Re: [PATCH 06/36] Add Cavium OCTEON processor CSR definitions

[Ralf Baechle <ralf@linux-mips.org>]

On Wed, Oct 29, 2008 at 12:18:47PM -0700, David Daney wrote:

> That file contains the bit definitions for all on-chip registers.
>
> We are interested in transforming this information into a form suitable  
> for inclusion in the kernel.  Any specific suggestions as to improve the  
> patch will be considered.
>
> Several possibilities are:
>
> 1) Don't typedef all the unions in  cvmx-csr-typedefs.h.  An rename the  
> file so it doesn't contain the reprehensible word 'typedef'
>
> 2) Break cvmx-csr-addresses.h and cvmx-csr-typedefs.h into several  
> parts, one for each functional block in the processor.
>
> There are obviously other options as well...

The use of bitfields also fires back:

+       struct cvmx_ciu_intx_en1_w1c_cn58xx {
+#if __BYTE_ORDER == __BIG_ENDIAN
+               uint64_t reserved_16_63:48;
+               uint64_t wdog:16;
+#else
+               uint64_t wdog:16;
+               uint64_t reserved_16_63:48;
+#endif

You see, everything was defined twice.  And gcc even recent gccs tend to
do silly stuff with bitfields when combined with volatile:

struct foo {
	unsigned int	x:1;
	unsigned int	y:4;
};

void bar(volatile struct foo *p)
{
	p->x = 1;
	p->y++;
}

which gcc 4.3 for a MIPS32R2 target will compile into:

	lw	$3, 0($4)
	li	$2, 1
	ins	$3, $2, 31, 1
	sw	$3, 0($4)
	lw	$2, 0($4)
	lw	$3, 0($4)
	ext	$2, $2, 27, 4
	addiu	$2, $2, 1
	ins	$3, $2, 27, 4
	sw	$3, 0($4)
	j	$31
	nop

Imagine struct foo was describing a hardware register so the pointer to it
was marked volatile.  A human coder wouldn't have done multiple loads /
stores.  Worse, if you actually want to change multiple fields in a
register atomically then with bitfields you have _no_ possibility to enforce
that.

The Linux programming programming model relies on accessor functions like
readl, ioread32 etc.  Those take addresses as arguments - but bitfields
don't have addresses in C ...

So exec summary: bitfields bad for such low-level stuff.

  Ralf