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

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Date:   Mon, 26 Mar 2018 11:18:33 +0530
From:   Rajneesh Bhardwaj <rajneesh.bhardwaj@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Anshuman Gupta <anshuman.gupta@...el.com>, glx@...utronix.de,
        x86@...nel.org, mingo@...hat.com, hpa@...or.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 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.

Ideally, we didnt want to add this BIOS check at all and only wanted to use
inb() approch but unfortunately, that too was broken for port 0x81.

@Rafael / Alan / Andy - Please add more or correct me in case of anything
missed  or not communicated fully.

> 
> Thanks,
> 
> 	tglx

-- 
Best Regards,
Rajneesh