linux-kernel - Re: [PATCH] mmc: omap_hsmmc: fix initialization order of mmc block devices

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Message-ID: <20151013140043.GT23893@bill-the-cat>
Date:	Tue, 13 Oct 2015 10:00:43 -0400
From:	Tom Rini <trini@...sulko.com>
To:	Nishanth Menon <nm@...com>
Cc:	Lokesh Vutla <a0131933@...com>, hs@...x.de,
	lkml <linux-kernel@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>, linux-mmc@...r.kernel.org,
	Dirk Behme <dirk.behme@...ch.com>, Georg.Soffel@...ch-si.com,
	linux-omap <linux-omap@...r.kernel.org>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	u-boot <u-boot@...ts.denx.de>
Subject: Re: [PATCH] mmc: omap_hsmmc: fix initialization order of mmc block
 devices

On Tue, Oct 13, 2015 at 08:24:08AM -0500, Nishanth Menon wrote:
> On Tue, Oct 13, 2015 at 3:03 AM, Lokesh Vutla <a0131933@...com> wrote:
> >
> >
> > On Tuesday 13 October 2015 01:14 PM, Heiko Schocher wrote:
> >> Hello Lokesh,
> >>
> >> Am 13.10.2015 um 08:46 schrieb Lokesh Vutla:
> >>> +Nishanth,
> >>>
> >>> On Tuesday 13 October 2015 10:59 AM, Heiko Schocher wrote:
> >>>> On embedded devices, often there is a combination of
> >>>> removable mmc devices (e.g. MMC/SD cards) and hard
> >>>> wired ones (e.g. eMMC). Depending on the hardware
> >>>> configuration, the 'mmcblkN' node might change if
> >>>> the removable device is available or not at boot time.
> >>>>
> >>>> E.g. if the removable device is attached at boot time,
> >>>> it might become mmxblk0. And the hard wired one mmcblk1.
> >>>> But if the removable device isn't there at boot time,
> >>>> the hard wired one will become mmcblk0. This makes it
> >>>> somehow difficult to hard code the root device to the
> >>>> non-removable device and boot fast.
> >>>
> >>> Why not use "root=PARTUUID=${uuid}" option instead of relying on
> >>> mmcblk no?
> >>> U-Boot can easily detect your partuuid. Refer to [1] on how TI platforms
> >>> does this in u-boot.
> >>
> >> Good tip ... I do not know, if it is possible to update U-Boot
> >> on this boards...
> >>
> >> Current U-Boot says:
> >> U-Boot 2013.01.01_heads/master-gc7900a0 (2015-05-06 - 20:37:15)
> >>
> >> I2C:   ready
> >> DRAM:  512 MiB
> >> [...]
> >> U-Boot# mmc rescan
> >> U-Boot# mmc part
> >>
> >> Partition Map for MMC device 0  --   Partition Type: DOS
> >>
> >> Part    Start Sector    Num Sectors     UUID            Type
> >>   1     63              144522          000ce343-01     0e Boot
> >>   2     144585          659861          000ce343-02     83
> >> U-Boot# part uuid mmc 0:2 uuid
> >> Unknown command 'part' - try 'help'
> >> U-Boot#
> >>
> >> So, if this patch has no chance for mainline, please let me
> >> know it, thanks!
> >>
> >
> > IIRC, Nishanth had posted a patch something similar but got rejected for
> > some reason. Probably Nishanth can comment more here.
> >
> 
> overall the feedback I received was for block devices, there is
> already an unique method(PARTUUID/uuid) of referencing required device
> and mmcxblky aliasing was not really needed - hence dropped my patch
> and switched over to partuuid.

Not telling the kernel what to do here but root=PARTUUID=$x is the long
standing portable multi-architecture (and storage medium) way to have a
stable name for your root device.  The automatic way of digging this
information out in U-Boot is dates back to Sept 2012 in mainline.

-- 
Tom

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