[PATCH v2] powerpc/papr_scm: Fix warning triggered by perf_stats_show()

[Vaibhav Jain]

A warning is reported by the kernel in case perf_stats_show() returns
an error code. The warning is of the form below:

 papr_scm ibm,persistent-memory:ibm,pmemory@44100001:
          Failed to query performance stats, Err:-10
 dev_attr_show: perf_stats_show+0x0/0x1c0 [papr_scm] returned bad count
 fill_read_buffer: dev_attr_show+0x0/0xb0 returned bad count

On investigation it looks like that the compiler is silently truncating the
return value of drc_pmem_query_stats() from 'long' to 'int', since the
variable used to store the return code 'rc' is an 'int'. This
truncated value is then returned back as a 'ssize_t' back from
perf_stats_show() to 'dev_attr_show()' which thinks of it as a large
unsigned number and triggers this warning..

To fix this we update the type of variable 'rc' from 'int' to
'ssize_t' that prevents the compiler from truncating the return value
of drc_pmem_query_stats() and returning correct signed value back from
perf_stats_show().

Fixes: 2d02bf835e573 ('powerpc/papr_scm: Fetch nvdimm performance
       stats from PHYP')
Signed-off-by: Vaibhav Jain <vaib...@linux.ibm.com>
---
Changelog:

v2: Added an explicit cast to the expression calling 'seq_buf_used()'
    and triggering this issue. [ Ira ]
---
 arch/powerpc/platforms/pseries/papr_scm.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/platforms/pseries/papr_scm.c 
b/arch/powerpc/platforms/pseries/papr_scm.c
index a88a707a608aa..5493bc847bd08 100644
--- a/arch/powerpc/platforms/pseries/papr_scm.c
+++ b/arch/powerpc/platforms/pseries/papr_scm.c
@@ -785,7 +785,8 @@ static int papr_scm_ndctl(struct nvdimm_bus_descriptor 
*nd_desc,
 static ssize_t perf_stats_show(struct device *dev,
                               struct device_attribute *attr, char *buf)
 {
-       int index, rc;
+       int index;
+       ssize_t rc;
        struct seq_buf s;
        struct papr_scm_perf_stat *stat;
        struct papr_scm_perf_stats *stats;
@@ -820,7 +821,7 @@ static ssize_t perf_stats_show(struct device *dev,
 
 free_stats:
        kfree(stats);
-       return rc ? rc : seq_buf_used(&s);
+       return rc ? rc : (ssize_t)seq_buf_used(&s);
 }
 DEVICE_ATTR_ADMIN_RO(perf_stats);
 
-- 
2.26.2