[PATCH v4 18/24] powerpc/pseries: Make caller pass buffer to plpks_read_var()

[Andrew Donnellan <ajd@linux.ibm.com>]

Currently, plpks_read_var() allocates a buffer to pass to the
H_PKS_READ_OBJECT hcall, then allocates another buffer, of the caller's
preferred size if necessary, into which the data is copied, and returns
that buffer to the caller.

This is a bit over the top - while we probably still want to allocate a
separate buffer to pass to the hypervisor in the hcall, we can let the
caller allocate the final buffer and specify the size.

Don't allocate var->data in plpks_read_var(), instead expect the caller to
allocate it. If the caller needs to discover the size, it can set
var->data to NULL and var->datalen will be populated. Update header file
to document this.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Russell Currey <ruscur@russell.cc>

---

v3: New patch (mpe)
---
 arch/powerpc/include/asm/plpks.h       | 12 ++++++++++++
 arch/powerpc/platforms/pseries/plpks.c | 11 ++++-------
 2 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/arch/powerpc/include/asm/plpks.h b/arch/powerpc/include/asm/plpks.h
index e7204e6c0ca4..0c49969b0864 100644
--- a/arch/powerpc/include/asm/plpks.h
+++ b/arch/powerpc/include/asm/plpks.h
@@ -88,16 +88,28 @@ int plpks_remove_var(char *component, u8 varos,
 
 /**
  * Returns the data for the specified os variable.
+ *
+ * Caller must allocate a buffer in var->data with length in var->datalen.
+ * If no buffer is provided, var->datalen will be populated with the object's
+ * size.
  */
 int plpks_read_os_var(struct plpks_var *var);
 
 /**
  * Returns the data for the specified firmware variable.
+ *
+ * Caller must allocate a buffer in var->data with length in var->datalen.
+ * If no buffer is provided, var->datalen will be populated with the object's
+ * size.
  */
 int plpks_read_fw_var(struct plpks_var *var);
 
 /**
  * Returns the data for the specified bootloader variable.
+ *
+ * Caller must allocate a buffer in var->data with length in var->datalen.
+ * If no buffer is provided, var->datalen will be populated with the object's
+ * size.
  */
 int plpks_read_bootloader_var(struct plpks_var *var);
 
diff --git a/arch/powerpc/platforms/pseries/plpks.c b/arch/powerpc/platforms/pseries/plpks.c
index 96a026a37285..5d9c6a3b2014 100644
--- a/arch/powerpc/platforms/pseries/plpks.c
+++ b/arch/powerpc/platforms/pseries/plpks.c
@@ -578,17 +578,14 @@ static int plpks_read_var(u8 consumer, struct plpks_var *var)
 		goto out_free_output;
 	}
 
-	if (var->datalen == 0 || var->datalen > retbuf[0])
+	if (!var->data || var->datalen > retbuf[0])
 		var->datalen = retbuf[0];
 
-	var->data = kzalloc(var->datalen, GFP_KERNEL);
-	if (!var->data) {
-		rc = -ENOMEM;
-		goto out_free_output;
-	}
 	var->policy = retbuf[1];
 
-	memcpy(var->data, output, var->datalen);
+	if (var->data)
+		memcpy(var->data, output, var->datalen);
+
 	rc = 0;
 
 out_free_output:
-- 
2.39.0