occ: Add support for GPU presence detection

On the Witherspoon platform, we need to distinguish between NVLink GPUs and
OpenCAPI accelerators. In order to do this, we first need to find out
whether the SXM2 socket is populated.

On Witherspoon, the SXM2 socket's presence detection pin is only visible
via I2C from the APSS, and thus can only be exposed to the host via the
OCC. The OCC, per OCC Firmware Interface Specification for POWER9 version
0.22, now exposes this to skiboot through a field in the dynamic data
shared memory.

Add the necessary dynamic data changes required to read the version and
GPU presence fields. Add a function, occ_get_gpu_presence(), that can be
used to check GPU presence.

If the OCC isn't reporting presence (old OCC firmware, or some other
reason), we default to assuming there is a device present and wait until
link training to fail.

This will be used in later patches to fix up the NPU2 probe path for
OpenCAPI support on Witherspoon.

Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Reviewed-by: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
Signed-off-by: Stewart Smith <stewart@linux.ibm.com>

1 files changed, 20 insertions, 3 deletions

diff --git a/hw/occ.c b/hw/occ.c
index fc95d39..10b2de0 100644
--- a/hw/occ.c
+++ b/hw/occ.c
@@ -229,10 +229,10 @@ struct occ_response_buffer {
  */
 struct occ_dynamic_data {
 	u8 occ_state;
+	u8 major_version;
+	u8 minor_version;
+	u8 gpus_present;
 	u8 spare1;
-	u8 spare2;
-	u8 spare3;
-	u8 spare4;
 	u8 cpu_throttle;
 	u8 mem_throttle;
 	u8 quick_pwr_drop;
@@ -1230,6 +1230,23 @@ exit:
 	unlock(&chip->queue_lock);
 }
 
+bool occ_get_gpu_presence(struct proc_chip *chip, int gpu_num)
+{
+	struct occ_dynamic_data *ddata;
+
+	assert(gpu_num <= 2);
+
+	ddata = get_occ_dynamic_data(chip);
+
+	if (ddata->major_version != 0 || ddata->minor_version < 1) {
+		prlog(PR_INFO, "OCC: OCC not reporting GPU slot presence, "
+		      "assuming device is present\n");
+		return true;
+	}
+
+	return (bool)(ddata->gpus_present & 1 << gpu_num);
+}
+
 static void occ_add_powercap_sensors(struct dt_node *power_mgt);
 static void occ_add_psr_sensors(struct dt_node *power_mgt);