linux-kernel - RE: [PATCH 2/2] ARM: dts: imx6ull-dhcor: Add Marantec maveo box

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Date:   Thu, 6 Apr 2023 08:37:20 +0000
From:   Christoph Niedermaier <cniedermaier@...electronics.com>
To:     Marek Vasut <marex@...x.de>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
CC:     Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Shawn Guo <shawnguo@...nel.org>, Li Yang <leoyang.li@....com>,
        Fabio Estevam <festevam@...x.de>,
        NXP Linux Team <linux-imx@....com>,
        kernel <kernel@...electronics.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>
Subject: RE: [PATCH 2/2] ARM: dts: imx6ull-dhcor: Add Marantec maveo box

From: Marek Vasut [mailto:marex@...x.de]
Sent: Wednesday, April 5, 2023 8:47 PM
> On 4/5/23 20:24, Christoph Niedermaier wrote:
>> From: Marek Vasut [mailto:marex@...x.de]
>> Sent: Wednesday, April 5, 2023 6:25 PM
>>> On 4/5/23 18:02, Christoph Niedermaier wrote:
>>>
>>> [...]
>>>
>>>> +/ {
>>>> +     model = "DH electronics i.MX6ULL DHCOR on maveo box";
>>>> +     compatible = "dh,imx6ull-dhcor-maveo-box", "dh,imx6ull-dhcor-som",
>>>> +                  "fsl,imx6ull";
>>>> +
>>>> +     aliases {
>>>> +             /delete-property/ mmc0; /* Avoid double definitions */
>>>> +             /delete-property/ mmc1;
>>>> +             mmc2 = &usdhc2; /* eMMC should be mmc2 */
>>>
>>> Why not mmc0 ?
>>>
>>> Use root=PARTUUID= when booting to avoid any dependency on
>>> root=/dev/mmcblk2pN enumeration.
>>
>> This is due to software interchangeability with the DHCOM
>> i.MX6ULL, where the eMMC is always mmc2.
> 
> +CC Ulf , I vaguely recall some discussion about this enumeration and I
> am not sure one can really depend on that.

That why I think it good to have a defined number for mmcblk devices
on an embedded system. An excerpt from [1]:

Alternative solutions like PARTUUIDs do not cover the case where multiple
mmcblk devices contain the same image. This is a common issue on devices
that can boot both from eMMC (for regular boot) and SD cards (as a
temporary boot medium for development). When a firmware image is
installed to eMMC after a test boot via SD card, there will be no
reliable way to refer to a specific device using (PART)UUIDs oder
LABELs

[1] https://patchwork.kernel.org/project/linux-mmc/patch/20200825134441.17537-2-matthias.schiffer@ew.tq-group.com/

So far I have never had a problem with numbering mmcblk devices via aliases.

Regards
Christoph