Re: [PATCH] drm/msm/dpu: Add missing safe_lut_tbl in sc8280xp catalog

[Johan Hovold <johan@kernel.org>]

On Mon, Oct 30, 2023 at 04:23:20PM -0700, Bjorn Andersson wrote:
> During USB transfers on the SC8280XP __arm_smmu_tlb_sync() is seen to
> typically take 1-2ms to complete. As expected this results in poor
> performance, something that has been mitigated by proposing running the
> iommu in non-strict mode (boot with iommu.strict=0).
> 
> This turns out to be related to the SAFE logic, and programming the QOS
> SAFE values in the DPU (per suggestion from Rob and Doug) reduces the
> TLB sync time to below 10us, which means significant less time spent
> with interrupts disabled and a significant boost in throughput.

I ran some tests with a gigabit ethernet adapter to get an idea of how
this performs in comparison to using lazy iommu mode ("non-strict"):

		6.6	6.6-lazy	6.6-dpu		6.6-dpu-lazy
iperf3 recv	114	941		941		941		MBit/s
iperf3 send	124	891		703		940		MBit/s

scp recv	14.6	110		110		111		MB/s
scp send	12.5	98.9		91.5		110		MB/s

This patch in itself indeed improves things quite a bit, but there is
still some performance that can be gained by using lazy iommu mode.

Notably, lazy mode with this patch applied appears to saturate the link
in both directions.

Tested-by: Johan Hovold <johan+linaro@kernel.org>

Johan