[U-Boot] [PATCH] spl: dm: Make it possible for the SPL to pick its own DTB from a FIT

[Jean-Jacques Hiblot <jjhiblot@ti.com>]

On 07/07/2017 16:30, Tom Rini wrote:
> On Fri, Jul 07, 2017 at 01:44:39PM +0200, Jean-Jacques Hiblot wrote:
>
>> u-boot can be embedded within a FIT image with multiple DTBs. It then
>> selects at run-time  which one is best suited for the platform.
>> Use the same principle here for the SPL: put the DTBs in a FIT image,
>> compress it (LZO, GZIP, or no compression) and append it at the end of the
>> SPL.
>>
>> Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
>> ---
>>
>> The impact in terms of boot time is not high when using LZO but I gues it can vary
>> from platform to platform.
>> The size of the SPL binary is increased (1.5kB more code on ARM), but the compression
>> really flattens the DTBS. so at the end of the day, enabling this option doesn't add much.
>>
>> Here are some sumbers with a DRA7 platform (numbers in bytes):
>>                            size  delta with ref
>>    MLO.reference         123450
>>    MLO.lzo_1_DTB         123715    +265
>>    MLO.lzo_4_DTB         124237    +787
>>    MLO.gzip_4_DTB        132006    +8556
>>    MLO.no_comp_4_DTB     134184    +10734
> Bearing in mind that you said in a follow up this is RFC, I'm ignoring
> all of the stuff that I believe you would fix in a v1.  At the heart of
> it, are you able to tell different boards before you have a DTB loaded?
In my case (DRA7) the board can be identified before the dtb is loaded. 
The identification
is done by reading an eeprom on I2c. It just can't be using DM I2C in 
the SPL.
> I keep coming back to the problem that for SPL it seems like we should
> be able to do what Franklin did for keystone
> (https://patchwork.ozlabs.org/patch/777242/) and have a 'fake' dts file
> that represents the SoC such that any real boards can get U-Boot loaded
> and from there have the real board dtb be used.  Or am I forgetting
> something?  Thanks!
That's more or less the situation now with DRA7. This idea comes from 
the need to accelerate the fastboot path by using the HS200 mode of the 
eMMC. There we need to configure the lines of the eMMC and this 
information is typically found in the DTB and differs from SOC to SOC. 
At the moment in the ti tree this is done by duplicating this 
information in C structures protected with #ifdef CONFIG_SPL_BUILD. 
While this works, it would be nicer to get it from the dtb as done in 
u-boot.

Jean-Jacques
>