Re: Fwd: Re: [patch] Add support for ARMv7M devices.

[Jonathan Larmour <jifl@eCosCentric.com>]

On 14/03/12 16:06, Pedro Alves wrote:
> On 03/11/2012 03:36 AM, Jonathan Larmour wrote:
>>
>> Can you just clarify to me how, for example, a program using VFP registers
>> (such as for Cortex-M4) would use the correct 'g' packet size? The
>> registers correspond to the tdesc, and not to either of the guessed sizes.
>> I guess if I could understand that example, I'll be happy. You can do this
>> off list if you like, to save others from boredom.
> 
> Even without a description, and before connecting to the remote side,
> GDB already has a clue of the target's architecture, inferred from the
> executable. GDB updates target_gdbarch based on that, and sets an initial
> expected size of the g packet based on the register set it things the
> target has (based on what it figured out from the executable).
> 
> static void *
> init_remote_state (struct gdbarch *gdbarch)
> {
> ...
>   /* Record the maximum possible size of the g packet - it may turn out
>      to be smaller.  */
>   rsa->sizeof_g_packet = map_regcache_remote_table (gdbarch, rsa->regs);
> 
> If it turns out to be smaller, GDB will re-adjust (process_g_packet).

Ah ok, that's what I was missing. And that uses gdbarch_num_regs() which
corresponds to arm-tdep.h's ARM_NUM_REGS which includes all possible
registers, both VFP and FPA. And later the 'g' packet is shrunk.

So, just to be clear, if neither the user nor target supplies a tdesc, the
block in arm_gdbarch_init() starting with:
   if (tdesc_has_registers (tdesc))
will never be run? If that's the case, no need to answer. Or is
arm_gdbarch_init() called a second time after the remote 'g' packet
guessing occurs (which has selected a tdesc)?

> Or did you mean, in the case where the target does send over a
> target description?

No I was thinking more of when GDB only has the executable to go on. But
thanks for that information - it fills other gaps in my knowledge.

Jifl
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