linux-kernel - Re: [PATCH v3 3/3] can: length: refactor frame lengths definition to add size in bits

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Message-ID: <CAMZ6RqK2vr0KRq76UNOSKzHMEfhz1YPFdg7CdQJqq4pBH3hj5w@mail.gmail.com>
Date:   Wed, 31 May 2023 02:29:43 +0900
From:   Vincent MAILHOL <mailhol.vincent@...adoo.fr>
To:     Simon Horman <simon.horman@...igine.com>
Cc:     Marc Kleine-Budde <mkl@...gutronix.de>, linux-can@...r.kernel.org,
        Thomas.Kopp@...rochip.com,
        Oliver Hartkopp <socketcan@...tkopp.net>,
        netdev@...r.kernel.org, marex@...x.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/3] can: length: refactor frame lengths definition to
 add size in bits

On Wed. 31 May 2023 at 00:56, Simon Horman <simon.horman@...igine.com> wrote:
> On Tue, May 30, 2023 at 11:46:37PM +0900, Vincent Mailhol wrote:
> > Introduce a method to calculate the exact size in bits of a CAN(-FD)
> > frame with or without dynamic bitsuffing.
> >
> > These are all the possible combinations taken into account:
> >
> >   - Classical CAN or CAN-FD
> >   - Standard or Extended frame format
> >   - CAN-FD CRC17 or CRC21
> >   - Include or not intermission
> >
> > Instead of doing several individual macro definitions, declare the
> > can_frame_bits() function-like macro. To this extent, do a full
> > refactoring of the length definitions.
> >
> > In addition add the can_frame_bytes(). This function-like macro
> > replaces the existing macro:
> >
> >   - CAN_FRAME_OVERHEAD_SFF: can_frame_bytes(false, false, 0)
> >   - CAN_FRAME_OVERHEAD_EFF: can_frame_bytes(false, true, 0)
> >   - CANFD_FRAME_OVERHEAD_SFF: can_frame_bytes(true, false, 0)
> >   - CANFD_FRAME_OVERHEAD_EFF: can_frame_bytes(true, true, 0)
> >
> > The different maximum frame lengths (maximum data length, including
> > intermission) are as follow:
> >
> >    Frame type                         bits    bytes
> >   -------------------------------------------------------
> >    Classic CAN SFF no-bitstuffing     111     14
> >    Classic CAN EFF no-bitstuffing     131     17
> >    Classic CAN SFF bitstuffing                135     17
> >    Classic CAN EFF bitstuffing                160     20
> >    CAN-FD SFF no-bitstuffing          579     73
> >    CAN-FD EFF no-bitstuffing          598     75
> >    CAN-FD SFF bitstuffing             712     89
> >    CAN-FD EFF bitstuffing             736     92
> >
> > The macro CAN_FRAME_LEN_MAX and CANFD_FRAME_LEN_MAX are kept as an
> > alias to, respectively, can_frame_bytes(false, true, CAN_MAX_DLEN) and
> > can_frame_bytes(true, true, CANFD_MAX_DLEN).
> >
> > In addition to the above:
> >
> >  - Use ISO 11898-1:2015 definitions for the name of the CAN frame
> >    fields.
> >  - Include linux/bits.h for use of BITS_PER_BYTE.
> >  - Include linux/math.h for use of mult_frac() and
> >    DIV_ROUND_UP(). N.B: the use of DIV_ROUND_UP() is not new to this
> >    patch, but the include was previously omitted.
> >  - Add copyright 2023 for myself.
> >
> > Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
>
> ...
>
> > +/**
> > + * can_bitstuffing_len() - Calculate the maximum length with bitsuffing
> > + * @bitstream_len: length of a destuffed bit stream
>
> Hi Vincent,
>
> it looks like an editing error has crept in here:
>
>         s/bitstream_len/destuffed_len/

Doh! Thanks for picking this up.

I already prepared a v4 locally. Before sending it, I will wait one
day to see if there are other comments.

> > + *
> > + * The worst bit stuffing case is a sequence in which dominant and
> > + * recessive bits alternate every four bits:
> > + *
> > + *   Destuffed: 1 1111  0000  1111  0000  1111
> > + *   Stuffed:   1 1111o 0000i 1111o 0000i 1111o
> > + *
> > + * Nomenclature
> > + *
> > + *  - "0": dominant bit
> > + *  - "o": dominant stuff bit
> > + *  - "1": recessive bit
> > + *  - "i": recessive stuff bit
> > + *
> > + * Aside of the first bit, one stuff bit is added every four bits.
> > + *
> > + * Return: length of the stuffed bit stream in the worst case scenario.
> > + */
> > +#define can_bitstuffing_len(destuffed_len)                   \
> > +     (destuffed_len + (destuffed_len - 1) / 4)