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Re: Cortex port

Hi Simon.

On Wed, 2008-09-10 at 15:36 +0200, wrote:

> As some of you know, I'm currently writing a port for the Cortex-M3, 
> specifically the STM32 CPU. I'm making good progress.

Good to know. I guess they must have payed you after all! :)
I started writing off a Cortex M3 port myself but will stall now
awaiting your contribution so that I can patch in bits for the Luminary
Micro Stellaris LM3S6965EVB board.

>  The following is 
> in place and working so far:
> * clock configuration using CDL (number of options might be a bit 
> excessive, but the STM32 offers quite a few possibilities for clock 
> configuration, see below)
> * basic initializiation of cpu and startup code (clocks, flash, 
> relocation etc.)
> * serial debug/diagnostic driver (ethernet is not available on STM32)
> * simple "framework" for GPIO configuration and manipulation
> * partly (as I go) written register definitions for the STM32 (the STM32 
> provided "header" files are pretty unusable)

That's awesome. I got a very basic arch+variant+platform definition
+init code and linker script going. The board I had access to was away
so I started using qemu instead and that helped.

> The next step will be the implementation of IRQ handling and context 
> switching, which is not done yet, and will be rather tricky. During 
> development a few questions came up, which I hope some of you might 
> comment on:
> * In quite a few places I have to set and clear individual bits of 
> registers. With the normal HAL_WRITE/READ_UINT32 access macros this is 
> obviously a bit tedious sometimes. I didn't found any SET/CLEAR BIT 
> macros. Is there a good reason for this? I have now written my own 
> macros and added them to the hal_io.h headerfile. Any objections?
> * I currently have prefixed all my STM32 register macros (var_io.h) with 
> CYGARC_HAL_STM32_. I wonder if this is necessary, or the right way. It 
> seems that different ports don't follow the same guidelines here. I'd 
> much prefer to just prefix them with STM32_ as it would make the 
> definitions rather a bit simpler to read and type.
> * I have prefixed "functional macros" in var_io.h with 
> HAL_CORTEXM_STM32_. This seems to be the preferred way. Anyway, is 
> var_io.h the right place for things like GPIO configuration and 
> manipulation, enabling/disabling clocks? As for example configuration of 
> GPIO ports is rather too complex to be done in a macro, I have 
> implemented a helper function in the stm32_misc.c of the variant. The 
> macro then points to this function. Is this OK, or should the complete 
> GPIO framework, as well as other helper for things like 
> enabling/disabling/reseting clocks be moved to stm32_misc?
> * I have currently added quite a few CDL options for the clock 
> configuration. Things like clock selections, prescalers, nominal clock 
> speeds etc. are all available for editing or viewing. Also the 
> configuration is checked with a few constraints, so that no invalid 
> clock configurations can be made. As there are about 20 options (10 of 
> them being read only, showing nominal clock frequencies), I wonder if 
> this is too excessive? I also wonder if there should be an API for 
> runtime clock reconfiguration?
> * The architecture CDL (as copied from the ARM architecture) still 
> contains options for THUMB and THUMB interworking. I think these can be 
> completely removed, as THUMB-2 is the only instructions set for the 
> Cortex-M, and the defines should only be used in the ARM architecture. 
> Right?

That's exactly what I did. I took the ARM9 variant definition and the
Integrator platform definition as a base since I had prior experience
with these.

>  Should we leave the big-endian options? The Cortex-M can access 
> data memory in big-endian format, but not the code.

After encountering this fact in the TRM, I opted not to support this but
I would leave this as a question for more experienced folks.

> * What about ARM-ICE Thread support? The ICE currently not supports 
> Cortex CPUs, but I guess they will in the future. I don't have an 
> ARM-ICE and will not be able to test.

I have access to an ARM Micro-link box as well as an ARM Multi-ICE box
so can possibly use these in time of need. I do not think that Cortex
specific support is a mandatory option as of now IMHO.

> These were just a few questions, I guess a lot more will arise sooner or 
> later. The code not nearly mature enough to post patches, but If anyone 
> is interested to help on the port, please let me know. I could also 
> setup SVN access for simpler development.

That would be much appreciated. Please let me know if you could set me
up with access and I'll try to pitch in bits.

Cheers and luck,

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