Re: [PATCH] mailbox: add API to query available TX queue slots

["Shah, Tanmay" <tanmays@amd.com>]

Tested-by: Tanmay Shah <tanmay.shah@amd.com>

On 2/9/2026 5:44 PM, jassisinghbrar@gmail.com wrote:
> From: Jassi Brar <jassisinghbrar@gmail.com>
> 
> Clients sometimes need to know whether the mailbox TX queue has room
> before posting a new message. Rather than exposing internal queue state
> through a struct field, provide a proper accessor function that returns
> the number of available slots for a given channel.
> 
> This lets clients choose to back off when the queue is full instead of
> hitting the -ENOBUFS error path and the misleading "Try increasing
> MBOX_TX_QUEUE_LEN" warning.
> 
> Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>
> ---
>  drivers/mailbox/mailbox.c      | 23 +++++++++++++++++++++++
>  include/linux/mailbox_client.h |  1 +
>  2 files changed, 24 insertions(+)
> 
> diff --git a/drivers/mailbox/mailbox.c b/drivers/mailbox/mailbox.c
> index 2acc6ec229a4..22eb8f3213be 100644
> --- a/drivers/mailbox/mailbox.c
> +++ b/drivers/mailbox/mailbox.c
> @@ -218,6 +218,29 @@ bool mbox_client_peek_data(struct mbox_chan *chan)
>  }
>  EXPORT_SYMBOL_GPL(mbox_client_peek_data);
>  
> +/**
> + * mbox_chan_tx_slots_available - Query the number of available TX queue slots.
> + * @chan: Mailbox channel to query.
> + *
> + * Clients may call this to check how many messages can be queued via
> + * mbox_send_message() before the channel's TX queue is full. This helps
> + * clients avoid the -ENOBUFS error without needing to increase
> + * MBOX_TX_QUEUE_LEN.
> + * This can be called from atomic context.
> + *
> + * Return: Number of available slots in the channel's TX queue.
> + */
> +unsigned int mbox_chan_tx_slots_available(struct mbox_chan *chan)
> +{
> +	unsigned int ret;
> +
> +	guard(spinlock_irqsave)(&chan->lock);
> +	ret = MBOX_TX_QUEUE_LEN - chan->msg_count;
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(mbox_chan_tx_slots_available);
> +
>  /**
>   * mbox_send_message -	For client to submit a message to be
>   *				sent to the remote.
> diff --git a/include/linux/mailbox_client.h b/include/linux/mailbox_client.h
> index c6eea9afb943..e5997120f45c 100644
> --- a/include/linux/mailbox_client.h
> +++ b/include/linux/mailbox_client.h
> @@ -45,6 +45,7 @@ int mbox_send_message(struct mbox_chan *chan, void *mssg);
>  int mbox_flush(struct mbox_chan *chan, unsigned long timeout);
>  void mbox_client_txdone(struct mbox_chan *chan, int r); /* atomic */
>  bool mbox_client_peek_data(struct mbox_chan *chan); /* atomic */
> +unsigned int mbox_chan_tx_slots_available(struct mbox_chan *chan); /* atomic */
>  void mbox_free_channel(struct mbox_chan *chan); /* may sleep */
>  
>  #endif /* __MAILBOX_CLIENT_H */