[Regression 5.5-rc1] Extremely low GPU performance on NVIDIA Tegra20/30

[Dmitry Osipenko]

Hello Thierry,

Commit [1] introduced a severe GPU performance regression on Tegra20 and
Tegra30 using.

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v5.5-rc1&id=fa6661b7aa0b52073681b0d26742650c8cbd30f3

Interestingly the performance is okay on Tegra30 if
CONFIG_TEGRA_HOST1X_FIREWALL=n, but that doesn't make difference for
Tegra20.

I was telling you about this problem on the #tegra IRC sometime ago and
you asked to report it in a trackable form, so finally here it is.

You could reproduce the problem by running [2] like this
`grate/texture-filter -f -s` which should produce over 100 FPS for 720p
display resolution and currently it's ~11 FPS.

[2]
https://github.com/grate-driver/grate/blob/master/tests/grate/texture-filter.c

Previously I was seeing some memory errors coming from Host1x DMA, but
don't see any errors at all right now.

I don't see anything done horribly wrong in the offending commit.

Unfortunately I couldn't dedicate enough time to sit down and debug the
problem thoroughly yet. Please let me know if you'll find a solution,
I'll be happy to test it. Thanks in advance!