[U-Boot] [U-boot][PATCH] keystone2: add support for UART download

[menon.nishanth at gmail.com]

On Wed, Feb 18, 2015 at 7:12 AM, Vitaly Andrianov <vitalya at ti.com> wrote:
>
>
> On 02/17/2015 05:47 PM, Nishanth Menon wrote:
>>
>> On Tue, Feb 17, 2015 at 4:27 PM, Murali Karicheri <m-karicheri2 at ti.com>
>> wrote:
>>>>>
>>>>> is complete the boot-loader sets the PC to the first MSMC address
>>>>> 0x0c000000. The u-boot.bin is linked to the address 0x0c001000.
>>>>
>>>>
>>>> why not just shift u-boot.bin to start of MSMC address?
>>>
>>>
>>> What is wrong with the current implementation? NAND and SPI NOR boot
>>> modes
>>> use the
>>> GPH headers that has the load address defined. But in the case of UART,
>>> RBL
>>
>>
>> So it GPH header has the load address defined, it does mean that we
>> could infact change the start address to 0xc000000 (instead of current
>> 0xc001000) and appropriately update the GPH headers to point there?
>> that way we can use 0xc000000 without padding on UART, as well as use
>> the same in NAND/SPI as well? correct?
>>
>>> loads it to start of MSMC and adding 1K of NOP just avoid a jump
>>> instruction
>>> at
>>> the start of the memory to jump to 0xc001000. This way we can keep the
>>> same
>>> start address across all boot modes.
>>
>>
>> Padding a 4kbytes (1K NOP at 32bits each) just because there is a
>> difference between linked address and start address in a specific mode
>> makes one wonder. This probably is not definitely a uniquely KS2 issue
>> - we probably have similar behavior on other platforms as well. what
>> if we chose a link address 2MB away (as an example)? agreed that the
>> specific usage has no such size story in place, but conceptually we
>> might be able to do better.
>>
>>>>> In order to use the u-boot.bin as an image for UART download, we need
>>>>> to
>>>>> add 4K zeros prefix that act as 1K NOP instructions before reaching
>>>>> 0xc001000.
>>>>
>>>>
>>>> OR, add a relocation logic which saves the 1k NOP and resultant load
>>>> time?
>>>
>>>
>>> What saving are you talking about? Miliseconds? seconds?
>>
>>
>> Maintainability? lets say we change link address tomorrow, we have to
>> adjust padding appropriately, further we just dont need padding when
>> we can just relocate self by being position independent in the first
>> place!.
>>
>> we have learnt that over years OMAP3 link address has gone through a
>> few transitions as we discovered better ways to do things. doing
>> padding based on link address does, on the first look, seem
>> unnecessary, makes sense only if all of the following are wrong:
>> a) cannot change start address to the common start address for all boot
>> modes.
>> b) cannot add relocation and position independent u-boot code.
>>
>> And even when we do need to add padding, it is not a good idea to hard
>> code the pad size, instead do it algorithmically (basically query the
>> start and add the delta) allowing changes to link address to be
>> something folks can do at a later point in time without
>> unintentionally breaking uart boot.
>>
[...]
> As I've already mentioned this patch is not about improving or changing
> current u-boot.bin, but just providing a way to download it over UART port.
> Any improvements, if they are required, shall be done in other patches.


We would not need a u-boot.uart if current u-boot.bin can do the job, do we?

--
Regards,
Nishanth Menon