Re: [PATCH] MIPS: Handle the DSP registers for bare metal

[Pedro Alves <palves@redhat.com>]

On 12/30/2014 12:10 PM, Maciej W. Rozycki wrote:
> On Tue, 30 Dec 2014, Pedro Alves wrote:
> 
>>> > >  I'm not sure offhand whether the piece of patch proposed you refer to 
>>> > > here is correct or not, but the overall scope of this and the other patch 
>>> > > Yao has mentioned yet outstanding is support for legacy bare-metal RSP 
>>> > > stubs that have no notion of target descriptions and may even predate 
>>> > > GDB's support for these descriptions, and yet they want to make all 
>>> > > processor registers available for inspection and modification by GDB.  
>>> > > This code comes from MIPS UK and dates back to early 2000s and I think it 
>>> > > would be good having it upstream so that standard GDB can talk to these 
>>> > > stubs.  The fixed layout of the g/G packet and corresponding p/P packet 
>>> > > offsets have been set by the bare-metal SDE toolchain years ago.
>> > 
>> > The way to handle that is still through target descriptions -- if a
>> > target doesn't send a target description, GDB maps known layouts to built-in
>> > target descriptions.  See mips_register_g_packet_guesses.

>  Register probing Yao mentioned is still needed, because in this fixed 
> packet format the whole set of architecturally defined register slots is 
> exchanged with the RSP stub, e.g. 8 * 32 = 256 CP0 registers, 8 * 32 = 256 
> CP2 registers, etc. (as documented by the change to mips-tdep.h proposed), 
> even if some are not present, e.g. not all CP0 register slots have already 
> been allocated in the architecture so far and the vast majority of them is 
> optional.

That's orthogonal.

> 
>  The information on which registers are present and which are not will not 
> be supplied by the target and has to be determined by gradual discovery, 
> that is poking at registers as they are determined to be present, i.e. 
> first CP0.PRId that is always there, then CP0.Config0 if present, then 
> CP0.Config1 if present, and so on.  Gaps will be present in the packets 
> exchanged for the absent registers as with the ACX registers discussed 
> previously.

> 
>  And if we were to define built-in target descriptions for the possible 
> variants of present register sets, then, as noted by Yao, we hit the issue 
> of having to make some 2^50 templates (slightly fewer actually, as there 

That'd be silly of course, but it's also not what I meant.  The built-in
description is used for the case of a legacy stubs that does not report a
target description, and those stubs you say use a fixed register layout.
So we're talking about _one_ built-in target description that describes
that fixed register layout.

> are some dependencies between the presence of some registers, e.g. you 
> can't have a processor that has a CP0.Config2 register, but does not have 
> CP0.Config1, the reverse is allowed; similarly a CPU can't have the ACX 
> register when it does not have CP0.Config3) which is of course beyond 
> technical capabilities as e.g. a 32-bit host will have fewer bytes of 
> addressable memory even.

>  NB `mips_register_g_packet_guesses' is not necessarily relevant here as 
> there may be registers accessible through the stub via the p/P packets 
> only and not included in the g/G packet layout for performance reasons 

If existing/legacy stubs are doing that in practice, then we can
make gdb pick the fallback description (in case in target doesn't
provide one) depending on selected osabi.

> (the packet may be shorter than the maximum defined) as to transfer all 
> the registers through a JTAG link every time the target is stopped may be 
> exceedingly slow, especially when single-stepping.

For the single-stepping case, the fix should be to include
all the registers GDB needs to compute whether the step finished
directly in the expedite stop reply, though.

Thanks,
Pedro Alves