[U-Boot] [RFC PATCH 1/5] spl: dfu: add dfu support in SPL

[B, Ravi]

Hi Lukasz

>> without fat/ext4, mmc support. But all device support may increase 
>> size.

> Ok.

> However, adding fat/ext4/mmc (and other) support should be on demand (and enabled by proper Kconfig options).
> This would allow others to add only what is really needed.

True, we provide incremental build option on need basis.  

>> 
>> >If yes, then even BBB's SPL can support DFU without any problems 
>> >(105KiB < 128 KiB).
>> 
>> You mean BBB must have 128KB ? Can you confirm.

>I didn't find any _hard_ rule about the size.

>In the am335x_evm.h file the spice reserved for SPL (on raw eMMC) is 128 KiB.

Are you referring to eMMC raw boot option ?

>> If BBB is support SPI boot, flashing MLO/U-boot to SPI-flash through 
>> SPL-DFU/SF would be sufficient right ?

>I don't know the exact use cases, but yes, BBB can boot from SPI flash.

>I'm just wondering - the use case for your board is to use USB to flash your device in u-boot SPL. 
>If I might ask, why cannot you wait for the U-Boot to use fully-fledged DFU for flashing?

Full-fledged DFU already supported in u-boot. 
The problem here is, how to flash the images first time to fresh boards to 	QSPI or eMMC device, where there is no MMC/SD boot option available. 
The solution to this problem is use peripheral USB boot mode (configuring sysboot switches), where the ROM loads the intial SPL(+DFU builtin for spi/eMMC) to IRAM, then run dfu/sf or dfu/eMMC to flash the binaries from PC using USB interface.
Refer to SPL-DFU support based on 2014.07 u-boot http://www.ti.com/lit/an/sprac33/sprac33.pdf. 

The SPL-DFU definitely helpful in production/development, where just connect EVM to PC through USB cable, and flash the MLO/U-BOOT, binaries to selected device (QSPI/eMMC).

I think based on discussion we had, some conclusion we could arrive, 
1) SPL size is constraint based on IRAM size, that all platform cannot be supported by default.
2) Kconfig option to compile only required device (eMMC, SPI)
3) Kconfig option for fat/ext4 support.
4) Try using common/spl/spl_fat{ext}.. to reduce footprint.

Regards
Ravi