Re: [PATCH v4 2/4] iommu: Add new iommu op to get iommu hardware information

[Jason Gunthorpe <jgg@nvidia.com>]

On Mon, Jul 31, 2023 at 08:33:55AM +0000, Liu, Yi L wrote:
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > Sent: Thursday, July 27, 2023 10:43 PM
> > 
> > On Thu, Jul 27, 2023 at 07:57:57AM +0000, Tian, Kevin wrote:
> > > > From: Liu, Yi L <yi.l.liu@intel.com>
> > > > Sent: Monday, July 24, 2023 7:00 PM
> > > >
> > > > @@ -252,11 +258,20 @@ struct iommu_iotlb_gather {
> > > >   * @remove_dev_pasid: Remove any translation configurations of a specific
> > > >   *                    pasid, so that any DMA transactions with this pasid
> > > >   *                    will be blocked by the hardware.
> > > > + * @hw_info_type: One of enum iommu_hw_info_type defined in
> > > > + *                include/uapi/linux/iommufd.h. It is used to tag the type
> > > > + *                of data returned by .hw_info callback. The drivers that
> > > > + *                support .hw_info callback should define a unique type
> > > > + *                in include/uapi/linux/iommufd.h. For the drivers that do
> > > > + *                not implement .hw_info callback, this field is
> > > > + *                IOMMU_HW_INFO_TYPE_NONE which is 0. Hence, such drivers
> > > > + *                do not need to care this field.
> > >
> > > every time looking at this field the same question came out why it is required
> > > (and looks I forgot your previous response).
> > >
> 
> The major reason is that not every driver implements the hw_info
> callback.
> 
> > > e.g. why cannot the type be returned in @hw_info():
> > >
> > > 	void *(*hw_info)(struct device *dev, u32 *length, int *type);
> > 
> > u32 *type
> > 
> > > NULL callback implies IOMMU_HW_INFO_TYPE_NONE.
> > 
> > If every one of these queries has its own type it makes sense
> > 
> > Though, is it not possible that we can have a type for the entire
> > driver?
> 
> Not quite sure if I got your point. Is it acceptable to define the
> callabck in the current version? or Kevin's suggestion makes
> more sense?

I'm trying to remember if there is a reason we need unique types for
the domain and the invalidation or if we can get bye with a single
type just for the whole iommu driver.

I suppose if we ever want to to "virtio-iommu invalidation" we'd want
to use a new type for it?

Jason