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

Vincent Mailhol
2025-01-15T03:36:31Z

On 15/01/2025 at 11:11, Ming Yu wrote:
> Vincent Mailhol <mailhol.vincent@wanadoo.fr> 於 2025年1月14日 週二 下午11:12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.

Yours sincerely,
Vincent Mailhol