linux-kernel - Re: [PATCH 0/4] PCI: Enable LTR/OBFF before device is used by driver

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Date:	Fri, 8 Jun 2012 12:02:55 -0600
From:	Myron Stowe <myron.stowe@...il.com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Xudong Hao <xudong.hao@...el.com>, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, avi@...hat.com,
	alex.williamson@...hat.com, ddutile@...hat.com,
	myron.stowe@...hat.com, xiantao.zhang@...el.com
Subject: Re: [PATCH 0/4] PCI: Enable LTR/OBFF before device is used by driver

On Fri, Jun 8, 2012 at 11:31 AM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
> On Fri, Jun 8, 2012 at 1:01 AM, Xudong Hao <xudong.hao@...el.com> wrote:
>> The series of patches enable LTR and OBFF before device is used by driver, and
>> introduce a couple of functions to save/restore LTR latency value.
>>
>> Patch 1/4 introduce new function pci_obff_supported() as pci_ltr_support().
>>
>> Patch 2/4 enable LTR(Latency tolerance reporting) before device is used by
>> driver.
>>
>> Patch 3/4 enable OBFF(optimized buffer flush/fill) before device is used by
>> driver.
>>
>> Patch 4/4 introduce a couple of functions pci_save_ltr_value() and
>> pci_restore_ltr_value() to save and restore LTR latency value, while device is
>> reset.
>
> We need some justification for these patches.  Why do we want them?
> Do they improve performance?  Reduce power consumption?  How have they
> been tested?  How can we be confident that these features work
> correctly on hardware in the field?  Should or could the BIOS enable
> them itself, based on OEM testing and desire to support these
> features?

I too am a little nervous about these changes due to Jesse's earlier response
(see http://marc.info/?l=linux-pci&m=133372610102933&w=2) where he indicated:
  "Given how device specific these extensions are, I'd expect you'd need
   to know about each specific device anyway, which is why I think the
   control belongs in the driver."

Having these features enabled by default may be too aggressive.  Not saying it
is not correct - something you may be able to inform us about, especially since
you are with Intel - just make me nervous without further information.

Myron

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