Re: /dev/disk0 vs /dev/mmc0

[Sascha Hauer <s.hauer@pengutronix.de>]

On Mon, Oct 07, 2013 at 11:57:35AM +0200, David Jander wrote:
> On Mon, 7 Oct 2013 08:41:11 +0200
> Sascha Hauer <s.hauer@pengutronix.de> wrote:
> 
> > On Mon, Oct 07, 2013 at 08:32:03AM +0200, David Jander wrote:
> > > 
> > > Dear Sascha,
> > > 
> > > On Sun, 6 Oct 2013 12:39:50 +0200
> > > Sascha Hauer <s.hauer@pengutronix.de> wrote:
> > > > 
> > > > It doesn't interfere with the kernel. The kernel currently ignores this
> > > > aliases. There are patches floating to let the kernel honor this
> > > > aliases, but then they should simply have the same effect as they have
> > > > in barebox.
> > > 
> > > That doesn't _feel_ right. Changing standard device names in Linux via
> > > aliases in the DT might be a fancy idea, but it'd have a different
> > > reason/use-case than in the case of barebox. IMHO using the same DT for
> > > both seems to be the Right Thing (tm) to do, but then the semantics must
> > > be the same also. If I need aliases in the DT only to be able to tell
> > > devices apart from each other in barebox, while in Linux the effect would
> > > only be a rather inconvenient renaming of devices with no other added
> > > value, I think we need a different way to differentiate devices in
> > > barebox. Why not just use a simple driver-provided prefix (mci, mmc, usb,
> > > sata, etc...) for now?
> > 
> > That's not enough. We also need a fixed numbering. Otherwise a
> > nonremovable eMMC and a removable SD card change their device names
> > depending on the detect order.
> 
> True. Can host->dev.id be used? Should be fixed AFAICS...

host->dev.id depends on the probe order.

> 
> > Using aliases to provide a numbering is done in the Kernel aswell at
> > least for gpios, uarts and i2c busses, so expanding this scheme to
> > mmc/sd slots doesn't feel too wrong to me.
> 
> Yes, but we are doing it in barebox now only because there is no other way to
> tell devices apart from each other. While the MMC device being called
> "mmcblk0" or whatever in Linux is perfectly fine (no need for alias),

Linux has exactly the same problem. There are enough systems on which
the eMMC changes its name depending on a SD card being plugged in or
not.

> in
> barebox the device is now named "disk0", and there is no way of knowing what
> "disk0" actually is. Using DT-aliases for that purpose seems wrong to me.
> Why was this changed anyway? Introduction of some common "disk" layer (like
> scsi-disk on Linux)? Or just for the sake of confusing it with other
> "disk"-like devices?

It has always been diskx on barebox, this behaviour hasn't changed.

Sascha

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