On Saturday 27 May 2006 15:55, Sergey Vlasov wrote:
> On Sat, 27 May 2006 13:27:15 +0200 Ivo van Doorn wrote:
>
> > I have a small question about the CRC16 usage in rt2x00
> > and what the netdev preferred method is for this.
> >
> > At the moment the rt2x00 drivers in wireless-dev use
> > its own CRC16 table and calculation inside the driver.
> > There already is a CRC16 library in the kernel
> > which uses the 0x8005 table, while rt2x00 uses the
> > 0x1021 polynomial table.
> >
> > Is keeping this version of CRC16 inside rt2x00 the most
> > sensible thing to do? Or should it be moved to the lib
> > folder of the kernel so other drivers could make use of it as well?
> > (So far I have not found a driver which makes use
> > of the same CRC16 table)
>
> However, fs/udf/crc.c has exactly the same CRC table as rt2x00:
>
> static uint16_t crc_table[256] = {
> 0x0000U, 0x1021U, 0x2042U, 0x3063U, 0x4084U, 0x50a5U, 0x60c6U,
> 0x70e7U,
> ...
> 0x6e17U, 0x7e36U, 0x4e55U, 0x5e74U, 0x2e93U, 0x3eb2U, 0x0ed1U, 0x1ef0U
> };
>
> /*
> * udf_crc
> *
> * PURPOSE
> * Calculate a 16-bit CRC checksum using ITU-T V.41 polynomial.
> *
> * DESCRIPTION
> * The OSTA-UDF(tm) 1.50 standard states that using CRCs is mandatory.
> * The polynomial used is: x^16 + x^12 + x^15 + 1
> *
> * PRE-CONDITIONS
> * data Pointer to the data block.
> * size Size of the data block.
> *
> * POST-CONDITIONS
> * <return> CRC of the data block.
> *
> * HISTORY
> * July 21, 1997 - Andrew E. Mileski
> * Adapted from OSTA-UDF(tm) 1.50 standard.
> */
>
> This is a bit-reversed form of CRC-CCITT supported by lib/crc-ccitt.c.
> Unfortunately, this does not help much, because converting one form to
> the other is too expensive (you would need to reverse bits in all data
> bytes, and then reverse bits in the 16-bit result).
>
> BTW, there is more CRC code duplication in drivers/bluetooth/hci_bcsp.c
> (bcsp_crc_update() seems to give the same result as crc_ccitt_byte()).
>
> In drivers/media/dvb/frontends/nxt200x.c, nxt200x_crc() seems to give
> the same result as rt2x00crc_byte() (but without using a table).
>
> drivers/net/wan/cycx_drv.c:checksum() is also some mutated version of
> CRC-CCITT (adding two additional zero bytes at the end of data makes it
> return the same result as rt2x00crc()).Interesting, so it would actually be usefull to move the nxt200x crc16 implemetation into the crc16 library (Is not using a table preferred over using a table?) And make rt2x00 and fs/udf/crc.c make use of this library? And perhaps hci_bcsp could be updated to use crc_ccitt_byte() instead? Regards, ivo
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