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

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Date:   Mon, 26 Mar 2018 11:11:51 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Rajneesh Bhardwaj <rajneesh.bhardwaj@...el.com>
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 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