RE: [PATCH v3 2/2] fpga: dfl: look for vendor specific capability

[matthew . gerlach]

On Wed, 2 Dec 2020, Wu, Hao wrote:

> > > > > > +		}
> > > > > > +
> > > > > > +		offset = dfl_res & PCI_VNDR_DFLS_RES_OFF_MASK;
> > > > > > +		if (offset >= len) {
> > > > > > +			dev_err(&pcidev->dev, "%s bad
> > offset %u >= %pa\n",
> > > > > > +				__func__, offset, &len);
> > > > > > +			return -EINVAL;
> > > > > > +		}
> > > > > > +
> > > > > > +		dev_dbg(&pcidev->dev, "%s BAR %d offset 0x%x\n",
> > > > > > __func__, bar, offset);
> > > > > > +
> > > > > > +		len -= offset;
> > > > > > +
> > > > > > +		start = pci_resource_start(pcidev, bar) + offset;
> > > > > > +
> > > > > > +		dfl_fpga_enum_info_add_dfl(info, start, len);
> > > > >
> > > > > That means everytime, we pass [start, endofbar] region to dfl core
> > > > > for enumeration, if there are multiple DFLs in one bar, then each range
> > > > > ends at the same endofbar, it seems fine as enumeration can be done
> > > > > one by one, but ideally the best case is that this capability can provide
> > > > > end address or size too, right? It is possible that information can be
> > > > > added to the capability as well? then we don't have such limitation.
> > > > >
> > > > > Hao
> > > >
> > > > I am not sure having more than one DFL in a bar serves any purpose over
> > a
> > > > single DFL.  Regardless, I think the consistency of just having Offset/BIR
> > > > in the VSEC is better than adding more infomation that has little or no
> > > > added value.
> > >
> > > Agreed. Can't you just link the DFLs in that case?
> >
> > I didn't see the value of more DFLs in one bar either. So I think we'd better
> > document it.
>
> Yes, it needs to be documented well, otherwise users may have their own
> choices, e.g. link 100 queues together by modify DFH registers of the
> queues one by one, or just have them done together in the VSEC. I am not
> sure which one is the easier way for logic developer, but at least we need to
> document what driver can support.
>
> Thanks
> Hao

I will update the documentation to clarify the basis of the rule of one 
DFL per bar.  Since we have a clearly stated rule, we can enforce the rule 
in the code and perform more error checking.

> > Thanks,
> > Yilun