linux-kernel - Re: [PATCH v10 4/6] powerpc/papr_scm: Improve error logging and handling papr_scm

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Date:   Fri, 5 Jun 2020 12:49:16 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     Vaibhav Jain <vaibhav@...ux.ibm.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        "Oliver O'Halloran" <oohall@...il.com>,
        Santosh Sivaraj <santosh@...six.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v10 4/6] powerpc/papr_scm: Improve error logging and
 handling papr_scm_ndctl()

On Fri, Jun 5, 2020 at 10:13 AM Ira Weiny <ira.weiny@...el.com> wrote:
>
> On Fri, Jun 05, 2020 at 05:11:34AM +0530, Vaibhav Jain wrote:
> > Since papr_scm_ndctl() can be called from outside papr_scm, its
> > exposed to the possibility of receiving NULL as value of 'cmd_rc'
> > argument. This patch updates papr_scm_ndctl() to protect against such
> > possibility by assigning it pointer to a local variable in case cmd_rc
> > == NULL.
> >
> > Finally the patch also updates the 'default' clause of the switch-case
> > block removing a 'return' statement thereby ensuring that value of
> > 'cmd_rc' is always logged when papr_scm_ndctl() returns.
> >
> > Cc: "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>
> > Cc: Dan Williams <dan.j.williams@...el.com>
> > Cc: Michael Ellerman <mpe@...erman.id.au>
> > Cc: Ira Weiny <ira.weiny@...el.com>
> > Signed-off-by: Vaibhav Jain <vaibhav@...ux.ibm.com>
> > ---
> > Changelog:
> >
> > v9..v10
> > * New patch in the series
>
> Thanks for making this a separate patch it is easier to see what is going on
> here.
>
> > ---
> >  arch/powerpc/platforms/pseries/papr_scm.c | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/powerpc/platforms/pseries/papr_scm.c b/arch/powerpc/platforms/pseries/papr_scm.c
> > index 0c091622b15e..6512fe6a2874 100644
> > --- a/arch/powerpc/platforms/pseries/papr_scm.c
> > +++ b/arch/powerpc/platforms/pseries/papr_scm.c
> > @@ -355,11 +355,16 @@ static int papr_scm_ndctl(struct nvdimm_bus_descriptor *nd_desc,
> >  {
> >       struct nd_cmd_get_config_size *get_size_hdr;
> >       struct papr_scm_priv *p;
> > +     int rc;
> >
> >       /* Only dimm-specific calls are supported atm */
> >       if (!nvdimm)
> >               return -EINVAL;
> >
> > +     /* Use a local variable in case cmd_rc pointer is NULL */
> > +     if (!cmd_rc)
> > +             cmd_rc = &rc;
> > +
>
> This protects you from the NULL.  However...
>
> >       p = nvdimm_provider_data(nvdimm);
> >
> >       switch (cmd) {
> > @@ -381,12 +386,13 @@ static int papr_scm_ndctl(struct nvdimm_bus_descriptor *nd_desc,
> >               break;
> >
> >       default:
> > -             return -EINVAL;
> > +             dev_dbg(&p->pdev->dev, "Unknown command = %d\n", cmd);
> > +             *cmd_rc = -EINVAL;
>
> ... I think you are conflating rc and cmd_rc...
>
> >       }
> >
> >       dev_dbg(&p->pdev->dev, "returned with cmd_rc = %d\n", *cmd_rc);
> >
> > -     return 0;
> > +     return *cmd_rc;
>
> ... this changes the behavior of the current commands.  Now if the underlying
> papr_scm_meta_[get|set]() fails you return that failure as rc rather than 0.
>
> Is that ok?

The expectation is that rc is "did the command get sent to the device,
or did it fail for 'transport' reasons". The role of cmd_rc is to
translate the specific status response of the command into a common
error code. The expectations are:

rc < 0: Error code, Linux terminated the ioctl before talking to hardware

rc == 0: Linux successfully submitted the command to hardware, cmd_rc
is valid for command specific response

rc > 0: Linux successfully submitted the command, but detected that
only a subset of the data was accepted for "write"-style commands, or
that only subset of data was returned for "read"-style commands. I.e.
short-write / short-read semantics. cmd_rc is valid in this case and
its up to userspace to determine if a short transfer is an error or
not.

> Also 'logging cmd_rc' in the invalid cmd case does not seem quite right unless
> you really want rc to be cmd_rc.
>
> The architecture is designed to separate errors which occur in the kernel vs
> errors in the firmware/dimm.  Are they always the same?  The current code
> differentiates them.

Yeah, they're distinct, transport vs end-point / command-specific
status returns.