Subject: Re: [PATCH v10 4/6] powerpc/papr_scm: Improve error logging and handling papr_scm_ndctl()

From: Vaibhav Jain <[email-masked]>
Date: Sat, 06 Jun 2020 16:51:41 +0530

Hi Ira and Dan,

Thanks for reviewing this patch. Have updated the patch based on your
feedback to upadate cmd_rc only when the nd_cmd was handled and return
'0' in that case.

Other errors in case the nd_cmd was unrecognized or invalid result in
error returned from this functions as you suggested.

~ Vaibhav

Dan Williams <[email-masked]> writes:

> On Fri, Jun 5, 2020 at 10:13 AM Ira Weiny <[email-masked]> 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" <[email-masked]>
>> > Cc: Dan Williams <[email-masked]>
>> > Cc: Michael Ellerman <[email-masked]>
>> > Cc: Ira Weiny <[email-masked]>
>> > Signed-off-by: Vaibhav Jain <[email-masked]>
>> > ---
>> > 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.

-- 
Cheers
~ Vaibhav