netdev - RE: [RFC] bmac:change to use bitrev8() generic function

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Date:	Mon, 10 Aug 2015 12:02:47 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Tobias Klauser' <tklauser@...tanz.ch>,
	yalin wang <yalin.wang2010@...il.com>
CC:	"fabf@...net.be" <fabf@...net.be>,
	"david@...son.dropbear.id.au" <david@...son.dropbear.id.au>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: RE: [RFC] bmac:change to use bitrev8() generic function

> From: Tobias Klauser
> Sent: 10 August 2015 12:49
> On 2015-08-10 at 11:53:41 +0200, yalin wang <yalin.wang2010@...il.com> wrote:
> > This change to use generic bitrev8() for bmac driver.
...
> > @@ -871,7 +860,7 @@ bmac_addhash(struct bmac_data *bp, unsigned char *addr)
> >
> >  	if (!(*addr)) return;
> >  	crc = bmac_crc((unsigned short *)addr) & 0x3f; /* Big-endian alert! */

Why not *((u8 *)addr + 1) & 0x3f

> > -	crc = reverse6[crc];	/* Hyperfast bit-reversing algorithm */
> > +	crc = bitrev8((u8)crc);
> 
> No, this won't work. reverse6 works on 6 bit values, while bitrev8 works
> on 8bit values, e.g. reverse6[1] = 0x20, while bitrev8(1) = 0x80

You could use bitrev8(n) >> 2.

But that is a 'strange' map of a 7-bit value to a 6-bit one.

I thought it was more common for ethernet hardware to use the
value of the crc register after all 6 bytes of the mac address
had been processed.

	David

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