On 04.11.2024 19:31:30, Vincent Mailhol wrote: > On Mon. 4 Nov. 2024 at 18:05, baozhu.liu <lucas.liu@xxxxxxxxxxxx> wrote: > > Since mb is a fixed-size two-dimensional array (u8 mb[2][512]), > > "priv->mb_count = sizeof(priv->regs->mb)/priv->mb_size;", > > this expression calculates mb_count correctly and is more concise. > > When using integers, > > (a1 / q) + (a2 / q) > > is not necessarily equal to > > (a1 + a2) / q. > > > If the decimal place of > > sizeof(priv->regs->mb[0]) / priv->mb_size > > were to be greater than or equal to 0.5, the result would have changed > because of the rounding. > > This is illustrated in https://godbolt.org/z/bfnhKcKPo. > > Here, luckily enough, the two valid results are, for CAN CC: > > sizeof(priv->regs->mb[0]) / priv->mb_size = 512 / 16 > = 64 > > and for CAN FD: > > sizeof(priv->regs->mb[0]) / priv->mb_size = 512 / 72 > = 14.22 > > and both of these have no rounding issues. > > I am not sure if we should take this patch. It is correct at the end > because you "won a coin flip", but the current code is doing the > correct logical calculation which would always yield the correct > result regardless of the rounding. Wow, that's an elaborative answer. Thanks Vincent! And yes the current code does the correct logical calculation because of the underlying restrictions of the hardware. A CAN-CC/FD frame cannot cross the boundary between the 2 memory areas. If you want to improve something, feel free to add a comment explaining the reasoning for the existing calculation. regards, Marc -- Pengutronix e.K. | Marc Kleine-Budde | Embedded Linux | https://www.pengutronix.de | Vertretung Nürnberg | Phone: +49-5121-206917-129 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-9 |
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