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

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Date:	Wed, 29 Jun 2016 17:21:27 -0400
From:	Sinan Kaya <okaya@...eaurora.org>
To:	"Rafael J. Wysocki" <rafael@...nel.org>
Cc:	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 6/29/2016 5:19 PM, Rafael J. Wysocki wrote:
> 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.

OK

> 
>>
>>>> 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"?

"later in that code path"

> 
> Thanks,
> Rafael
> 


-- 
Sinan Kaya
Qualcomm Technologies, Inc. on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project