linux-kernel - Re: [PATCH v1 1/1] fpga: dfl: pci: Use pci_find_vsec

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Date:   Wed, 10 Nov 2021 11:44:57 +0800
From:   Xu Yilun <yilun.xu@...el.com>
To:     matthew.gerlach@...ux.intel.com
Cc:     Tom Rix <trix@...hat.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Moritz Fischer <mdf@...nel.org>, linux-fpga@...r.kernel.org,
        linux-kernel@...r.kernel.org, Wu Hao <hao.wu@...el.com>
Subject: Re: [PATCH v1 1/1] fpga: dfl: pci: Use pci_find_vsec_capability()
  when looking for DFL

On Tue, Nov 09, 2021 at 10:51:33AM -0800, matthew.gerlach@...ux.intel.com wrote:
> 
> 
> 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

I agree u16 is OK.

A minor concern, is it better we also change the dfl_res_off to u16?
dfl_res_off & voff are the same type of variables needed on positioning
the DFL, so I'd like them listed together.

> 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.  Do we need to change this a

I agree. The vendor_id should be checked before VSEC ID is meaningful,
and now this Vendor/VSEC pair is the only supported one, so this piece of
code is good to me.

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

A generic Vendor/VSEC pair means all vendors must use the unified
vendor_id if they want to use DFL. I'm not sure if this is proper.

Thanks,
Yilun

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