linux-kernel - Re: [PATCH V2] AHCI: Workaround for ThunderX Errata#22536

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Date:	Tue, 16 Feb 2016 22:14:10 +0100
From:	Robert Richter <rric@...nel.org>
To:	Tirumalesh Chalamarla <tchalamarla@...iumnetworks.com>
Cc:	David Daney <ddaney@...iumnetworks.com>, Tejun Heo <tj@...nel.org>,
	linux-ide@...r.kernel.org, stripathi@....com,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH V2] AHCI: Workaround for ThunderX Errata#22536

On 16.02.16 11:50:44, Tirumalesh Chalamarla wrote:
> 
> 
> On 02/16/2016 11:38 AM, David Daney wrote:
> >On 02/16/2016 11:14 AM, Tirumalesh Chalamarla wrote:
> >>
> >>
> >>On 02/16/2016 06:42 AM, Robert Richter wrote:
> >>>On 15.02.16 13:30:41, Tejun Heo wrote:
> >>>>On Sun, Feb 14, 2016 at 07:36:18PM -0800, Tirumalesh Chalamarla wrote:
> >>>>>There is no need for special Driver, AHCI is sufficient for
> >>>>>ThunderX, the
> >>>>>file only contains this interrupt handler,
> >>>>>is it preferable if this interrupt handler in libahci.c with others,
> >>>>>instead
> >>>>>of separate file?
> >>>>
> >>>>Yeap, just fold it in ahci.c with surrounding #ifdef guard.
> >>>
> >>>Yes, please use #ifdef CONFIG_CAVIUM_ERRATUM_22536 ... and add a
> >>>kconfig entry for this to arch/arm64/Kconfig.
> >>>
> >>Are you sure, this is not a workaround that is based on alternative
> >>framework rather on pci device and vendor
> >>
> >>do you think CONFIG_ARCH_THUNDER a good alternative?
> >
> >No.  CONFIG_ARCH_THUNDER should be removed all together.
> >
> >Grouping a bunch of unrelated features under a single config variable
> >creates a very brittle system.  What are you going to do when a new
> >hardware revision is released?  Create CONFIG_ARCH_THUNDER2?  Which one
> >of these two would you select if building a kernel?  It is a choice that
> >we don't want to force users (kernel builders) to have to waste mental
> >energy on.
> >
> >Instead, let's try to make things work out of the box without having to
> >set a bunch of random config variables.
> >
> >If a generic arm64 kernel won't get too bloated, I would suggest just
> >enabling the compilation of the code unconditionally (at least for
> >arm64).  The use of the code would still be gated by the PCI version
> >probe that is part of the patch.
> >
> 
> exactly, that is my initial choice with v1, and only depends on vendor and
> device id.
> 
> but it seems a config is needed. how about ARCH_ARM64 then?

CONFIG_CAVIUM_ERRATUM_22536 is exactly that you need. It is not only
used for core interrupts, e.g. also for gicv3 devices (and now also
for ahci). Non-core errata (e.g. CONFIG_CAVIUM_ERRATUM_23144) are not
enabled in the arm64 cpu errata framework (not handled in
arch/arm64/kernel/cpu_errata.c).

Thus,

#ifdef CONFIG_CAVIUM_ERRATUM_22536
	if (pdev->vendor == 0x177d && pdev->device == 0xa01c)
		ahci_thunderx_init(&pdev->dev, hpriv);
#endif

is the correct enablement of the workaround by device id.

And, CAVIUM_ERRATUM_* is very easy to handle, enable and document.

-Robert