linux-kernel - Re: [PATCH v2 0/4] Patches to allow consistent mmc / mmcblk numbering w/ device tree

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Date:	Fri, 29 Apr 2016 13:04:48 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Ulf Hansson <ulf.hansson@...aro.org>,
	Jaehoon Chung <jh80.chung@...sung.com>,
	Shawn Lin <shawn.lin@...k-chips.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Stefan Agner <stefan@...er.ch>,
	"linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
	Brian Norris <computersforpeace@...il.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Heiko Stuebner <heiko@...ech.de>,
	Jisheng Zhang <jszhang@...vell.com>,
	"open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
	devicetree-spec@...r.kernel.org,
	Mark Rutland <mark.rutland@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Venu Byravarasu <vbyravarasu@...dia.com>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Jon Hunter <jonathanh@...dia.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Grant Grundler <grundler@...omium.org>,
	Kumar Gala <galak@...eaurora.org>,
	"Luca Porzio (lporzio)" <lporzio@...ron.com>,
	Rob Herring <robh+dt@...nel.org>,
	Chaotian Jing <chaotian.jing@...iatek.com>,
	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
	Sudeep Holla <sudeep.holla@....com>,
	zhonghui.fu@...ux.intel.com, kirill.shutemov@...ux.intel.com
Subject: Re: [PATCH v2 0/4] Patches to allow consistent mmc / mmcblk numbering
 w/ device tree

Russell,

On Fri, Apr 29, 2016 at 12:57 PM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
>> * Presumably on a PC you've got an extra bit in the middle (like grub
>> or something like that) that can help you resolve your UUIDs even if
>> you get your kernel from somewhere else.
>
> You are over-estimating what grub does.  Grub doesn't resolve UUIDs at
> all.  Grub just passes the kernel arguments in its configuration file
> for the entry it is booting to the kernel.  It's a static configuration
> found in /boot/grub/grub.conf.
>
> It doesn't probe devices for UUIDs.

OK.  The point was: if folks on PCs have a workflow that works for
them, wonderful.  That workflow doesn't work so great for me.  My
workflow doesn't hurt them.  Why is it bad?


>> * Presumably in the non-embedded world kernel hackers have a different
>> workflow.  They probably don't swap between different devices with
>> different configurations on an hourly basis.  They're not in the habit
>> of totally reimaging their system periodically.  Etc.  Trying to force
>> the workflow of a PC kernel hacker and an embedded kernel hacker to be
>> the same doesn't seem like a worthwhile goal.
>
> In _my_ world with the "embedded" devices I have, I mount by UUID on
> platforms which have multiple MMC devices to avoid exactly the problem
> you're having.  This works fine.
>
> If I were to switch the SD card, and I wanted to avoid changing the
> boot loader configuration, I'd use label instead, and I'd label all
> the SD card rootfs using the same label so I could just swap the cards.

OK.  The point was: if you have a workflow that works for you,
wonderful.  That workflow doesn't work so great for me.  My workflow
doesn't hurt you.  Why is it bad?


>> * Presumably an embedded kernel hacker running with ATA / SCSI could
>> _usually_ assume that "sda" is his/her root filesystem.  It's unlikely
>> an embedded system would have more than one "sda" disk builtin and
>> it's nearly guaranteed (I think) that a builtin ATA / SCSI controller
>> would probe before any USB based devices.
>
> You've got a funny view again.  N2100 has two hard disks.  The clearfog
> board from SolidRun has two mini-PCIe slots, each of which can have two
> SATA interfaces... If you want to use it as a server-type platform with
> lots of disks...

OK.  The point was: if you have a workflow that works for you,
wonderful.  That workflow doesn't work so great for me.  My workflow
doesn't hurt you.  Why is it bad?


>> Sure, if your root
>> filesystem is USB based (really?) and you've got additional USB
>> storage devices then you're SOL.  Sorry.
>
> One of my Versatile Express platforms boots from USB, and has a MMC
> slot...  So this argument does not stack up.

OK.  The point was: if you have a workflow that works for you,
wonderful.  That workflow doesn't work so great for me.  My workflow
doesn't hurt you.  Why is it bad?


-Doug