Re: [PATCH] socfpga: Find partition with environment via device tree

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

On Fri, Dec 11, 2015 at 08:05:00PM +0000, Trent Piepho wrote:
> On Fri, 2015-12-11 at 10:30 +0100, Sascha Hauer wrote:
> > On Fri, Dec 11, 2015 at 02:13:21AM +0000, Trent Piepho wrote:
> > The "barebox,environment" compatible describes a binding which takes
> > partition described in device-path as a raw file containing a barebox
> > envfs.
> 
> Well, I redefined it to have the device-path be the device containing
> the environment as raw data or the one containing a filesystem with the
> environment as a file if the architecture expects that.
> 
> That doesn't seem like a bad definition.

No, indeed not. It's more the implementation I have a problem with. The
implementation should be done in the environment driver.

> 
> > What you have here is a partition which contains a FAT which has a
> > barebox envfs in a file. I think it's fine to extend the binding (and
> > the barebox env driver) to handle that case.  What you are doing here is
> > to smuggle code behind the driver and bend the environment path to
> > somewhere else. That is not ok. I think what we can do is to add some
> > file-path property which contains the path and tells the barebox
> > environment driver to not use the whole partition but instead to mount
> > it and use the file specified in the file-path property.
> > 
> > As an alternative which I might even like better is to extend the
> > partition binding to be able to describe a file in a partition, like:
> 
> This won't work.  mmc devices, which all boards that use a file for the
> env are using, don't have partitions in the device tree, since they are
> created dynamically from the partition table.  There is nowhere to put
> the file node.

Just because they are dynamically created from information found on the
MMC card doesn't mean we can't describe them in the device tree. We can
for example specify "The second partition" or "The partition with UUID
xy".
We have the same problem sometimes with USB or PCI devices aswell. The
busses are autoprobable, so the devices are usually not described in the
device tree. Nevertheless sometimes a USB device has an additional reset
pin or regulator which is not autodetectable (worse even, the device may
not show up on the bus without handling the regulator first). For these
reasons having a descripion of devices on autoprobable busses is
desirable. I think partitions fall into the same category.

> 
> I did think about extending the env driver to support files and put the
> mounting code in there.  It would bloat it for all those platforms that
> don't use files (but it would mean all platforms could use files and
> socfpga could use a raw device...)

If it bloats the code too much we could make it configurable, that
shouldn't be a problem.

> 
> It is also problematic for my board.  I need to change the env path
> based on data in an eeprom that selects which partition to use.  Other
> boards that have multiple environments select them in a
> postcore_initcall using boot strapping pins that aren't part of a
> device.  I'm using an eeprom so I need to do the selection later in the
> init process.

Ok, so you couldn't you do the same?

> 
> Maybe extend load_environment to do the mount?  const char
> *default_environment_path would have to change to something that could
> support the concept of a device + optional filename.  This way one could
> change the env path upto environment_initcall() time.

My original plan was to extend this:

	device-path = &mmc, "partname:1";

with a filename, like this:

	device-path = &mmc, "partname:1", "/bla/env";

I found out that this is not how the device tree should be like. It
looks and feels better when the phandle directly points to a node and
the node describes a partition (or file in this case).

Sascha

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