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[pcnet32] Eliminate redundant register read 

The value of ( ( x & 0x0c00 ) | 0x0c00 ) is always 0x0c00 regardless
of the value of x, and so the read_csr() is redundant.  (There are no
read side effects for this register, according to the datasheet.)

This line of code originated in Linux kernel 2.3.19pre1 as

  a->write_csr(ioaddr, 80, a->read_csr(ioaddr, 80) | 0x0c00);

and was modified in kernel 2.3.41pre4 to read

  a->write_csr(ioaddr, 80, (a->read_csr(ioaddr, 80) & 0x0C00) | 0x0c00);

In the absence of commit messages, the intention of the code is
unclear.  However, the logic resulting in a fixed value of 0x0c00 has
remained unaltered for over 17 years, and can probably be assumed to
have the correct overall result.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
pull/58/merge
Michael Brown 2017-03-23 17:35:10 +02:00
parent 1ff1eebcf7
commit 19d3e966d9
1 changed files with 1 additions and 2 deletions
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3
src/drivers/net/pcnet32.c
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@ -414,8 +414,7 @@ pcnet32_chip_detect ( struct pcnet32_private *priv )
if (fset) {
a->write_bcr ( ioaddr, 18,
( a->read_bcr ( ioaddr, 18 ) | 0x0860 ) );
a->write_csr ( ioaddr, 80,
( a->read_csr ( ioaddr, 80 ) & 0x0C00) | 0x0C00 );
a->write_csr ( ioaddr, 80, 0x0c00 );
}
priv->full_duplex = fdx;