linux-kernel - Re: [PATCH] mailbox: check mailbox queue is full or not

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Message-ID: <CABb+yY0MzJWOGtS=ocphpUVcJ-gb6_ENCjSmJQiWfRiOhxrpCg@mail.gmail.com>
Date: Tue, 30 Sep 2025 09:11:26 -0500
From: Jassi Brar <jassisinghbrar@...il.com>
To: Tanmay Shah <tanmay.shah@....com>
Cc: andersson@...nel.org, mathieu.poirier@...aro.org, 
	linux-kernel@...r.kernel.org, linux-remoteproc@...r.kernel.org
Subject: Re: [PATCH] mailbox: check mailbox queue is full or not

On Thu, Sep 25, 2025 at 1:51 PM Tanmay Shah <tanmay.shah@....com> wrote:
>
> Sometimes clients need to know if mailbox queue is full or not before
> posting new message via mailbox. If mailbox queue is full clients can
> choose not to post new message. This doesn't mean current queue length
> should be increased, but clients may want to wait till previous Tx is
> done. This API can help avoid false positive warning from mailbox
> framework "Try increasing MBOX_TX_QUEUE_LEN".
>
> Signed-off-by: Tanmay Shah <tanmay.shah@....com>
> ---
>  drivers/mailbox/mailbox.c               | 24 ++++++++++++++++++++++++
>  drivers/remoteproc/xlnx_r5_remoteproc.c |  4 ++++
>  include/linux/mailbox_client.h          |  1 +
>  3 files changed, 29 insertions(+)
>
> diff --git a/drivers/mailbox/mailbox.c b/drivers/mailbox/mailbox.c
> index 5cd8ae222073..7afdb2c9006d 100644
> --- a/drivers/mailbox/mailbox.c
> +++ b/drivers/mailbox/mailbox.c
> @@ -217,6 +217,30 @@ bool mbox_client_peek_data(struct mbox_chan *chan)
>  }
>  EXPORT_SYMBOL_GPL(mbox_client_peek_data);
>
> +/**
> + * mbox_queue_full - check if mailbox queue is full or not
> + * @chan: Mailbox channel assigned to this client.
> + *
> + * Clients can choose not to send new msg if mbox queue is full.
> + *
> + * Return: true if queue is full else false. < 0 for error
> + */
> +int mbox_queue_full(struct mbox_chan *chan)
> +{
> +       unsigned long flags;
> +       int res;
> +
> +       if (!chan)
> +               return -EINVAL;
> +
> +       spin_lock_irqsave(&chan->lock, flags);
> +       res = (chan->msg_count == (MBOX_TX_QUEUE_LEN - 1));
>
1) If we really need this, it should be
        res = (chan->msg_count == MBOX_TX_QUEUE_LEN);

2) I am thinking instead, introduce a
       struct mbox_client.msg_slots_ro;
  Which is a read-only field for the client, denoting how many message
slots are currently available.
  The mailbox api will always adjust it when msg_count changes...
      chan->cl->msg_slots_ro = MBOX_TX_QUEUE_LEN - chan->msg_count;

-jassi