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

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Date:   Tue, 23 Nov 2021 08:44:32 +0100
From:   Jimmy Assarsson <extja@...ser.com>
To:     Vincent MAILHOL <mailhol.vincent@...adoo.fr>,
        Oliver Hartkopp <socketcan@...tkopp.net>
Cc:     Marc Kleine-Budde <mkl@...gutronix.de>, linux-can@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] can: bittiming: replace CAN units with the SI metric

On 2021-11-22 03:22, Vincent MAILHOL wrote:
> Le lun. 22 nov. 2021 à 03:27, Oliver Hartkopp <socketcan@...tkopp.net> a écrit :
>> On 19.11.21 17:18, Vincent Mailhol wrote:
>>> In [1], we introduced a set of units in linux/can/bittiming.h. Since
>>> then, generic SI prefix were added to linux/units.h in [2]. Those new
>>> prefix can perfectly replace the CAN specific units.
>>>
>>> This patch replaces all occurrences of the CAN units with their
>>> corresponding prefix according to below table.
>>>
>>>    CAN units   SI metric prefix
>>>    -------------------------------
>>>    CAN_KBPS    KILO
>>>    CAN_MBPS    MEGA
>>>    CAM_MHZ     MEGA
>>>
>>> The macro declarations are then removed from linux/can/bittiming.h
>>>
>>> [1] commit 1d7750760b70 ("can: bittiming: add CAN_KBPS, CAN_MBPS and
>>> CAN_MHZ macros")
>>>
>>> [2] commit 26471d4a6cf8 ("units: Add SI metric prefix definitions")
>>>
>>> Suggested-by: Jimmy Assarsson <extja@...ser.com>
>>> Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
>>> ---
>>>    drivers/net/can/dev/bittiming.c           | 5 +++--
>>>    drivers/net/can/usb/etas_es58x/es581_4.c  | 5 +++--
>>>    drivers/net/can/usb/etas_es58x/es58x_fd.c | 5 +++--
>>>    include/linux/can/bittiming.h             | 7 -------
>>>    4 files changed, 9 insertions(+), 13 deletions(-)
>>>
>>> diff --git a/drivers/net/can/dev/bittiming.c b/drivers/net/can/dev/bittiming.c
>>> index 0509625c3082..a5c9f973802a 100644
>>> --- a/drivers/net/can/dev/bittiming.c
>>> +++ b/drivers/net/can/dev/bittiming.c
>>> @@ -4,6 +4,7 @@
>>>     * Copyright (C) 2008-2009 Wolfgang Grandegger <wg@...ndegger.com>
>>>     */
>>>
>>> +#include <linux/units.h>
>>>    #include <linux/can/dev.h>
>>>
>>>    #ifdef CONFIG_CAN_CALC_BITTIMING
>>> @@ -81,9 +82,9 @@ int can_calc_bittiming(struct net_device *dev, struct can_bittiming *bt,
>>>        if (bt->sample_point) {
>>>                sample_point_nominal = bt->sample_point;
>>>        } else {
>>> -             if (bt->bitrate > 800 * CAN_KBPS)
>>> +             if (bt->bitrate > 800 * KILO)
>>>                        sample_point_nominal = 750;
>>> -             else if (bt->bitrate > 500 * CAN_KBPS)
>>> +             else if (bt->bitrate > 500 * KILO)
>>>                        sample_point_nominal = 800;
>>>                else
>>>                        sample_point_nominal = 875;
>>> diff --git a/drivers/net/can/usb/etas_es58x/es581_4.c b/drivers/net/can/usb/etas_es58x/es581_4.c
>>> index 14e360c9f2c9..ed340141c712 100644
>>> --- a/drivers/net/can/usb/etas_es58x/es581_4.c
>>> +++ b/drivers/net/can/usb/etas_es58x/es581_4.c
>>> @@ -10,6 +10,7 @@
>>>     */
>>>
>>>    #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?
> 
> 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.

Came across linux/units.h when looking at a different driver, and thought
that it was also possible to utilize them in the CAN drivers.

I've no strong opinion about any of the suggested solutions.
I'm fine with keeping it as it is, just wanted to raise the question :)

Best regards,
jimmy

>> 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
> 
>>>        .ctrlmode_supported = CAN_CTRLMODE_CC_LEN8_DLC,
>>>        .tx_start_of_frame = 0xAFAF,
>>>        .rx_start_of_frame = 0xFAFA,
>>> diff --git a/drivers/net/can/usb/etas_es58x/es58x_fd.c b/drivers/net/can/usb/etas_es58x/es58x_fd.c
>>> index 4f0cae29f4d8..aec299bed6dc 100644
>>> --- a/drivers/net/can/usb/etas_es58x/es58x_fd.c
>>> +++ b/drivers/net/can/usb/etas_es58x/es58x_fd.c
>>> @@ -12,6 +12,7 @@
>>>     */
>>>
>>>    #include <linux/kernel.h>
>>> +#include <linux/units.h>
>>>    #include <asm/unaligned.h>
>>>
>>>    #include "es58x_core.h"
>>> @@ -522,8 +523,8 @@ const struct es58x_parameters es58x_fd_param = {
>>>         * Mbps work in an optimal environment but are not recommended
>>>         * for production environment.
>>>         */
>>> -     .bitrate_max = 8 * CAN_MBPS,
>>> -     .clock = {.freq = 80 * CAN_MHZ},
>>> +     .bitrate_max = 8 * MEGA,
>>> +     .clock = {.freq = 80 * MEGA},
>>>        .ctrlmode_supported = CAN_CTRLMODE_LOOPBACK | CAN_CTRLMODE_LISTENONLY |
>>>            CAN_CTRLMODE_3_SAMPLES | CAN_CTRLMODE_FD | CAN_CTRLMODE_FD_NON_ISO |
>>>            CAN_CTRLMODE_CC_LEN8_DLC | CAN_CTRLMODE_TDC_AUTO,
>>> diff --git a/include/linux/can/bittiming.h b/include/linux/can/bittiming.h
>>> index 20b50baf3a02..a81652d1c6f3 100644
>>> --- a/include/linux/can/bittiming.h
>>> +++ b/include/linux/can/bittiming.h
>>> @@ -12,13 +12,6 @@
>>>    #define CAN_SYNC_SEG 1
>>>
>>>
>>> -/* Kilobits and Megabits per second */
>>> -#define CAN_KBPS 1000UL
>>> -#define CAN_MBPS 1000000UL
>>> -
>>> -/* Megahertz */
>>> -#define CAN_MHZ 1000000UL
>>
>> So what about
>>
>> #define CAN_KBPS KILO /* kilo bits per second */
>> #define CAN_MBPS MEGA /* mega bits per second */
>>
>> #define CAN_MHZ MEGA /* mega hertz */
>>
>>
>> ??
>>
>> Regards,
>> Oliver