Re: [PATCH v6 2/6] iommufd: Add IOMMU_HWPT_INVALIDATE

[Nicolin Chen <nicolinc@nvidia.com>]

On Fri, Nov 24, 2023 at 02:36:29AM +0000, Tian, Kevin wrote:

> > > > > > >> + * @out_driver_error_code: Report a driver speicifc error code
> > upon
> > > > > > failure.
> > > > > > >> + *                         It's optional, driver has a choice to fill it or
> > > > > > >> + *                         not.
> > > > > > >
> > > > > > > Being optional how does the user tell whether the code is filled or
> > not?
> > > >
> > > > Well, naming it "error_code" indicates zero means no error while
> > > > non-zero means something? An error return from this ioctl could
> > > > also tell the user space to look up for this driver error code,
> > > > if it ever cares.
> > >
> > > probably over-thinking but I'm not sure whether zero is guaranteed to
> > > mean no error in all implementations...
> >
> > Well, you are right. Usually HW conveniently raises a flag in a
> > register to indicate something wrong, yet it is probably unsafe
> > to say it definitely.
> >
> 
> this reminds me one open. What about an implementation having
> a hierarchical error code layout e.g. one main error register with
> each bit representing an error category then multiple error code
> registers each for one error category? In this case probably
> a single out_driver_error_code cannot carry that raw information.

Hmm, good point.

> Instead the iommu driver may need to define a customized error
> code convention in uapi header which is converted from the
> raw error information.
> 
> From this angle should we simply say that the error code definition
> must be included in the uapi header? If raw error information can
> be carried by this field then this hw can simply say that the error
> code format is same as the hw spec defines.
> 
> With that explicit information then the viommu can easily tell
> whether error code is filled or not based on its own convention.

That'd be to put this error_code field into the driver uAPI
structure right?

I also thought about making this out_driver_error_code per HW.
Yet, an error can be either per array or per entry/quest. The
array-related error should be reported in the array structure
that is a core uAPI, v.s. the per-HW entry structure. Though
we could still report an array error in the entry structure
at the first entry (or indexed by "array->entry_num")?

Thanks
Nic