Re: [PATCH v5 4/7] can: Add Nuvoton NCT6694 CAN support

Ming Yu
2025-01-15T05:35:24Z

Vincent Mailhol <mailhol.vincent@wanadoo.fr> 於 2025年1月15日 週三 上午11:36寫道：
> >>>>> +static int nct6694_can_get_berr_counter(const struct net_device *ndev,
> >>>>> + struct can_berr_counter *bec)
> >>>>> +{
> >>>>> + struct nct6694_can_priv *priv = netdev_priv(ndev);
> >>>>> + struct nct6694_can_event *evt = priv->rx->event;
> >>>>> + struct nct6694_cmd_header cmd_hd;
> >>>>> + u8 mask = NCT6694_CAN_EVENT_REC | NCT6694_CAN_EVENT_TEC;
> >>>>> + int ret;
> >>>>> +
> >>>>> + guard(mutex)(&priv->lock);
> >>>>> +
> >>>>> + cmd_hd = (struct nct6694_cmd_header) {
> >>>>> + .mod = NCT6694_CAN_MOD,
> >>>>> + .cmd = NCT6694_CAN_EVENT,
> >>>>> + .sel = NCT6694_CAN_EVENT_SEL(priv->can_idx, mask),
> >>>>> + .len = cpu_to_le16(sizeof(priv->rx->event))
> >>>>> + };
> >>>>> +
> >>>>> + ret = nct6694_read_msg(priv->nct6694, &cmd_hd, evt);
> >>>>> + if (ret < 0)
> >>>>> + return ret;
> >>>>
> >>>> You are holding the priv->lock mutex before calling
> >>>> nct6694_read_msg(). But nct6694_read_msg() then holds the
> >>>> nct6694->access_lock mutex. Why do you need a double mutex here? What
> >>>> kind of race scenario are you trying to prevent here?
> >>>>
> >>>
> >>> I think priv->lock need to be placed here to prevent priv->rx from
> >>> being assigned by other functions, and nct6694->access_lock ensures
> >>> that the nct6694_read_msg() transaction is completed.
> >>> But in this case, cmd_hd does not need to be in priv->lock's scope.
> >>
> >> So, the only reason for holding priv->lock is because priv->rx is shared
> >> between functions.
> >>
> >> struct nct6694_can_event is only 8 bytes. And you only need it for the
> >> life time of the function so it can simply be declared on the stack:
> >>
> >> struct nct6694_can_event evt;
> >>
> >> and with this, no more need to hold the lock. And the same thing also
> >> applies to the other functions.
> >>
> >> Here, by trying to optimize the memory for only a few bytes, you are
> >> getting a huge penalty on the performance by putting locks on all the
> >> functions. This is not a good tradeoff.
> >>
> >
> > Since nct6694_read_msg()/nct6694_write_msg() process URBs via
> > usb_bulk_msg(), the transferred data must not be located on the stack.
> > For more details about allocating buffers for transmitting data,
> > please refer to the link:
> > https://lore.kernel.org/linux-can/20241028-observant-gentle-doberman-0a2baa-mkl@pengutronix.de/
>
> Ack, I forgot that you can not use stack memory in usb_bulk_msg().
>
> Then, instead, you can either:
>
> - do a dynamic memory allocation directly in the function (good for
> when you are outside of the hot path, for example struct
> nct6694_can_setting)
>
> - and for the other structures which are part of the hot path
> (typically struct nct6694_can_frame) continue to use a dynamically
> allocated buffer stored in your priv but change the type of
> nct6694_can_tx and nct6694_can_rx from union to structures.
>
> And no more overlaps, thus no more need for the mutex.
>

Understood, I will remove the unions and add members to private
structure in the next patch.
e.g.
struct nct6694_can_priv {
struct can_priv can;
...
struct nct6694_can_frame tx;
struct nct6694_can_frame rx;
};
And do dynamic memory allocation for struct nct6694_can_setting and
struct nct6694_can_information.

In addition, I would like to know your thoughts on how struct
nct6694_can_event[2] should be handled?
It is utilized in both nct6694_can_get_berr_counter() and
nct6694_can_irq(), with the latter being called more frequently during
runtime.

Thanks,
Ming