powerpc/pseries: Add papr-vpd character driver for VPD retrieval

PowerVM LPARs may retrieve Vital Product Data (VPD) for system
components using the ibm,get-vpd RTAS function.

We can expose this to user space with a /dev/papr-vpd character
device, where the programming model is:

  struct papr_location_code plc = { .str = "", }; /* obtain all VPD */
  int devfd = open("/dev/papr-vpd", O_RDONLY);
  int vpdfd = ioctl(devfd, PAPR_VPD_CREATE_HANDLE, &plc);
  size_t size = lseek(vpdfd, 0, SEEK_END);
  char *buf = malloc(size);
  pread(devfd, buf, size, 0);

When a file descriptor is obtained from ioctl(PAPR_VPD_CREATE_HANDLE),
the file contains the result of a complete ibm,get-vpd sequence. The
file contents are immutable from the POV of user space. To get a new
view of the VPD, the client must create a new handle.

This design choice insulates user space from most of the complexities
that ibm,get-vpd brings:

* ibm,get-vpd must be called more than once to obtain complete
  results.

* Only one ibm,get-vpd call sequence should be in progress at a time;
  interleaved sequences will disrupt each other. Callers must have a
  protocol for serializing their use of the function.

* A call sequence in progress may receive a "VPD changed, try again"
  status, requiring the client to abandon the sequence and start
  over.

The memory required for the VPD buffers seems acceptable, around 20KB
for all VPD on one of my systems. And the value of the
/rtas/ibm,vpd-size DT property (the estimated maximum size of VPD) is
consistently 300KB across various systems I've checked.

I've implemented support for this new ABI in the rtas_get_vpd()
function in librtas, which the vpdupdate command currently uses to
populate its VPD database. I've verified that an unmodified vpdupdate
binary generates an identical database when using a librtas.so that
prefers the new ABI.

Along with the papr-vpd.h header exposed to user space, this
introduces a common papr-miscdev.h uapi header to share a base ioctl
ID with similar drivers to come.

Tested-by: Michal Suchánek <[email protected]>
Signed-off-by: Nathan Lynch <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>
Link: https://msgid.link/20231212-papr-sys_rtas-vs-lockdown-v6-9-e9eafd0c8c6c@linux.ibm.com

Author: nathanlynch

Documentation/userspace-api/ioctl/ioctl-number.rst

@@ -349,6 +349,8 @@ Code  Seq#    Include File                                           Comments
                                                                      <mailto:[email protected]>
 0xB1  00-1F                                                          PPPoX
                                                                      <mailto:[email protected]>
+0xB2  00     arch/powerpc/include/uapi/asm/papr-vpd.h                powerpc/pseries VPD API
+                                                                     <mailto:linuxppc-dev>
 0xB3  00     linux/mmc/ioctl.h
 0xB4  00-0F  linux/gpio.h                                            <mailto:[email protected]>
 0xB5  00-0F  uapi/linux/rpmsg.h                                      <mailto:[email protected]>

arch/powerpc/include/uapi/asm/papr-miscdev.h

@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef _UAPI_PAPR_MISCDEV_H_
+#define _UAPI_PAPR_MISCDEV_H_
+
+enum {
+	PAPR_MISCDEV_IOC_ID = 0xb2,
+};
+
+#endif /* _UAPI_PAPR_MISCDEV_H_ */

arch/powerpc/include/uapi/asm/papr-vpd.h

@@ -0,0 +1,22 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef _UAPI_PAPR_VPD_H_
+#define _UAPI_PAPR_VPD_H_
+
+#include <asm/ioctl.h>
+#include <asm/papr-miscdev.h>
+
+struct papr_location_code {
+	/*
+	 * PAPR+ v2.13 12.3.2.4 Converged Location Code Rules - Length
+	 * Restrictions. 79 characters plus nul.
+	 */
+	char str[80];
+};
+
+/*
+ * ioctl for /dev/papr-vpd. Returns a VPD handle fd corresponding to
+ * the location code.
+ */
+#define PAPR_VPD_IOC_CREATE_HANDLE _IOW(PAPR_MISCDEV_IOC_ID, 0, struct papr_location_code)
+
+#endif /* _UAPI_PAPR_VPD_H_ */

arch/powerpc/platforms/pseries/Makefile

@@ -4,6 +4,7 @@ ccflags-$(CONFIG_PPC_PSERIES_DEBUG)	+= -DDEBUG
 
 obj-y			:= lpar.o hvCall.o nvram.o reconfig.o \
 			   of_helpers.o rtas-work-area.o papr-sysparm.o \
+			   papr-vpd.o \
 			   setup.o iommu.o event_sources.o ras.o \
 			   firmware.o power.o dlpar.o mobility.o rng.o \
 			   pci.o pci_dlpar.o eeh_pseries.o msi.o \