linux-kernel - Re: [PATCH v3 1/2] mailbox: check mailbox queue is full or not

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Message-ID: <75024300-f576-4f06-83a7-6f0c424e6fc7@amd.com>
Date: Fri, 30 Jan 2026 12:13:13 -0600
From: "Shah, Tanmay" <tanmays@....com>
To: Jassi Brar <jassisinghbrar@...il.com>, 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 v3 1/2] mailbox: check mailbox queue is full or not

Hi Jassi,

Thank You for reviews.

I agree to all your comments, and I will address them in the next
revision. I am returning from the leave now hence delay in the response.

Tanmay

On 1/18/2026 1:53 PM, Jassi Brar wrote:
> On Wed, Dec 17, 2025 at 3:27 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. Introduce variable per channel to track available msg slots.
>> Clients can check this variable and decide not to send new message if
>> it is 0. This will 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      | 3 +++
>>  include/linux/mailbox_client.h | 2 ++
>>  2 files changed, 5 insertions(+)
>>
>> diff --git a/drivers/mailbox/mailbox.c b/drivers/mailbox/mailbox.c
>> index 2acc6ec229a4..b7ae5c173143 100644
>> --- a/drivers/mailbox/mailbox.c
>> +++ b/drivers/mailbox/mailbox.c
>> @@ -36,6 +36,7 @@ static int add_to_rbuf(struct mbox_chan *chan, void *mssg)
>>         idx = chan->msg_free;
>>         chan->msg_data[idx] = mssg;
>>         chan->msg_count++;
>> +       chan->cl->msg_slot_ro = (MBOX_TX_QUEUE_LEN - chan->msg_count);
>>
> msg_count is protected by a lock and limited within MBOX_TX_QUEUE_LEN,
> so how about simply   chan->cl->msg_slot_ro--;
> 
>>         if (idx == MBOX_TX_QUEUE_LEN - 1)
>>                 chan->msg_free = 0;
>> @@ -71,6 +72,7 @@ static void msg_submit(struct mbox_chan *chan)
>>                 if (!err) {
>>                         chan->active_req = data;
>>                         chan->msg_count--;
>> +                       chan->cl->msg_slot_ro = (MBOX_TX_QUEUE_LEN - chan->msg_count);
>>
> Similarly   chan->cl->msg_slot_ro++  ?
> 
>>                 }
>>         }
>>
>> @@ -321,6 +323,7 @@ static int __mbox_bind_client(struct mbox_chan *chan, struct mbox_client *cl)
>>                 chan->msg_count = 0;
>>                 chan->active_req = NULL;
>>                 chan->cl = cl;
>> +               chan->cl->msg_slot_ro = MBOX_TX_QUEUE_LEN;
>>                 init_completion(&chan->tx_complete);
>>
>>                 if (chan->txdone_method == TXDONE_BY_POLL && cl->knows_txdone)
>> diff --git a/include/linux/mailbox_client.h b/include/linux/mailbox_client.h
>> index c6eea9afb943..a073fb0c03d1 100644
>> --- a/include/linux/mailbox_client.h
>> +++ b/include/linux/mailbox_client.h
>> @@ -17,6 +17,7 @@ struct mbox_chan;
>>   * @dev:               The client device
>>   * @tx_block:          If the mbox_send_message should block until data is
>>   *                     transmitted.
>> + * @msg_slot_ro:       msg slots remaining for this client's channel.
>>   * @tx_tout:           Max block period in ms before TX is assumed failure
>>   * @knows_txdone:      If the client could run the TX state machine. Usually
>>   *                     if the client receives some ACK packet for transmission.
>> @@ -29,6 +30,7 @@ struct mbox_chan;
>>  struct mbox_client {
>>         struct device *dev;
>>         bool tx_block;
>> +       unsigned int msg_slot_ro;
>>
> maybe call it 'tx_slots_available_ro' ?
> 
> Thanks