Re: [PATCH] can: bittiming: replace CAN units with the SI metric

[Oliver Hartkopp]

Hi Vincent,

On 22.11.21 03:22, Vincent MAILHOL wrote:
> Le lun. 22 nov. 2021 à 03:27, Oliver Hartkopp <socketcan@xxxxxxxxxxxx> a écrit :


> > >    #include <linux/kernel.h>
> > > +#include <linux/units.h>
> > >    #include <asm/unaligned.h>
> > >
> > >    #include "es58x_core.h"
> > > @@ -469,8 +470,8 @@ const struct es58x_parameters es581_4_param = {
> > >        .bittiming_const = &es581_4_bittiming_const,
> > >        .data_bittiming_const = NULL,
> > >        .tdc_const = NULL,
> > > -     .bitrate_max = 1 * CAN_MBPS,
> > > -     .clock = {.freq = 50 * CAN_MHZ},
> > > +     .bitrate_max = 1 * MEGA,
> > > +     .clock = {.freq = 50 * MEGA},
> >
> > IMO we are losing information here.
> >
> > It feels you suggest to replace MHz with M.
>
> When I introduced the CAN_{K,M}BPS and CAN_MHZ macros, my primary
> intent was to avoid having to write more than five zeros in a
> row (because the human brain is bad at counting those). And the
> KILO/MEGA prefixes perfectly cover that intent.
>
> You are correct to say that the information of the unit is
> lost. But I assume this information to be implicit (frequencies
> are in Hz, baudrate are in bits/second). So yes, I suggest
> replacing MHz with M.
>
> Do you really think that people will be confused by this change?

It is not about confusing people but about the quality of documentation 
and readability.

> I am not strongly opposed to keeping it either (hey, I was the
> one who introduced it in the first place). I just think that
> using linux/units.h is sufficient.
>
> > So where is the Hz information then?
>
> It is in the comment of can_clock:freq :)
>
> https://elixir.bootlin.com/linux/v5.15/source/include/uapi/linux/can/netlink.h#L63

Haha, you are funny ;-)

But the fact that you provide this URL shows that the information is not 
found or easily accessible when someone reads the code here.

> > > -     .bitrate_max = 8 * CAN_MBPS,
> > > -     .clock = {.freq = 80 * CAN_MHZ},
> > > +     .bitrate_max = 8 * MEGA,
> > > +     .clock = {.freq = 80 * MEGA},

What about

+     .bitrate_max = 8 * MEGA, /* bits per second */
+     .clock = {.freq = 80 * MEGA}, /* Hz */

which uses the SI constants but maintains the unit?

Best regards,
Oliver