linux-kernel - Re: x86/PCI: Scan all functions during probing

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Message-ID: <73341fe4-9017-7dcb-7ae3-a993428c11fc@siemens.com>
Date:   Wed, 24 Aug 2016 07:13:12 -0400
From:   Jan Kiszka <jan.kiszka@...mens.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Bjorn Helgaas <helgaas@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        Benedikt Spranger <b.spranger@...utronix.de>,
        Lukas Wunner <lukas@...ner.de>
Subject: Re: x86/PCI: Scan all functions during probing

On 2016-08-24 04:39, Thomas Gleixner wrote:
> On Thu, 18 Aug 2016, Bjorn Helgaas wrote:
>> I looked up the spec: PCI (not PCIe) r3.0, sec 3.2.2.3.4, says:
>>
>>   A single-function device may optionally respond to all function
>>   numbers as the same function or may ... respond only to function 0
>>   and not respond to the other function numbers.
>>
>> I'm concerned that a single-function device that responds to all
>> function numbers might break with this patch.
>>
>>   [multi-function devices] are also required to always implement
>>   function 0 in the device.
>>
>> Here's the reason we can advance by 8 in the "Go find them" loop.
>>
>>   If a single function device is detected (i.e., bit 7 in the Header
>>   Type register of function 0 is 0), no more functions for that Device
>>   Number will be checked.  If a multi-function device is detected
>>   (i.e., bit 7 in the Header Type register of function 0 is 1), then
>>   all remaining Function Numbers will be checked.
>>
>> This patch does the opposite of what the first sentence recommends.
> 
> Fair enough. We'll need to find a way to deal with that in jailhouse then.

Wouldn't it also be an option to have this fine-grained scanning only
activated if we detect to run over Jailhouse (which we have to anyway)?
Such code hasn't been proposed for upstream yet, but we will eventually.

Jan

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