docs: Add support for TPM devices over I2C bus

This is a documentation change for I2C TPM device support.

Qemu already supports devices attached to ISA and sysbus.
This drop adds support for the I2C bus attached TPM devices.

Signed-off-by: Ninad Palsule <ninad@linux.ibm.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Message-id: 20230414220754.1191476-2-ninadpalsule@us.ibm.com

docs/specs/tpm.rst

@@ -21,12 +21,16 @@ QEMU files related to TPM TIS interface:
  - ``hw/tpm/tpm_tis_common.c``
  - ``hw/tpm/tpm_tis_isa.c``
  - ``hw/tpm/tpm_tis_sysbus.c``
+ - ``hw/tpm/tpm_tis_i2c.c``
  - ``hw/tpm/tpm_tis.h``
 
 Both an ISA device and a sysbus device are available. The former is
 used with pc/q35 machine while the latter can be instantiated in the
 Arm virt machine.
 
+An I2C device support is also provided which can be instantiated in the Arm
+based emulation machines. This device only supports the TPM 2 protocol.
+
 CRB interface
 -------------
 
@@ -348,6 +352,23 @@ In case an Arm virt machine is emulated, use the following command line:
     -drive if=pflash,format=raw,file=flash0.img,readonly=on \
     -drive if=pflash,format=raw,file=flash1.img
 
+In case a ast2600-evb bmc machine is emulated and you want to use a TPM device
+attached to I2C bus, use the following command line:
+
+.. code-block:: console
+
+  qemu-system-arm -M ast2600-evb -nographic \
+    -kernel arch/arm/boot/zImage \
+    -dtb arch/arm/boot/dts/aspeed-ast2600-evb.dtb \
+    -initrd rootfs.cpio \
+    -chardev socket,id=chrtpm,path=/tmp/mytpm1/swtpm-sock \
+    -tpmdev emulator,id=tpm0,chardev=chrtpm \
+    -device tpm-tis-i2c,tpmdev=tpm0,bus=aspeed.i2c.bus.12,address=0x2e
+
+  For testing, use this command to load the driver to the correct address
+
+  echo tpm_tis_i2c 0x2e > /sys/bus/i2c/devices/i2c-12/new_device
+
 In case SeaBIOS is used as firmware, it should show the TPM menu item
 after entering the menu with 'ESC'.