On Sun, Jul 22, 2018 at 3:10 AM, Marek Vasut [off-list ref] wrote:

On 07/22/2018 12:23 AM, Olof Johansson wrote:
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You end up with user space tools trying to parse the kernel command
line to figure out what's on the flash, and other really bad habits.
:(

You should just read /proc/mtd , see
http://www.linux-mtd.infradead.org/doc/general.html

Sure, I know that but not everybody does, and they do it in bad ways
if given the opportunity.

That's what the documentation is for. I assume whoever is using MTD is
also capable of reading the documentation and thus avoiding such basic
mistakes.

Why read the docs when the information obviously is there when you grep it?

To do things correctly of course.

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I'm agreeing with you -- I've just seen it done wrong so many times.
Making it harder to make mistakes is always a good thing.

If you follow this argument through, you could say we should start
plucking out random files from /proc because someone might cluelessly
misuse them. This might lead to a system that's hard to misuse indeed,
but likely also quite useless.

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I'd strongly advice you to keep this in the board files, unless you
have an actual real motivation for changing it. This patch does not
provide one.

Partitioning is not hardware description, it should not be in DT.

Read my reply again. It's part of _platform_ description, and belongs in DT.

I can repartition the flash whichever way I want for my purposes, just
like I can repartition a harddrive. We don't have harddrive partitioning
in the DT, why should we have flash partitioning there ? How do those
differ ?

A drive can be auto-probed. We have partition tables on the disk that
self-describes the contents.

That doesn't exist on flashes, at least not usually. This is really
unfortunate, I wish we could change _that_ instead of argue here.

Technically it does. Consider the fact that those flashes have between
4-64 MiB in size and sector size of 256 kiB . If you waste one sector on
the 4 MiB flash for some sort of partition table, you waste 1/16th of
the flash, which is a lot.

The bootloader needs to know the flash layout and it must be in sync
with the kernel, so there has to be some sort of communication channel
between those two. I don't want to add handler for every possible
bootloader that can be used on those platforms into the kernel though.

The standard argument here, is always to have the bootloader patch up
the device tree to make it represent the actual device. We do this all
the time for things such as command line, memory sizes, ethernet
addresses, etc. This is no different.

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If you repartition your disk, you need to change the command line
arguments to the kernel too. It's definitely not an argument to
keeping it on the command line instead.

How so ? /dev/sdxy remains /dev/sdxy . Sure, you can use UUID and change
it, but that's a specific case.

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Also, you now have a dependencies between hardware description, probe
order and stable naming in the kernel, and the command line. With DT,
you have it all in one place, right next to the flash part. It doesn't
matter what name it gets probed at, and nobody needs to know the
mapping to get it right.

In this case it doesn't matter either, since you have only one flash device.

In this very narrow case maybe, but in general it's a horrible idea to
encourage people to use the command line instead of device tree for
this.

I'm still waiting on a compelling argument to motivate why this patch
should be applied. I don't think there is one, so let's not apply it.


-Olof