RE: [PATCH] hw/arm/stm32f405: Add preliminary RCC emulation support

[Stephanos Ioannidis]

> Is it portable, though? I thought C bitfield order and packing was 
> implementation-defined, which would mean that you can't guarantee that this 
> union will give you the required thing in the uint32_t half.

They are indeed implementation defined. As for packing, it is often 
controllable; as for order, I do not recall seeing a compiler that dares to 
change the order by which the fields are specified; though
I understand the concern if QEMU policy is to be as portable as possible.

Does QEMU have any form of bit field manipulation macros (e.g. GET_BIT_RANGE(a, 
b), SET_BIT_RANGE(a, b), ...)?

-----Original Message-----
From: Peter Maydell <address@hidden> 
Sent: Friday, March 6, 2020 4:30 AM
To: Alistair Francis <address@hidden>
Cc: Stephanos Ioannidis <address@hidden>; Alistair Francis <address@hidden>; 
open list:All patches CC here <address@hidden>; open list:ARM TCG CPUs 
<address@hidden>
Subject: Re: [PATCH] hw/arm/stm32f405: Add preliminary RCC emulation support

On Thu, 5 Mar 2020 at 19:24, Alistair Francis <address@hidden> wrote:
>
> On Sat, Feb 29, 2020 at 6:12 AM Stephanos Ioannidis <address@hidden> wrote:

> > +typedef union {
> > +    struct {
> > +        uint32_t hsion : 1;
> > +        uint32_t hsirdy : 1;
> > +        uint32_t reserved0 : 1;
> > +        uint32_t hsitrim : 5;
> > +        uint32_t hsical : 8;
> > +        uint32_t hseon : 1;
> > +        uint32_t hserdy : 1;
> > +        uint32_t hsebyp : 1;
> > +        uint32_t csson : 1;
> > +        uint32_t reserved1 : 4;
> > +        uint32_t pllon : 1;
> > +        uint32_t pllrdy : 1;
> > +        uint32_t plli2son : 1;
> > +        uint32_t plli2srdy : 1;
> > +        uint32_t reserved2 : 4;
> > +    };
> > +    uint32_t reg;
> > +} RccCrType;
>
> This is a pretty interesting way to represent the registers

Is it portable, though? I thought C bitfield order and packing was 
implementation-defined, which would mean that you can't guarantee that this 
union will give you the required thing in the uint32_t half.

I think it would be better to do this the way that other device models do it, 
and avoid bitfields.

thanks
-- PMM