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

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Date:   Tue, 9 Nov 2021 10:51:33 -0800 (PST)
From:   matthew.gerlach@...ux.intel.com
To:     Tom Rix <trix@...hat.com>
cc:     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>,
        Xu Yilun <yilun.xu@...el.com>
Subject: Re: [PATCH v1 1/1] fpga: dfl: pci: Use pci_find_vsec_capability()
 when looking for DFL



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.  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?

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