hwtest.c

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/* Tests for presence or absence of hardware registers.
 * This code was originally in atari/config.c, but I noticed
 * that it was also in drivers/nubus/nubus.c and I wanted to
 * use it in hp300/config.c, so it seemed sensible to pull it
 * out into its own file.
 *
 * The test is for use when trying to read a hardware register
 * that isn't present would cause a bus error. We set up a
 * temporary handler so that this doesn't kill the kernel.
 *
 * There is a test-by-reading and a test-by-writing; I present
 * them here complete with the comments from the original atari
 * config.c...
 *                -- PMM <[email protected]>, 05/1998
 */

/* This function tests for the presence of an address, specially a
 * hardware register address. It is called very early in the kernel
 * initialization process, when the VBR register isn't set up yet. On
 * an Atari, it still points to address 0, which is unmapped. So a bus
 * error would cause another bus error while fetching the exception
 * vector, and the CPU would do nothing at all. So we needed to set up
 * a temporary VBR and a vector table for the duration of the test.
 */

#include <linux/module.h>

int hwreg_present( volatile void *regp )
{
    int ret = 0;
    long    save_sp, save_vbr;
    long    tmp_vectors[3];

    __asm__ __volatile__
    (   "movec  %/vbr,%2\n\t"
        "movel  #Lberr1,%4@(8)\n\t"
                "movec  %4,%/vbr\n\t"
        "movel  %/sp,%1\n\t"
        "moveq  #0,%0\n\t"
        "tstb   %3@\n\t"
        "nop\n\t"
        "moveq  #1,%0\n"
                "Lberr1:\n\t"
        "movel  %1,%/sp\n\t"
        "movec  %2,%/vbr"
        : "=&d" (ret), "=&r" (save_sp), "=&r" (save_vbr)
        : "a" (regp), "a" (tmp_vectors)
                );

    return( ret );
}
EXPORT_SYMBOL(hwreg_present);

/* Basically the same, but writes a value into a word register, protected
 * by a bus error handler. Returns 1 if successful, 0 otherwise.
 */

int hwreg_write( volatile void *regp, unsigned short val )
{
    int     ret;
    long    save_sp, save_vbr;
    long    tmp_vectors[3];

    __asm__ __volatile__
    (   "movec  %/vbr,%2\n\t"
        "movel  #Lberr2,%4@(8)\n\t"
        "movec  %4,%/vbr\n\t"
        "movel  %/sp,%1\n\t"
        "moveq  #0,%0\n\t"
        "movew  %5,%3@\n\t"
        "nop    \n\t"   /* If this nop isn't present, 'ret' may already be
                 * loaded with 1 at the time the bus error
                 * happens! */
        "moveq  #1,%0\n"
    "Lberr2:\n\t"
        "movel  %1,%/sp\n\t"
        "movec  %2,%/vbr"
        : "=&d" (ret), "=&r" (save_sp), "=&r" (save_vbr)
        : "a" (regp), "a" (tmp_vectors), "g" (val)
    );

    return( ret );
}
EXPORT_SYMBOL(hwreg_write);