[PATCH v4 1/2] drm/nouveau/pci: use config-space MSI rearm on MCP79/MCP7A (NVAC)

[Marek Czernohous <mczernohous@gmail.com>]

From: Marek Czernohous <marek@czernohous.de>

NVAC (MCP79/MCP7A) uses g94_pci_func, whose .msi_rearm is
nv40_pci_msi_rearm(): a re-arm write through the MMIO mirror of PCI
config space.  On this IGP that path is unreliable; when a re-arm is
missed the interrupt line stays dead, command submission times out and
the GPU appears hung until reboot.  On an Apple Mac mini (early 2009,
MCP79, boot0 0x0ac080b1) this showed as sporadic fifo timeouts and GPU
hangs under load unless MSI was disabled via config=NvMSI=0.

Give NVAC its own pci func that re-arms through real PCI config space
(nv46_pci_msi_rearm) instead.  This follows existing precedent: nv46.c
documents the MMIO-mirror re-arm as broken on several related parts,
and commit 5112abc6a433 ("drm/nouveau/pci/g92: Fix rearm") fixed g92
the same way while moving the remaining chipsets, NVAC included, into
the newly added shared g94 table, where NVAC stayed on the MMIO path.
This change completes that fix for NVAC.  The sibling IGP NVAA
(MCP77/MCP78) has MSI disabled entirely as "reported broken" in
nvkm_pci_new_(); NVAC works correctly once the re-arm goes through
config space, so disabling MSI is not necessary.

Only NVAC is switched: that is the hardware this has been validated
on.  The other users of g94_pci_func (G94/G96/G98/GT2xx and the
MCP77/MCP89 IGPs) keep their current behavior; MCP77 and MCP89
plausibly want the same treatment but were not tested.

Tested on the Mac mini as a daily driver for two months with MSI
enabled and zero fifo timeouts.  Independently confirmed stable on an
iMac9,1 (MCP79) running 6.12.90 with the v1 form of this change (the
same one-line functional switch, applied to that kernel's g94
implementation).

Fixes: 5112abc6a433 ("drm/nouveau/pci/g92: Fix rearm")
Cc: <stable@vger.kernel.org> # v6.16+
Tested-by: Fab Stz <fabstz-it@yahoo.fr>
Assisted-by: Claude:claude-opus-4-7
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Marek Czernohous <marek@czernohous.de>
---
 .../gpu/drm/nouveau/include/nvkm/subdev/pci.h |  1 +
 .../gpu/drm/nouveau/nvkm/engine/device/base.c |  2 +-
 .../gpu/drm/nouveau/nvkm/subdev/pci/Kbuild    |  1 +
 .../gpu/drm/nouveau/nvkm/subdev/pci/mcp79.c   | 35 +++++++++++++++++++
 4 files changed, 38 insertions(+), 1 deletion(-)
 create mode 100644 drivers/gpu/drm/nouveau/nvkm/subdev/pci/mcp79.c

