recommended action for bootloaders regarding modifying device-tree nodes

On Thu, Jan 30, 2014 at 1:15 PM, Jason Gunthorpe
[off-list ref] wrote:

On Thu, Jan 30, 2014 at 03:45:58PM -0500, Jason Cooper wrote:

quoted

This is more of a process question:  Is there any information captured
in your EEPROM that can't be represented in the dtb?  iow, at the point
when you write the EEPROM, why not write the dtb to it as configured?

I can share what we do here.. In our systems the serial EEPROM is only
256 bytes, so storing things in DT format would be challenging.

What we do is have a master DTB that has the union of all our
configurations. The boot process has a very simple bit of code that
runs down the DTB in binary format and replaces entire
OF_DT_BEGIN_NODE->OF_DT_END_NODE regions with OF_DT_NOP.

The NOP approach is very simple, no other changes (eg offset
recalculation) needs to be done to the DT, so we can do this process
with a very small code footprint and without libfdt.

Choosing which sections to drop is done with some combination of
hardwired code and searching for specific property patterns. There are
also a few places where placeholder sections are directly fixed up, eg
a mac address is written into a placeholder of 0s, etc.

So an example might be

   optional_peripheral at 10000 {
      orc,board-style = <1>;
      [..]
   }

Eg The board-style number comes from the EEPROM and if board-style !=
1 then the entire stanza is replaced with NOP.

Jason

Jason,

Sounds pretty much like what we are doing.  I am using u-boot and my
current code looks like this:

        /*
         * Peripheral Config:
         *  remove nodes by alias path if EEPROM config tells us the
         *  peripheral is not loaded on the board.
         */
        if (!test_bit(EECONFIG_ETH0, info->config))
                fdt_del_node_and_alias(blob, "ethernet0");
        if (!test_bit(EECONFIG_ETH1, info->config))
                fdt_del_node_and_alias(blob, "ethernet1");
        if (!test_bit(EECONFIG_HDMI_OUT, info->config))
                fdt_del_node_and_alias(blob, "hdmi_out");
        if (!test_bit(EECONFIG_SATA, info->config))
                fdt_del_node_and_alias(blob, "ahci0");
        if (!test_bit(EECONFIG_PCIE, info->config))
                fdt_del_node_and_alias(blob, "pcie");
        if (!test_bit(EECONFIG_SSI0, info->config))
                fdt_del_node_and_alias(blob, "ssi0");
        if (!test_bit(EECONFIG_SSI1, info->config))
                fdt_del_node_and_alias(blob, "ssi1");
...

I've submitted my code to u-boot and have been asked if its more
appropriate to remove nodes as I'm doing above or to mark them as
'disabled'.  From what I can tell there really isn't a rule or
recommendation for this so I think I'll keep doing what I'm doing
above.

Thanks!

Tim