netdev - Re: [PATCH v3 3/4] can: skb:: move can_dropped_invalid_skb and can_skb_headroom

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Date:   Tue, 17 May 2022 15:35:03 +0200
From:   Oliver Hartkopp <socketcan@...tkopp.net>
To:     Max Staudt <max@...as.org>, Marc Kleine-Budde <mkl@...gutronix.de>
Cc:     Vincent MAILHOL <mailhol.vincent@...adoo.fr>,
        linux-can@...r.kernel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH v3 3/4] can: skb:: move can_dropped_invalid_skb and
 can_skb_headroom_valid to skb.c



On 5/17/22 14:39, Max Staudt wrote:
> On Tue, 17 May 2022 14:21:53 +0200
> Marc Kleine-Budde <mkl@...gutronix.de> wrote:
> 
>> On 17.05.2022 14:14:04, Max Staudt wrote:
>>>> After looking through drivers/net/can/Kconfig I would probably
>>>> phrase it like this:
>>>>
>>>> Select CAN devices (hw/sw) -> we compile a can_dev module. E.g.
>>>> to handle the skb stuff for vcan's.
>>>>
>>>> Select hardware CAN devices -> we compile the netlink stuff into
>>>> can_dev and offer CAN_CALC_BITTIMING and CAN_LEDS to be compiled
>>>> into can_dev too.
>>>>
>>>> In the latter case: The selection of flexcan, ti_hecc and
>>>> mcp251xfd automatically selects CAN_RX_OFFLOAD which is then also
>>>> compiled into can_dev.
>>>>
>>>> Would that fit in terms of complexity?
>>>
>>> IMHO these should always be compiled into can-dev. Out of tree
>>> drivers are fairly common here, and having to determine which kind
>>> of can-dev (stripped or not) the user has on their system is a
>>> nightmare waiting to happen.
>>
>> I personally don't care about out-of-tree drivers.
> 
> I know that this is the official stance in the kernel.
> 
> But out-of-tree drivers do happen on a regular basis, even when
> developing with the aim of upstreaming. And if a developer builds a
> minimal kernel to host a CAN driver, without building in-tree hardware
> CAN drivers, then can-dev will be there but behave differently from
> can-dev in a full distro. Leading to heisenbugs and wasting time. The
> source of heisenbugs really are the suggested *hidden* Kconfigs.
> 
> 
> On another note, is the module accounting overhead in the kernel for
> two new modules with relatively little code in each, code that almost
> always is loaded when CAN is used, really worth it?

Oh, I didn't want to introduce two new kernel modules but to have 
can_dev in different 'feature levels'.

I would assume a distro kernel to have everything enabled with a full 
featured can_dev - which is likely the base for out-of-tree drivers too.

But e.g. the people that are running Linux instances in a cloud only 
using vcan and vxcan would not need to carry the entire infrastructure 
of CAN hardware support and rx-offload.

Best regards,
Oliver