linux-kernel - Re: [PATCH 2/2] nouveau: fix dependencies for DEVICE

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Date:   Fri, 8 May 2020 22:39:38 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Jason Gunthorpe <jgg@...lanox.com>
Cc:     Ben Skeggs <bskeggs@...hat.com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Dan Williams <dan.j.williams@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        ML nouveau <nouveau@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] nouveau: fix dependencies for DEVICE_PRIVATE

On Fri, May 8, 2020 at 9:04 PM Jason Gunthorpe <jgg@...lanox.com> wrote:
>
> On Fri, May 08, 2020 at 05:05:03PM +0200, Arnd Bergmann wrote:
> > On Fri, May 8, 2020 at 5:00 PM Jason Gunthorpe <jgg@...lanox.com> wrote:
> > >
> > > On Fri, May 08, 2020 at 04:40:09PM +0200, Arnd Bergmann wrote:
> > > > CONFIG_DEVICE_PRIVATE cannot be selected in configurations
> > > > without ZONE_DEVICE:
> > >
> > > It is kind of unfortunate to lift dependencies from DEVICE_PRIVATE
> > > into the users, is this really how kconfig is supposed to work or is
> > > something else wrong here?
> >
> > Usually the problem is trying to use 'select' for something that can
> > be done with 'depends on'. I have actually no idea what
> > CONFIG_DEVICE_PRIVATE does, as it lacks a help text
> > and is a rather generic term.
>
> It is a sub mode of ZONE_DEVICE, ie DEVICE_PRIVATE turns on a certain
> kind of ZONE_DEVICE page.
>
> Both ZONE_DEVICE and DEVICE_PRIVATE are APIs families drivers use,
> there is no reason for a user to select either of these directly.

Ok, then how about making ZONE_DEVICE a hidden symbol and adding
something like

config ZONE_DEVICE_POSSIBLE
        def_bool y
        depends on MEMORY_HOTPLUG
        depends on MEMORY_HOTREMOVE
        depends on SPARSEMEM_VMEMMAP
        depends on ARCH_HAS_PTE_DEVMAP

then drivers that want it can do

config FOO
        tristate "user visible option"
        depends on ZONE_DEVICE_POSSIBLE
        select ZONE_DEVICE

      Arnd