) + if (offset < hole_start + aligned_size) break; - offset -= obj->base.size; + offset -= aligned_size; } if (!drm_mm_node_allocated(&vma->node) || @@ -506,22 +517,25 @@ static int fill_hole(struct i915_address_space *vm, } if (p->step > 0) { - if (offset + obj->base.size > hole_end) + if (offset + aligned_size > hole_end) break; - offset += obj->base.size; + offset += aligned_size; } } offset = p->offset; list_for_each_entry_reverse(obj, &objects, st_link) { + u64 aligned_size = round_up(obj->base.size, + min_alignment); + vma = i915_vma_instance(obj, vm, NULL); if (IS_ERR(vma)) continue; if (p->step < 0) { - if (offset < hole_start + obj->base.size) + if (offset < hole_start + aligned_size) break; - offset -= obj->base.size; + offset -= aligned_size; } err = i915_vma_pin(vma, 0, 0, offset | flags); @@ -543,22 +557,25 @@ static int fill_hole(struct i915_address_space *vm, i915_vma_unpin(vma); if (p->step > 0) { - if (offset + obj->base.size > hole_end) + if (offset + aligned_size > hole_end) break; - offset += obj->base.size; + offset += aligned_size; } } offset = p->offset; list_for_each_entry_reverse(obj, &objects, st_link) { + u64 aligned_size = round_up(obj->base.size, + min_alignment); + vma = i915_vma_instance(obj, vm, NULL); if (IS_ERR(vma)) continue; if (p->step < 0) { - if (offset < hole_start + obj->base.size) + if (offset < hole_start + aligned_size) break; - offset -= obj->base.size; + offset -= aligned_size; } if (!drm_mm_node_allocated(&vma->node) || @@ -579,9 +596,9 @@ static int fill_hole(struct i915_address_space *vm, } if (p->step > 0) { - if (offset + obj->base.size > hole_end) + if (offset + aligned_size > hole_end) break; - offset += obj->base.size; + offset += aligned_size; } } } @@ -611,6 +628,7 @@ static int walk_hole(struct i915_address_space *vm, const u64 hole_size = hole_end - hole_start; const unsigned long max_pages = min_t(u64, ULONG_MAX - 1, hole_size >> PAGE_SHIFT); + unsigned long min_alignment; unsigned long flags; u64 size; @@ -620,6 +638,8 @@ static int walk_hole(struct i915_address_space *vm, if (i915_is_ggtt(vm)) flags |= PIN_GLOBAL; + min_alignment = i915_vm_min_alignment(vm, INTEL_MEMORY_SYSTEM); + for_each_prime_number_from(size, 1, max_pages) { struct drm_i915_gem_object *obj; struct i915_vma *vma; @@ -638,7 +658,7 @@ static int walk_hole(struct i915_address_space *vm, for (addr = hole_start; addr + obj->base.size < hole_end; - addr += obj->base.size) { + addr += round_up(obj->base.size, min_alignment)) { err = i915_vma_pin(vma, 0, 0, addr | flags); if (err) { pr_err("%s bind failed at %llx + %llx [hole %llx- %llx] with err=%d\n", @@ -690,6 +710,7 @@ static int pot_hole(struct i915_address_space *vm, { struct drm_i915_gem_object *obj; struct i915_vma *vma; + unsigned int min_alignment; unsigned long flags; unsigned int pot; int err = 0; @@ -698,6 +719,8 @@ static int pot_hole(struct i915_address_space *vm, if (i915_is_ggtt(vm)) flags |= PIN_GLOBAL; + min_alignment = i915_vm_min_alignment(vm, INTEL_MEMORY_SYSTEM); + obj = i915_gem_object_create_internal(vm->i915, 2 * I915_GTT_PAGE_SIZE); if (IS_ERR(obj)) return PTR_ERR(obj); @@ -710,13 +733,13 @@ static int pot_hole(struct i915_address_space *vm, /* Insert a pair of pages across every pot boundary within the hole */ for (pot = fls64(hole_end - 1) - 1; - pot > ilog2(2 * I915_GTT_PAGE_SIZE); + pot > ilog2(2 * min_alignment); pot--) { u64 step = BIT_ULL(pot); u64 addr; - for (addr = round_up(hole_start + I915_GTT_PAGE_SIZE, step) - I915_GTT_PAGE_SIZE; - addr <= round_down(hole_end - 2*I915_GTT_PAGE_SIZE, step) - I915_GTT_PAGE_SIZE; + for (addr = round_up(hole_start + min_alignment, step) - min_alignment; + addr <= round_down(hole_end - (2 * min_alignment), step) - min_alignment; addr += step) { err = i915_vma_pin(vma, 0, 0, addr | flags); if (err) { @@ -761,6 +784,7 @@ static int drunk_hole(struct i915_address_space *vm, unsigned long end_time) { I915_RND_STATE(prng); + unsigned int min_alignment; unsigned int size; unsigned long flags; @@ -768,15 +792,18 @@ static int drunk_hole(struct i915_address_space *vm, if (i915_is_ggtt(vm)) flags |= PIN_GLOBAL; + min_alignment = i915_vm_min_alignment(vm, INTEL_MEMORY_SYSTEM); + /* Keep creating larger objects until one cannot fit into the hole */ for (size = 12; (hole_end - hole_start) >> size; size++) { struct drm_i915_gem_object *obj; unsigned int *order, count, n; struct i915_vma *vma; - u64 hole_size; + u64 hole_size, aligned_size; int err = -ENODEV; - hole_size = (hole_end - hole_start) >> size; + aligned_size = max_t(u32, ilog2(min_alignment), size); + hole_size = (hole_end - hole_start) >> aligned_size; if (hole_size > KMALLOC_MAX_SIZE / sizeof(u32)) hole_size = KMALLOC_MAX_SIZE / sizeof(u32); count = hole_size >> 1; @@ -816,7 +843,7 @@ static int drunk_hole(struct i915_address_space *vm, GEM_BUG_ON(vma->size != BIT_ULL(size)); for (n = 0; n < count; n++) { - u64 addr = hole_start + order[n] * BIT_ULL(size); + u64 addr = hole_start + order[n] * BIT_ULL(aligned_size); err = i915_vma_pin(vma, 0, 0, addr | flags); if (err) { @@ -868,11 +895,14 @@ static int __shrink_hole(struct i915_address_space *vm, { struct drm_i915_gem_object *obj; unsigned long flags = PIN_OFFSET_FIXED | PIN_USER; + unsigned int min_alignment; unsigned int order = 12; LIST_HEAD(objects); int err = 0; u64 addr; + min_alignment = i915_vm_min_alignment(vm, INTEL_MEMORY_SYSTEM); + /* Keep creating larger objects until one cannot fit into the hole */ for (addr = hole_start; addr < hole_end; ) { struct i915_vma *vma; @@ -913,7 +943,7 @@ static int __shrink_hole(struct i915_address_space *vm, } i915_vma_unpin(vma); - addr += size; + addr += round_up(size, min_alignment); /* * Since we are injecting allocation faults at random intervals,