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

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Message-ID: <73c3b9cb-3b46-1523-d926-4bdf86de3fb8@hartkopp.net>
Date:   Tue, 23 Nov 2021 21:53:28 +0100
From:   Oliver Hartkopp <socketcan@...tkopp.net>
To:     Vincent MAILHOL <mailhol.vincent@...adoo.fr>
Cc:     Marc Kleine-Budde <mkl@...gutronix.de>, linux-can@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jimmy Assarsson <extja@...ser.com>
Subject: Re: [PATCH] can: bittiming: replace CAN units with the SI metric

Hi Vincent,

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