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SubjectRe: [PATCH 09/13] Core WQE/CQE Types
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On Thu, 2006-11-16 at 20:45 -0800, Roland Dreier wrote:
> > +struct t3_send_wr {
> > + struct fw_riwrh wrh; /* 0 */
> > + union t3_wrid wrid; /* 1 */
> > +
> > + enum t3_rdma_opcode rdmaop:8;
> > + u32 reserved:24; /* 2 */
>
> Does this do the right thing wrt endianness? I'd be more comfortable
> with something like
>
> u8 rdmaop;
> u8 reserved[3];
>
> (although the __attribute__((packed)) on enum t3_rdma_opcode does make
> it OK to use here, I guess)
>
> > + u32 rem_stag; /* 2 */
> > + u32 plen; /* 3 */
> > + u32 num_sgle;
> > + struct t3_sge sgl[T3_MAX_SGE]; /* 4+ */
> > +};


I don't really like the bit fields either. I inherited these structs and
I'm not adverse to changing them as you suggest to get rid of bit
fields. But I think they are correct wrt endianness. I wrote a test
program and on a LE machine it put the u8 first in memory followed by
the 24 bit reserved. However, I think if you use bit fields less than 8
bits its not endian safe.

BTW: I don't have a PPC system (yet) to test this code on BE...

Here's a dumb program that plays around with bit fields...

#include <sys/types.h>
#include <inttypes.h>
#include <stdint.h>
#include <stdio.h>

struct foo {
uint32_t a:8;
uint32_t b:24;
uint32_t c:16;
uint32_t d:8;
uint32_t e:8;
};
struct bar {
uint8_t a;
uint8_t b[3];
uint16_t c;
uint8_t d;
uint8_t e;
};
struct bits {
#if 0 /* BE */
uint32_t a:4;
uint32_t b:4;
#else /* LE */
uint32_t b:4;
uint32_t a:4;
#endif
uint32_t c:8;
uint32_t d:8;
uint32_t e:8;
};
main()
{
struct foo foo;
struct bar bar;
struct bits bits;
uint8_t *cp;
int i;
foo.a = 0x01;
foo.b = 0x020304;
foo.c = 0x0506;
foo.d = 0x07;
foo.e = 0x08;

printf("foo cpu: 0x%" PRIx64 "\n", *(uint64_t *)&foo);
printf("foo mem: ");
cp = (uint8_t *)&foo;
for (i=0; i<8; i++)
printf("%02x", *cp++);
printf("\n");

bar.a = 0x01;
bar.b[0] = 0x02;
bar.b[1] = 0x03;
bar.b[2] = 0x04;
bar.c = 0x0506;
bar.d = 0x07;
bar.e = 0x08;

printf("bar cpu: 0x%" PRIx64 "\n", *(uint64_t *)&bar);
printf("bar mem: ");
cp = (uint8_t *)&bar;
for (i=0; i<8; i++)
printf("%02x", *cp++);
printf("\n");


bits.a = 0x1;
bits.b = 0x2;
bits.c = 0x3;
bits.d = 0x4;
bits.e = 0x5;

printf("bits cpu: 0x%08x\n", *(uint32_t *)&bits);
printf("bar mem: ");
cp = (uint8_t *)&bits;
for (i=0; i<4; i++)
printf("%02x", *cp++);
printf("\n");
}



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