RE: [PATCH v1 1/1] fpga: dfl: pci: Use pci_find_vsec_capability() when looking for DFL

From: "Wu, Hao" <hao.wu@intel.com>
To: "matthew.gerlach@linux.intel.com"
	<matthew.gerlach@linux.intel.com>, "Tom Rix" <trix@redhat.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Moritz Fischer <mdf@kernel.org>,
	"linux-fpga@vger.kernel.org" <linux-fpga@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"Xu, Yilun" <yilun.xu@intel.com>
Subject: RE: [PATCH v1 1/1] fpga: dfl: pci: Use pci_find_vsec_capability() when looking for DFL
Date: Wed, 10 Nov 2021 03:51:46 +0000	[thread overview]
Message-ID: <DM6PR11MB381971C2FDA4AF1D024C4D1985939@DM6PR11MB3819.namprd11.prod.outlook.com> (raw)
In-Reply-To: <alpine.DEB.2.22.394.2111091044060.1548144@rhweight-WRK1>

> On Tue, 9 Nov 2021, Tom Rix wrote:
> 
> >
> > On 11/9/21 10:05 AM, Andy Shevchenko wrote:
> >> On Tue, Nov 09, 2021 at 07:55:43AM -0800, Tom Rix wrote:
> >>> On 11/9/21 7:41 AM, Andy Shevchenko wrote:
> >>>> Currently the find_dfls_by_vsec() opens code pci_find_vsec_capability().
> >>>> Refactor the former to use the latter. No functional change intended.
> >> Thanks for review, my answers below.
> >>
> >> ...
> >>
> >>>> +	u16 voff;
> >>> The later use of voff in pci_read_config_dword is of type 'int', it may be
> >>> better to keep voff as an int.
> >> I don't think so. The rule of thumb that the types should match the value
> >> they
> >> got in the first place. In this case it's u16. Compiler will implicitly
> >> cast it
> >> to whatever is needed as long as the type is good for integer promotion.
> >>
> 
> I think u16 is more precise than int, but I think it'll get promoted to an
> int anywhen when used with calls to pci_read_config_dword().  Was this
> change tested on real or emulated HW?
> 
> >> ...
> >>
> >>>> +	voff = pci_find_vsec_capability(dev, PCI_VENDOR_ID_INTEL,
> >>>> PCI_VSEC_ID_INTEL_DFLS);
> >>> This may be a weakness in the origin code, but intel isn't the exclusive
> >>> user of DFL.
> >> This does not change the original code. If you think so, this can be
> >> extended
> >> later on.
> >
> > I would rather see this fixed now or explained why this isn't a problem.
> 
> I agree that a single Vendor/VSEC id being supported is a problem,
> but I think fixing it should be a separate patch.  

Yes, I think that should be a separate patch.

> Do we need to change
> this a table lookup of Vendor/VSEC id's, or do we need to reserve a more
> generic Vendor/VSEC pair?

No, we don't want to maintain another table in DFL code. Let everybody apply
some new ids sounds like waste of resources too.

What about using DVSEC here? then other devices can reuse the same DVSEC
without conflicts. This is what is done for other Intel tech, e.g. Intel SIOV. But
anyway this must be captured in DFL spec firstly.

How do you guys think?

Thanks
Hao
> 
> >
> > Tom
> >
> >>
> >>>>    	if (!voff) {
> >>>>    		dev_dbg(&pcidev->dev, "%s no DFL VSEC found\n", __func__);
> >>>>    		return -ENODEV;
> >
> >