Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) client-ip=2620:137:e000::1:18;
MIME-Version: 1.0
References: <20220513201243.2381133-1-vladimir.oltean@nxp.com>
 <CAGETcx9vjrh_ORhGq0g5oH5kUE8MbcyEW4Mv9i=S8m9PLzBkhA@mail.gmail.com>
 <20220519151529.qkhlsjfkh6mebpqw@skbuf> <CAGETcx_eiuKnyTgzxGBzgakV7reQOEdfpsvVjAsUXVmCSvSttQ@mail.gmail.com>
In-Reply-To: <CAGETcx_eiuKnyTgzxGBzgakV7reQOEdfpsvVjAsUXVmCSvSttQ@mail.gmail.com>
From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Fri, 20 May 2022 09:02:05 +0200
Message-ID: <CAMuHMdUso+j5K8VYxTofg7Pw4jdH2xMMGBXGwBxpzRiQWX+ZNA@mail.gmail.com>
Subject: Re: [RFC PATCH devicetree] of: property: mark "interrupts" as
 optional for fw_devlink
To:     Saravana Kannan <saravanak@google.com>
Cc:     Vladimir Oltean <vladimir.oltean@nxp.com>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Robin Murphy <robin.murphy@arm.com>,
        Geert Uytterhoeven <geert+renesas@glider.be>,
        Rob Herring <robh+dt@kernel.org>,
        Frank Rowand <frowand.list@gmail.com>,
        John Stultz <john.stultz@linaro.org>,
        =?UTF-8?B?QWx2aW4g4pS8w6FpcHJhZ2E=?= <alsi@bang-olufsen.dk>,
        Florian Fainelli <f.fainelli@gmail.com>,
        Andrew Lunn <andrew@lunn.ch>,
        Russell King <rmk+kernel@armlinux.org.uk>,
        Heiner Kallweit <hkallweit1@gmail.com>,
        Android Kernel Team <kernel-team@android.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

Hi Saravana,

On Thu, May 19, 2022 at 10:00 PM Saravana Kannan <saravanak@google.com> wrote:
> There's still work to be done that might make this easier/cleaner in the future:
> 1. Adding a DT property that explicitly marks device A as not
> dependent on B (Rob was already open to this -- with additional
> context I don't want to type up here).
> 2. Adding kernel command line options that might allow people to say
> stuff like "Device A doesn't depend on Device B independent of what DT
> might say".

There are clearly cases where the hardware defines if the property
is optional or required, and a DT property would work.  However, in
general relaxing such dependencies involves a complex mix of hardware
capabilities, driver support, and system policies.
Examples:
  - A hardware block may support both DMA and PIO, or can require DMA,
  - DMA-capable devices can typically work without an IOMMU,
    unless all RAM lies outside the address space addressable by
    the DMA controller,
  - A driver may fall back to PIO if DMA is not available (yet),
    but doing so may not meet the required performance target,
  - Not using the IOMMU may violate anti-tampering policies.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds