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

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Message-ID: <3ecf0bb8-a1f1-498f-8b7d-39483a67cbfc@amd.com>
Date: Tue, 30 Sep 2025 11:52:50 -0500
From: Tanmay Shah <tanmay.shah@....com>
To: Jassi Brar <jassisinghbrar@...il.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

Hi Jassi,

Please find my comments below.

On 9/30/25 9:11 AM, Jassi Brar wrote:
> 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);
> 

Ack here.

> 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;
> 

It's not possible to make msg_slots_ro true Read-Only. Nothing prevents 
clients to write to that variable as far as I know. Also, we have 
msg_free variable that can be used to implement similar concept as 
msg_slots_ro. But that also has same problem, not possible to implement 
it in true read-only fashion.

I can instead implement API msg_queue_empty() and return msg_free 
variable via that API.


> -jassi