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From: Arnd Bergmann <arnd@arndb.de>
Date: Mon, 11 Dec 2017 14:40:43 +0100
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Subject: Re: [PATCH 1/6] ARM: stm32: prepare stm32 family to welcome armv7 architecture
To: Linus Walleij <linus.walleij@linaro.org>
Cc: Ludovic Barre <ludovic.Barre@st.com>,
        Russell King <linux@armlinux.org.uk>, Rob Herring <robh+dt@kernel.org>,
        Maxime Coquelin <mcoquelin.stm32@gmail.com>,
        Alexandre Torgue <alexandre.torgue@st.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>
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On Mon, Dec 11, 2017 at 11:25 AM, Linus Walleij
<linus.walleij@linaro.org> wrote:
> On Fri, Dec 8, 2017 at 3:11 PM, Ludovic Barre <ludovic.Barre@st.com> wrote:
>
>> From: Ludovic Barre <ludovic.barre@st.com>
>>
>> This patch prepares the STM32 machine for the integration of Cortex-A
>> based microprocessor (MPU), on top of the existing Cortex-M
>> microcontroller family (MCU). Since both MCUs and MPUs are sharing
>> common hardware blocks we can keep using ARCH_STM32 flag for most of
>> them. If a hardware block is specific to one family we can use either
>> ARCH_STM32_MCU or ARCH_STM32_MPU flag.
>>
>> Signed-off-by: Ludovic Barre <ludovic.barre@st.com>

To what degree do we need to treat them as separate families
at all then? I wonder if the MCU/MPU distinction is always that
clear along the Cortex-M/Cortex-A separation, especially if
we ever get to a chip that has both types of cores. What
exactly would we miss if we do away with the ARCH_STM32_MCU
symbol here?

> So yesterdays application processors are todays MCU processors.
>
> I said this on a lecture for control systems a while back and
> stated it as a reason I think RTOSes are not really seeing a bright
> future compared to Linux.
>
> It happened quicker than I thought though, interesting.

I think there is still lots of room for smaller RTOS in the long run,
but it's likely that the 'MPU + external DRAM' design point will
shift further to Linux, as there isn't really a benefit in squeezing
in anything smaller when the minimum is 32MB or 128MB of
RAM, depending on the interface.

For on-chip eDRAM or SRAM based MPUs, that doesn't hold
true, the memory size is what drives the cost here.

        Arnd