Re: [ECOS] CGY_SWAP16() seems broken to me.

[Sergei Gavrikov <w3sg@SoftHome.net>]

On Sat, Sep 22, 2007 at 02:56:33AM +0000, Grant Edwards wrote:
> unsigned u = CYG_SWAP16(0x1234);
> 
> What would a reasonable person (who hasn't looked at
> hal_endian.h) expect u to be?
> 
> I certainly expected it to be 0x3412. 
> 
> But it's not.  It's 0x123412.  

That is exists. I got same result

#include <cyg/hal/hal_endian.h>
#include <sys/param.h>
#include <sys/endian.h>

#include <stdio.h>

int
main (void)
{
    printf ("HAL swap16(0x1234) = 0x%x\n", CYG_SWAP16 (0x1234));
    printf ("NET swap16(0x1234) = 0x%x\n", swap16 (0x1234));
    return 0;
}

HAL swap16(0x1234) = 0x123412
NET swap16(0x1234) = 0x3412

> IMO, that's just plain broken.

... I don't know. What do eCos experts think about?

> Here's the definition from hal_endian.h:
> 
> # define CYG_SWAP16(_x_)                                        \
>     ({ cyg_uint16 _x = (_x_); ((_x << 8) | (_x >> 8)); })
> 
> Why isn't it this?
> 
> # define CYG_SWAP16(_x_)                                        \
>     ({ cyg_uint16 _x = (_x_); (cyg_uint16)((_x << 8) | (_x >> 8)); })
> 
> Why would anybody expect a macro named CYG_SWAP16() to return a
> "24-bit value" when passed a "16-bit value"?

Usually, they use yet masking in same macros. Good byteorder examples
are

net/bsd_tcpip/current/include/sys/endian.h
net/tcpip/current/include/sys/endian.h

It seems, that eCos HAL's CYG_SWAP16() either assumes what it will be to
live in "16-bit world" or it assumes that will be a developers's care. I
did find only 3 places where CYG_SWAP16() macro is used. For example,
CYG_SWAP16() macro uses itself in the eCos drivers for 16-bit Ethernet
controllers (cs8900a, ec555) "16-bit world". And I found that RedBoot's
conf_endian_fixup() (fconfig.c) uses eCos HAL's CYG_SWAP16() "as is",
because that was a "developer's care", there is there

                u16 = CYG_SWAP16(u16);

but, not :-)

		unsigned u = CYG_SWAP16(0x1234);

In any case, thank you for your report, we will be to know about.


	Sergei


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