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

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Message-ID: <50c88cc4-e046-6c43-2d35-116d1d4ea2f8@denx.de>
Date:   Thu, 6 Apr 2023 16:10:16 +0200
From:   Marek Vasut <marex@...x.de>
To:     Christoph Niedermaier <cniedermaier@...electronics.com>,
        "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

On 4/6/23 10:37, Christoph Niedermaier wrote:
> 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]:

I might be misremembering this, but could it be that, if any non-OF 
SDMMC controller probes early and hogs the /dev/mmcblk2 before the OF 
ones have a chance to probe, then the OF ones would fail to probe ?

> Alternative solutions like PARTUUIDs do not cover the case where multiple
> mmcblk devices contain the same image.

I agree, this is indeed a downside of PARTUUID .

> 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

This can be solved by the installer updating the PARTUUID on the eMMC 
however.

> [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.

Based on the above, I don't think either the aliases or PARTUUID is a 
perfect solution, but the aliases should be fine for mx6ull at least?
So I think we can conclude this discussion thread ?