linux-kernel - Re: [PATCH 2/2] pci: Update VPD size with correct length

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Date:	Fri, 18 Dec 2015 15:14:22 +0100
From:	Hannes Reinecke <hare@...e.de>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	Bjorn Helgaas <bhelgaas@...gle.com>,
	Michal Kubecek <mkubecek@...e.com>,
	"Shane M. Seymour" <shane.seymour@....com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Bjorn Helgaas <helgaas@...nel.org>
Subject: Re: [PATCH 2/2] pci: Update VPD size with correct length

On 12/18/2015 03:02 PM, Alexander Duyck wrote:
> On Fri, Dec 18, 2015 at 5:57 AM, Hannes Reinecke <hare@...e.de> wrote:
>> On 12/18/2015 02:49 PM, Alexander Duyck wrote:
>>>
>>> On Fri, Dec 18, 2015 at 12:35 AM, Hannes Reinecke <hare@...e.de> wrote:
>>>>
>>>> PCI-2.2 VPD entries have a maximum size of 32k, but might actually
>>>> be smaller than that. To figure out the actual size one has to read
>>>> the VPD area until the 'end marker' is reached.
>>>> Trying to read VPD data beyond that marker results in 'interesting'
>>>> effects, from simple read errors to crashing the card. And to make
>>>> matters worse not every PCI card implements this properly, leaving
>>>> us with no 'end' marker or even completely invalid data.
>>>> This path modifies the size of the VPD attribute to the available
>>>> size, or set it to '0' if no valid data could be read.
>>>
>>>
>>> This isn't what I had in mind.  There is no need to add an f0 version
>>> of the size function.  The size for all functions other than function
>>> 0 when the F0 flag is set is 0.  We aren't going to be reading their
>>> VPD, we only read the VPD region of function 0.
>>>
>> Ah. (I'm a bit confused about the proposed action for VPD other than
>> function 0).
>> So the idea here is to _disallow_ access to VPDs from functions other than
>> '0' unless these functions have different PCI IDs?
>
> If you take a look at the F0 functions what they do is bypass the VPD
> of the functions other than function 0.  As such setting the size to 0
> should really have no effect since the VPD of the function isn't
> actually read if the F0 flag is set.
>
Setting the size to '0' effectively inhibits you to read the VPD 
data. So if we were to return '0' for PCI devices with the F0 bit 
set we will never ever to be able to read (or write) _any_ VPD data 
for that PCI device/function.
Which would be rendering all these F0 accessors pointless, and we 
might as well remove them.

Hence my confusion.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		               zSeries & Storage
hare@...e.de			               +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton
HRB 21284 (AG Nürnberg)
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