[rpi bsp] add cmdline options

[Pavel Pisa]

Hello Qiao Yang,

On Friday 24 of July 2015 18:04:47 QIAO YANG wrote:
> Some updates for you. I've found out that my previous problem with cmdline
> due to my minicom didn't wrap up the lines.... I've chosen the interface of
> vc to retrieve cmdline so that we don't need to parse the atag nor
> devicetree. Now we can use --video=...  to choose resolution and
> --console=... to choose console.

I have done only some short test attempt of local build on Friday
but without serial port and I get to more probably only on Wednesday.

As for the interface to command line through VC, I expect that
this interface allows access only to value set in config.txt
but it is not changed by U-boot if used. If that is true
then use of ATAG or device tree would be better for flexibility.
It would be reusable on other ARM targets as well.
But that this can be tried in some follow up project.

> Some notes:
>
> 1. the rpi's cmdline can be setup by a plain-text, single-line file
> cmdline.txt.

Yes that is good/easy to use.

> 2. I've implemented only 32-bpp. So the bpp is ignored. I may 
> add other pixel drawing mode if necessary.

May it be 16-bpp has some value for some cases. It takes less memory
and can be faster. But 32-bpp (or 24-bpp aligned to 32-bits) is most
common need.

> 3. If no device connected to 
> hdmi port, the display size would be 656x416. I use this as a probe.

> 4. --console=fbcons will select the fb console. Otherwise we will use the
> default option (serial console).

> 5. If we pass supported parameters to  videocore, it will correct
> it and return the value configured. So I didn't port the part of
> code which select resolution from a list, but this lead to 
> a behavior that we can pass some parameters like 12800x645  and get
> everything displayed.

That is OK, I expect that most common use is to define "auto"
or nothing. But I prefer if keyword "auto" is supported.
I have not looked if you support that.

> I wonder if we have to limit the resolution in a mode_list.

If videocore behaves sane and rounds/adjusts parameters
then I think that more processing/checking is not
required in your code.

Best wishes,

               Pavel