diff --git a/drivers/gpu/drm/nouveau/include/nvkm/subdev/pci.h b/drivers/gpu/drm/nouveau/include/nvkm/subdev/pci.h
index 112b674ed9c8..0172e0d200f8 100644
--- a/drivers/gpu/drm/nouveau/include/nvkm/subdev/pci.h
+++ b/drivers/gpu/drm/nouveau/include/nvkm/subdev/pci.h
@@ -46,6 +46,7 @@ int nv4c_pci_new(struct nvkm_device *, enum nvkm_subdev_type, int inst, struct n
 int g84_pci_new(struct nvkm_device *, enum nvkm_subdev_type, int inst, struct nvkm_pci **);
 int g92_pci_new(struct nvkm_device *, enum nvkm_subdev_type, int inst, struct nvkm_pci **);
 int g94_pci_new(struct nvkm_device *, enum nvkm_subdev_type, int inst, struct nvkm_pci **);
+int mcp79_pci_new(struct nvkm_device *, enum nvkm_subdev_type, int inst, struct nvkm_pci **);
 int gf100_pci_new(struct nvkm_device *, enum nvkm_subdev_type, int inst, struct nvkm_pci **);
 int gf106_pci_new(struct nvkm_device *, enum nvkm_subdev_type, int inst, struct nvkm_pci **);
 int gk104_pci_new(struct nvkm_device *, enum nvkm_subdev_type, int inst, struct nvkm_pci **);
diff --git a/drivers/gpu/drm/nouveau/nvkm/engine/device/base.c b/drivers/gpu/drm/nouveau/nvkm/engine/device/base.c
index ea62dc97f118..f7e02a16e5bc 100644
--- a/drivers/gpu/drm/nouveau/nvkm/engine/device/base.c
+++ b/drivers/gpu/drm/nouveau/nvkm/engine/device/base.c
@@ -1237,7 +1237,7 @@ nvac_chipset = {
 	.mc       = { 0x00000001, g98_mc_new },
 	.mmu      = { 0x00000001, mcp77_mmu_new },
 	.mxm      = { 0x00000001, nv50_mxm_new },
-	.pci      = { 0x00000001, g94_pci_new },
+	.pci      = { 0x00000001, mcp79_pci_new },
 	.therm    = { 0x00000001, g84_therm_new },
 	.timer    = { 0x00000001, nv41_timer_new },
 	.volt     = { 0x00000001, nv40_volt_new },
diff --git a/drivers/gpu/drm/nouveau/nvkm/subdev/pci/Kbuild b/drivers/gpu/drm/nouveau/nvkm/subdev/pci/Kbuild
index a14ea0f7b1c8..90f03baeb22a 100644
--- a/drivers/gpu/drm/nouveau/nvkm/subdev/pci/Kbuild
+++ b/drivers/gpu/drm/nouveau/nvkm/subdev/pci/Kbuild
@@ -9,6 +9,7 @@ nvkm-y += nvkm/subdev/pci/nv4c.o
 nvkm-y += nvkm/subdev/pci/g84.o
 nvkm-y += nvkm/subdev/pci/g92.o
 nvkm-y += nvkm/subdev/pci/g94.o
+nvkm-y += nvkm/subdev/pci/mcp79.o
 nvkm-y += nvkm/subdev/pci/gf100.o
 nvkm-y += nvkm/subdev/pci/gf106.o
 nvkm-y += nvkm/subdev/pci/gk104.o
diff --git a/drivers/gpu/drm/nouveau/nvkm/subdev/pci/mcp79.c b/drivers/gpu/drm/nouveau/nvkm/subdev/pci/mcp79.c
new file mode 100644
index 000000000000..e2ae242f1aa7
--- /dev/null
+++ b/drivers/gpu/drm/nouveau/nvkm/subdev/pci/mcp79.c
@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: MIT
+/*
+ * Copyright 2026 Marek Czernohous
+ *
+ * MCP79/MCP7A (NVAC): like g94, but MSI re-arm goes through real PCI
+ * config space.  The MMIO-mirror re-arm is unreliable on this IGP and a
+ * missed re-arm kills the interrupt line (see the nv46 comment; g92
+ * already re-arms through config space for the same reason).
+ */
+#include "priv.h"
+
+static const struct nvkm_pci_func
+mcp79_pci_func = {
+	.cfg = { .addr = 0x088000, .size = 0x1000 },
+
+	.init = g84_pci_init,
+	.msi_rearm = nv46_pci_msi_rearm,
+
+	.pcie.init = g84_pcie_init,
+	.pcie.set_link = g84_pcie_set_link,
+
+	.pcie.max_speed = g84_pcie_max_speed,
+	.pcie.cur_speed = g84_pcie_cur_speed,
+
+	.pcie.set_version = g84_pcie_set_version,
+	.pcie.version = g84_pcie_version,
+	.pcie.version_supported = g92_pcie_version_supported,
+};
+
+int
+mcp79_pci_new(struct nvkm_device *device, enum nvkm_subdev_type type, int inst,
+	      struct nvkm_pci **ppci)
+{
+	return nvkm_pci_new_(&mcp79_pci_func, device, type, inst, ppci);
+}
-- 
2.54.0