Re: [patch] Add support for VFP d16 layout for Cortex-M4

[Jonathan Larmour <jifl at eCosCentric dot com>]

On 19/04/12 17:18, Terry Guo wrote:
> On Thu, Apr 19, 2012 at 11:57 PM, Jonathan Larmour <jifl@ecoscentric.com> wrote:
>>
>> STM32F4 is likely to follow very soon after. Ironically, it's not likely
>> to be committed until we have some confidence that what goes into GDB will
>> match up with what the stub is doing! We wouldn't want to commit something
>> that has to change in an incompatible way.
> 
> Agree. I am also worrying about the gdb stub. If they return something
> different from what we currently expect, we will have to make another
> change.

I think there's probably very little variation in what would make sense.
Ultimately the layout used on the wire reflects the realities of the real
register set: the standard Cortex-M regs plus the VFP ones. Theoretically
it's possible someone might like to do away with the VFP pseudo regs and
make the format be 32 single precision registers; but if you did, it would
need a fair bit more GDB work for no actual practical benefit.

So I expect the format I've proposed is the natural format everyone would
want to use.

> So I am talking to OpenOCD to share with them what the gdb is
> going to do. There are many other gdb stub vendors like JLink. I plan
> to talk with them and hope to achieve some consensus between GDB and
> stub.

Because they run on the host, something that's easier for them to do is
supply their own target description XML over the wire. It's also easier to
just update and run a new version if they need to. In contrast, eCos's
stub is programmed into target flash and run on the target, and as you
know, many of the Cortex-M implementations out there have very limited
memory - bytes count. Using a GDB stub on the target is hard enough,
without increasing the footprint further with that. So speaking selfishly,
it doesn't matter so much about OpenOCD or J-link compared to things which
are programmed directly onto hardware, which are harder to change. They
won't care as much about the format as it's easier for them to adapt.

Jifl
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