[PATCH 3/3] drm/nouveau/tegra: Keep the fuse clock enabled while the GPU is powered

[Sasha Levin <sashal@kernel.org>]

Since the GPU on the Jetson TX1 was enabled in commit d1b725476104
("arm64: tegra: p2180: Explicitly enable GPU"), nouveau often fails to
bring up the GM20B. Either BOOT_0 reads back garbage ("unknown chipset
(5ffefa47)"), or PRI reads of the GPC PLL time out ("MMIO read of
00000000 FAULT at 137000 [ TIMEOUT ]"), or the first register reads in
nvkm_device_ctor() stall until an asynchronous SError. How often depends
on the board; one board fails on every boot.

nouveau powers the GPU up with the fuse clock gated: since commit
24a15252ff04 ("soc/tegra: fuse: Add runtime PM support"), the Tegra fuse
driver only enables that clock around its own fuse reads. NVIDIA's
downstream nvgpu driver enables the fuse clock along with the other GPU
clocks whenever the GPU is powered. Do the same: take an optional "fuse"
clock and keep it enabled from before the GPU rail is turned on until
after it is turned off.

Tested on eight Jetson TX1 devkits, on mainline and on v6.18. Without
this patch, the GPU failed to come up in 27 of 109 boots; with it, in 11
of 291, none of them with a garbage BOOT_0. On the board that failed on
every boot, 38 of 40 boots now succeed. The remaining failures are
mostly a register read that stalls until an SError; wherever the console
shows when, nouveau had already finished probing. Such stalls also occur
without this patch and are a separate problem. The Jetson Nano, which
shares the Tegra210 GPU node, still works (24 of 24 boots).

Fixes: d1b725476104 ("arm64: tegra: p2180: Explicitly enable GPU")
Assisted-by: LLM
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/gpu/drm/nouveau/include/nvkm/core/tegra.h  |  1 +
 drivers/gpu/drm/nouveau/nvkm/engine/device/tegra.c | 13 +++++++++++++
 2 files changed, 14 insertions(+)

diff --git a/drivers/gpu/drm/nouveau/include/nvkm/core/tegra.h b/drivers/gpu/drm/nouveau/include/nvkm/core/tegra.h
index 6aaa30ef167f..0189043382d0 100644
--- a/drivers/gpu/drm/nouveau/include/nvkm/core/tegra.h
+++ b/drivers/gpu/drm/nouveau/include/nvkm/core/tegra.h
@@ -15,6 +15,7 @@ struct nvkm_device_tegra {
 	struct clk *clk;
 	struct clk *clk_ref;
 	struct clk *clk_pwr;
+	struct clk *clk_fuse;
 
 	struct regulator *vdd;
 
diff --git a/drivers/gpu/drm/nouveau/nvkm/engine/device/tegra.c b/drivers/gpu/drm/nouveau/nvkm/engine/device/tegra.c
index 3c8d0878891a..31f8eaa9d650 100644
--- a/drivers/gpu/drm/nouveau/nvkm/engine/device/tegra.c
+++ b/drivers/gpu/drm/nouveau/nvkm/engine/device/tegra.c
@@ -32,6 +32,10 @@ nvkm_device_tegra_power_up(struct nvkm_device_tegra *tdev)
 {
 	int ret;
 
+	ret = clk_prepare_enable(tdev->clk_fuse);
+	if (ret)
+		return ret;
+
 	if (tdev->vdd) {
 		ret = regulator_enable(tdev->vdd);
 		if (ret)
@@ -76,6 +80,7 @@ nvkm_device_tegra_power_up(struct nvkm_device_tegra *tdev)
 	if (tdev->vdd)
 		regulator_disable(tdev->vdd);
 err_power:
+	clk_disable_unprepare(tdev->clk_fuse);
 	return ret;
 }
 
@@ -95,6 +100,8 @@ nvkm_device_tegra_power_down(struct nvkm_device_tegra *tdev)
 			return ret;
 	}
 
+	clk_disable_unprepare(tdev->clk_fuse);
+
 	return 0;
 }
 
@@ -308,6 +315,12 @@ nvkm_device_tegra_new(const struct nvkm_device_tegra_func *func,
 		goto free;
 	}
 
+	tdev->clk_fuse = devm_clk_get_optional(&pdev->dev, "fuse");
+	if (IS_ERR(tdev->clk_fuse)) {
+		ret = PTR_ERR(tdev->clk_fuse);
+		goto free;
+	}
+
 	tdev->pmc = devm_tegra_pmc_get(&pdev->dev);
 	if (IS_ERR(tdev->pmc)) {
 		ret = PTR_ERR(tdev->pmc);
-- 
2.43.0