linux-kernel - Re: [PATCH v3 2/2] fpga: dfl: look for vendor specific capability

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Date:   Wed, 2 Dec 2020 10:00:03 +0800
From:   Xu Yilun <yilun.xu@...el.com>
To:     Moritz Fischer <mdf@...nel.org>
Cc:     matthew.gerlach@...ux.intel.com, "Wu, Hao" <hao.wu@...el.com>,
        "linux-fpga@...r.kernel.org" <linux-fpga@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "trix@...hat.com" <trix@...hat.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "corbet@....net" <corbet@....net>
Subject: Re: [PATCH v3 2/2] fpga: dfl: look for vendor specific capability

> > > 
> > > > +		}
> > > > +
> > > > +		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.

Thanks,
Yilun