[U-Boot-Users] Question about CFG_ENV_ADDR during RAMBOOT

[Håvard Skinnemoen]

On 5/28/07, Ladislav Michl <ladis at linux-mips.org> wrote:
> Principle described above has few serious drawbacks. Storage is no
> longer storage, but randomly mapped memory. As a u-boot programer you
> have to determine such memory map and as a u-boot user you have to
> remember it.

Exactly. Giving storage devices which aren't memory mapped a "virtual"
address is a bad idea. And the database example is crap because it
depends on hardware paging, which isn't being proposed here (how many
architectures supported by u-boot can do 64-bit virtual address in
hardware?)

That said, I do think Ulf has a point -- being able to compare a block
of data in nand- or dataflash with something in memory (or other kinds
of storage) is a pretty neat feature. But it should not be done by
using "virtual" addresses (due to the problems described above), and
it should be implemented in a generic way. Pouring #ifdefs all over
cmd_mem.c is short-sighted and selfish.

I'm not sure if anyone's suggested this before, or whether it was
well-received or not, but why don't we generalize the cmd_mem syntax a
little? For example, let every memory address can be prefixed by an
optional "storage specifier" and a ':' character. The default "storage
specifier" is "mem", so that if it is omitted everything will work as
before, but you can also do things like

  cp.b df0:0x4200 0x85000000
  cmp.l 0x24000000 nand1.1:0x123456789

and so on. Whatever comes after ':' is passed to the storage handler
as a string, so if some storage types need 64-bit offsets, the handler
can support it without disturbing anything else.

What do you think about something like that? The actual syntax
probably needs some work to get right, and we need to take care to
ensure backwards compatibility of course.

This does actually come quite close to Ulf's proposal, but instead of
requiring the user to remember a "virtual address", we instead require
him to enter a logical device and an offset, ie.. kind of a simple VFS
without any actual files involved...

Haavard