linux-kernel - Re: [PATCH 09/10] powerpc: Enable OpenCAPI Storage Class Memory driver on bare metal

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Date:   Thu, 30 Jan 2020 21:14:33 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Alastair D'Silva" <alastair@....ibm.com>
Cc:     Frederic Barrat <fbarrat@...ux.ibm.com>,
        Oscar Salvador <osalvador@...e.com>,
        Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        David Hildenbrand <david@...hat.com>,
        Wei Yang <richard.weiyang@...il.com>,
        Keith Busch <keith.busch@...el.com>,
        Linux MM <linux-mm@...ck.org>, Michal Hocko <mhocko@...e.com>,
        Paul Mackerras <paulus@...ba.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Dave Jiang <dave.jiang@...el.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Anju T Sudhakar <anju@...ux.vnet.ibm.com>,
        Mahesh Salgaonkar <mahesh@...ux.vnet.ibm.com>,
        Andrew Donnellan <ajd@...ux.ibm.com>,
        Arnd Bergmann <arnd@...db.de>, Greg Kurz <groug@...d.org>,
        Qian Cai <cai@....pw>,
        Cédric Le Goater <clg@...d.org>,
        Hari Bathini <hbathini@...ux.ibm.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Vasant Hegde <hegdevasant@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH 09/10] powerpc: Enable OpenCAPI Storage Class Memory
 driver on bare metal

On Thu, Jan 30, 2020 at 8:57 PM Alastair D'Silva <alastair@....ibm.com> wrote:
>
> On Fri, 2019-11-08 at 08:10 +0100, Frederic Barrat wrote:
> >
> > Le 25/10/2019 à 06:47, Alastair D'Silva a écrit :
> > > From: Alastair D'Silva <alastair@...ilva.org>
> > >
> > > Enable OpenCAPI Storage Class Memory driver on bare metal
> > >
> > > Signed-off-by: Alastair D'Silva <alastair@...ilva.org>
> > > ---
> > >   arch/powerpc/configs/powernv_defconfig | 4 ++++
> > >   1 file changed, 4 insertions(+)
> > >
> > > diff --git a/arch/powerpc/configs/powernv_defconfig
> > > b/arch/powerpc/configs/powernv_defconfig
> > > index 6658cceb928c..45c0eff94964 100644
> > > --- a/arch/powerpc/configs/powernv_defconfig
> > > +++ b/arch/powerpc/configs/powernv_defconfig
> > > @@ -352,3 +352,7 @@ CONFIG_KVM_BOOK3S_64=m
> > >   CONFIG_KVM_BOOK3S_64_HV=m
> > >   CONFIG_VHOST_NET=m
> > >   CONFIG_PRINTK_TIME=y
> > > +CONFIG_OCXL_SCM=m
> > > +CONFIG_DEV_DAX=y
> > > +CONFIG_DEV_DAX_PMEM=y

This specific line is not needed since DEV_DAX_PMEM already defaults to DEV_DAX.

> > > +CONFIG_FS_DAX=y
> >
> > If this really the intent or do we want to activate DAX only if
> > CONFIG_OCXL_SCM is enabled?
> >
> >    Fred
>
> We had a bit of a play around with reworking this the other day.
>
> Putting them in as depends didn't make sense, as they are "soft"
> dependancies - the driver works and you can do some things without DAX.
>
> Adding them as selects was rejected as selecting symbols that can also
> be manually select is discouraged.
>
> We ended up going full circle and adding them back to the defconfig.

This dovetails with a suggestion Dave made a while back [1]. Given all
the pieces that need to be turned on to have a "feature complete"
persistent memory enabled build it would be nice to have general
config symbols that go and select all the necessary dependencies for
DAX, and let the rest happen by default.

[1]: https://lore.kernel.org/lkml/20161129021052.GF28177@dastard/