[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 10/07/2017 18:34, Franklin S Cooper Jr wrote:
>
> On 07/07/2017 10:00 AM, Jean-Jacques Hiblot wrote:
>>
>> 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
> It would great if you can show the difference in time between todays,
> approach, your FIT approach gzipped and your FIT approach using lzo in
> terms of time between power on to SPL handling things off to U-boot.
I'll add the measurements in the next version.
> Also would it make sense to sacrifice saving some space by not
> compressing the entire FIT but instead compressing dtbs individually and
> storing it in an uncompressed FIT? This way any additional boot time is
> limited to uncompressing the DTB you need. It would be especially
> helpful in the case of a large amount of dtbs being stored, slower
> devices, or using compression algorithms that save more space but its
> decompression is slower.
It makes sense. I'll look into it.
I wasn't aware of your work in this area (CONFIG_FIT_EMBED). Now that is 
in mainline u-boot, I'll leverage on it as it mostly does the same thing.
At the end, I may just end up removing the restriction that prevents 
using CONFIG_FIT_EMBED in the SPL and move the decompression stage at 
the very end of fdtdec_setup()

JJ
>
>>> 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.
> Yeah I think thats the toughest thing with my approach. It only works in
> situations where loading the "next stage" is consistent between boards.
> In my case Keystone doesn't use SPL so I benefited from the ROM handling
> any board specific logic to load U-boot. Not sure if it will work well
> in SPL since loading U-boot can vary quite a bit.
>> Jean-Jacques