i2c-imc-skylake: retract the CF8/CFC claim, harden the state machine

v2 said firmware filters CF8/CFC config writes to this function and that
the registers are reachable only through ECAM.  That was never tested; the
only evidence was the boot line "PCI: Using configuration type 1", which
names the accessor and says nothing about a write failing.

Measured on the board, driver unloaded:

  # setpci -s 16:1e.5 9c.L=20085002    # read SPD 0x50 register 0x02
  # setpci -s 16:1e.5 9c.L
  20005002                             # GO consumed by the engine
  # setpci -s 16:1e.5 a8.L
  0000500c                             # READ_DONE, no error
  # setpci -s 16:1e.5 b4.L
  0000000c                             # SPD byte 2, DDR4 type code

The command executes and returns correct data through the ordinary
accessors, and MSR_SMI_COUNT does not move across it.  So the driver now
uses pci_read_config_dword() and pci_write_config_dword(), the submission
is a single patch again, and nothing in it touches the PCI core.  The
cover letter retracts the claim explicitly rather than dropping it
quietly, because v2 was posted with it.

Correctness work in the same pass:

  - the completion bits are latched, so a command word that never reaches
    the register is indistinguishable from a completed transfer and the
    caller gets the previous transaction's data.  Read the command word
    back and require it to be the one just written; the engine only ever
    clears GO, which the measurement above shows directly;
  - compare the command registers after every transfer against what the
    driver left in them and fail with -EAGAIN on a mismatch.  The original
    iMC driver had this check and it is what makes an unarbitrated system
    detectable instead of silently wrong;
  - use the i2c core's suspended-adapter mechanism rather than a private
    flag and -EHOSTDOWN;
  - use devm_mutex_init(), drop the unnecessary pci_enable_device(), and
    return -ENODEV rather than -EPERM from the default-off gate, which was
    printing a driver-core error on every boot;
  - mask GO when restoring a saved command word.

An earlier version of the latched-DONE check required the GO bit to be
seen set or the status word to change.  That was wrong: repeated transfers
with the same outcome leave STATUS bit for bit identical, and GO is
consumed faster than a config read-back can observe it.  It produced false
positives on 3 of 15 smoke-test items before being replaced.

Add Documentation/i2c/busses/i2c-imc-skylake.rst, which every other PC
SMBus driver has and which was asked for on the 2020 iMC thread, and the
tools/ scripts for the hardware measurements: probe-cf8-write.sh and
probe-cf8-transaction.sh produced the result above, dump-pcu-config.sh is
the search for a TSOD polling control, watch-engine.sh is the firmware
activity measurement.

Smoke test on the X299 system with the current module: 15 checks, 0
failures, dmesg clean.

.github/workflows/ci.yml

@@ -43,8 +43,6 @@ jobs:
           sudo apt-get update
           sudo apt-get install -y build-essential clang llvm linux-headers-$(uname -r)
 
-      # A stock kernel has no pci_mmcfg_read_config() yet, so this exercises
-      # the compat/ shim path. The in-tree job below covers the real thing.
       - name: Build module
         run: make W=1 KCFLAGS=-W ${{ matrix.make_args }}
 
@@ -69,7 +67,7 @@ jobs:
           if-no-files-found: error
 
   in-tree:
-    name: Current i2c-host integration (2-patch series)
+    name: Current i2c-host integration
     runs-on: ubuntu-latest
     timeout-minutes: 45
 
@@ -91,12 +89,6 @@ jobs:
       - name: Prepare current maintainer tree
         run: |
           git clone --depth=1 --branch i2c/i2c-host "$I2C_TREE" linux-i2c
-          # Patch 1/2: the PCI/x86 accessor the driver depends on. If this
-          # stops applying, the tracked copy is stale and must be refreshed
-          # from the kernel tree - that is a real failure, not CI noise.
-          git -C linux-i2c apply --verbose \
-            ../docs/submission/0001-pci-mmcfg-accessor.patch
-          # Patch 2/2: the driver itself.
           cp i2c-imc-skylake.c linux-i2c/drivers/i2c/busses/
           cat docs/submission/Kconfig.kernel >> linux-i2c/drivers/i2c/busses/Kconfig
           printf '%s\n' 'obj-$(CONFIG_I2C_IMC_SKYLAKE) += i2c-imc-skylake.o' \
@@ -110,15 +102,10 @@ jobs:
           scripts/config --file ../linux-out/.config --enable I2C --module I2C_IMC_SKYLAKE
           make O=../linux-out olddefconfig
           grep '^CONFIG_I2C_IMC_SKYLAKE=m' ../linux-out/.config
-          grep '^CONFIG_PCI_MMCONFIG=y' ../linux-out/.config
 
-      - name: Build the series in-tree
+      - name: Build driver in-tree
         working-directory: linux-i2c
-        run: |
-          make O=../linux-out W=1 \
-            drivers/pci/access.o \
-            arch/x86/pci/mmconfig-shared.o \
-            drivers/i2c/busses/i2c-imc-skylake.o
+        run: make O=../linux-out W=1 drivers/i2c/busses/i2c-imc-skylake.o
 
       - name: Build arm64 COMPILE_TEST with Clang
         working-directory: linux-i2c
@@ -130,15 +117,12 @@ jobs:
           make O=../linux-out-arm64 ARCH=arm64 LLVM=1 olddefconfig
           grep '^CONFIG_I2C_IMC_SKYLAKE=m' ../linux-out-arm64/.config
           make O=../linux-out-arm64 ARCH=arm64 LLVM=1 W=1 \
-            drivers/pci/access.o \
             drivers/i2c/busses/i2c-imc-skylake.o
 
       - name: Run Sparse in-tree
         working-directory: linux-i2c
         run: |
           make O=../linux-out C=2 CF=-D__CHECK_ENDIAN__ \
-            drivers/pci/access.o \
-            arch/x86/pci/mmconfig-shared.o \
             drivers/i2c/busses/i2c-imc-skylake.o
 
       - name: Run current-tree checkpatch
@@ -146,8 +130,6 @@ jobs:
         run: |
           scripts/checkpatch.pl --strict --no-tree -f \
             drivers/i2c/busses/i2c-imc-skylake.c
-          scripts/checkpatch.pl --strict \
-            ../docs/submission/0001-pci-mmcfg-accessor.patch
 
       - name: Build the documentation
         working-directory: linux-i2c