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

[Yi Liu]

On 2023/11/28 03:53, Nicolin Chen wrote:
> 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?

looks to be. Then it would be convenient to reserve a code for
the case of no error (either no error happened or just not used)

> 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")?

per-entry error code seems like to be a completion code. Each
entry in the array can have a corresponding code (0 for succ,
others for failure). do you already have such a need?

--
Regards,
Yi Liu