Currently, we don't call dma_set_max_seg_size() for i915 because we intentionally do not limit the segment length that the device supports. However, this results in a warning being emitted if we try to map anything larger than SZ_64K on a kernel with CONFIG_DMA_API_DEBUG_SG enabled:
[ 7.751926] DMA-API: i915 0000:00:02.0: mapping sg segment longer than device claims to support [len=98304] [max=65536] [ 7.751934] WARNING: CPU: 5 PID: 474 at kernel/dma/debug.c:1220 debug_dma_map_sg+0x20f/0x340
This was originally brought up on https://bugs.freedesktop.org/show_bug.cgi?id=108517 , and the consensus there was it wasn't really useful to set a limit (and that dma-debug isn't really all that useful for i915 in the first place). Unfortunately though, CONFIG_DMA_API_DEBUG_SG is enabled in the debug configs for various distro kernels. Since a WARN_ON() will disable automatic problem reporting (and cause any CI with said option enabled to start complaining), we really should just fix the problem.
Note that as me and Chris Wilson discussed, the other solution for this would be to make DMA-API not make such assumptions when a driver hasn't explicitly set a maximum segment size. But, taking a look at the commit which originally introduced this behavior, commit 78c47830a5cb ("dma-debug: check scatterlist segments"), there is an explicit mention of this assumption and how it applies to devices with no segment size:
Conversely, devices which are less limited than the rather conservative defaults, or indeed have no limitations at all (e.g. GPUs with their own internal MMU), should be encouraged to set appropriate dma_parms, as they may get more efficient DMA mapping performance out of it.
So unless there's any concerns (I'm open to discussion!), let's just follow suite and call dma_set_max_seg_size() with UINT_MAX as our limit to silence any warnings.
Signed-off-by: Lyude Paul lyude@redhat.com Cc: Chris Wilson chris@chris-wilson.co.uk Cc: stable@vger.kernel.org # v4.18+ --- drivers/gpu/drm/i915/i915_gem_gtt.c | 5 +++++ 1 file changed, 5 insertions(+)
diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c index 0b81e0b64393..a1475039d182 100644 --- a/drivers/gpu/drm/i915/i915_gem_gtt.c +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c @@ -3152,6 +3152,11 @@ static int ggtt_probe_hw(struct i915_ggtt *ggtt, struct intel_gt *gt) if (ret) return ret;
+ /* We don't have a max segment size, so set it to the max so sg's + * debugging layer doesn't complain + */ + dma_set_max_seg_size(ggtt->vm.dma, UINT_MAX); + if ((ggtt->vm.total - 1) >> 32) { DRM_ERROR("We never expected a Global GTT with more than 32bits" " of address space! Found %lldM!\n",