linux-kernel - Re: [PATCH V2 1/4] ACPI,PCI,IRQ: factor in PCI possible

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Date:	Wed, 29 Jun 2016 23:19:38 +0200
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Sinan Kaya <okaya@...eaurora.org>
Cc:	"Rafael J. Wysocki" <rafael@...nel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Timur Tabi <timur@...eaurora.org>,
	Christopher Covington <cov@...eaurora.org>,
	Jon Masters <jcm@...hat.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	eric.auger@...hat.com, Linux PCI <linux-pci@...r.kernel.org>,
	Andy Gross <agross@...eaurora.org>,
	linux-arm-msm@...r.kernel.org,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>, wim@....tudelft.nl,
	perex@...ex.cz, tiwai@...e.com,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <lenb@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 1/4] ACPI,PCI,IRQ: factor in PCI possible

On Wed, Jun 29, 2016 at 8:47 PM, Sinan Kaya <okaya@...eaurora.org> wrote:
> On 6/29/2016 9:13 AM, Rafael J. Wysocki wrote:
>> On Wed, Jun 29, 2016 at 10:27 AM, Sinan Kaya <okaya@...eaurora.org> wrote:
>>> The change introduced in commit 103544d86976 ("ACPI,PCI,IRQ: reduce
>>> resource requirements") omitted the initially assigned POSSIBLE penalty
>>> when the IRQ is active.
>>
>> It would be good to say what can go wrong with that here.
>>
>
> I can add more description. Here is a first attempt.
>
> Incorrect calculation of penalty leads to ACPI code assigning the wrong
> interrupt number to PCI INTx interrupts.
>
> This would not be as bad as it sounds in theory. You would just cause the
> interrupts to be shared and observe performance penalty.
>
> However, some drivers like parallel port driver doesn't like interrupt
> sharing as in this example and causes all other PCI drivers sharing the interrupt
> to malfunction.
>
> The issue has not been caught because the behavior is platform specific
> and depends on the peripheral drivers sharing the IRQ.
>
> I can claim that this could be a BIOS bug. if interrupt 7 is not good for PCI,
> it shouldn't have been listed in the possible PCI interrupts to begin with.
> Given this is an existing platform, I don't think we have the luxury to request
> all BIOS to be updated. This bugfix is needed to support existing platforms.
>
>
> Feel free to request more information if the above description is not clear.

It is clear enough.  I can add it to the changelog when applying the patch.

>
>>> The original code would assign the POSSIBLE value divided by the number
>>> of possible IRQs during initialization.
>>>
>>> Later, if the IRQ is chosen as the active IRQ or if the IRQ is in use
>>> by ISA; additional penalties get added.

Does "later" here mean "later in that code path" or "in a later patch"?

Thanks,
Rafael