linux-kernel - Re: [PATCH v3] x86: i8237: Register based on FADT legacy boot flag

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Date:   Mon, 26 Mar 2018 03:34:44 -0700
From:   hpa@...or.com
To:     Thomas Gleixner <tglx@...utronix.de>,
        Rajneesh Bhardwaj <rajneesh.bhardwaj@...el.com>
CC:     Anshuman Gupta <anshuman.gupta@...el.com>, glx@...utronix.de,
        x86@...nel.org, mingo@...hat.com, rjw@...ysocki.net,
        andriy.shevchenko@...ux.intel.com, alan@...ux.intel.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] x86: i8237: Register based on FADT legacy boot flag

On March 26, 2018 2:11:51 AM PDT, Thomas Gleixner <tglx@...utronix.de> wrote:
>On Mon, 26 Mar 2018, Rajneesh Bhardwaj wrote:
>
>> On Sun, Mar 25, 2018 at 01:50:40PM +0200, Thomas Gleixner wrote:
>> > On Thu, 22 Mar 2018, Anshuman Gupta wrote:
>> > 
>> > > From: Rajneesh Bhardwaj <rajneesh.bhardwaj@...el.com>
>> > > 
>> > > >From Skylake onwards, the platform controller hub (Sunrisepoint
>PCH) does
>> > > not support legacy DMA operations to IO ports 81h-83h, 87h,
>89h-8Bh, 8Fh.
>> > > Currently this driver registers as syscore ops and its resume
>function is
>> > > called on every resume from S3. On Skylake and Kabylake, this
>causes a
>> > > resume delay of around 100ms due to port IO operations, which is
>a problem.
>> > > 
>> > > This change allows to load the driver only when the platform bios
>> > > explicitly supports such devices or has a cut-off date earlier
>than 2017.
>> > 
>> > Please explain WHY 2017 is the cut-off date. I still have no clue
>how that
>> > is decided aside of being a random number.
>> 
>> Hello Thomas,
>> 
>> We tested on few Intel platforms such as Skylake, Kabylake,
>Geminilake etc
>> and realized that the BIOS always sets the FADT flag to be true
>though the
>> device may not be physically present on the SoC. This is a BIOS bug.
>To keep
>> the impact minimum, we decided to add a cut-off date since we are not
>aware
>> of any BIOS (other than the coreboot link provided in the commit msg)
>that
>> properly sets this field. SoCs released after Skylake will not have
>this DMA
>> device on the PCH. So, because of these two reasons, we decided to
>add a
>> cut-off date as 2017.
>> 
>> Please let us know if you feel strongly about it and we can change it
>or
>> remove it if you feel so.
>
>I don't feel strongly about the cut off itself, but I want a reasonable
>explanation in the changelog or code comment because half a year from
>now
>nobody remembers ....
>
>Thanks,
>
>	tglx

Can we probe safely for this device?
-- 
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