linux-kernel - Re: [PATCH v23 1/2] mailbox/pcc: support mailbox management of the shared buffer

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Message-ID: <1ec0cb87-463c-4321-a1c7-05f120c607aa@amperemail.onmicrosoft.com>
Date: Mon, 8 Sep 2025 10:58:55 -0400
From: Adam Young <admiyo@...eremail.onmicrosoft.com>
To: Sudeep Holla <sudeep.holla@....com>, admiyo@...amperecomputing.com
Cc: Jassi Brar <jassisinghbrar@...il.com>,
 "Rafael J. Wysocki" <rafael@...nel.org>, Len Brown <lenb@...nel.org>,
 Robert Moore <robert.moore@...el.com>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, Jeremy Kerr <jk@...econstruct.com.au>,
 Matt Johnston <matt@...econstruct.com.au>,
 "David S . Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>,
 Jonathan Cameron <Jonathan.Cameron@...wei.com>,
 Huisong Li <lihuisong@...wei.com>
Subject: Re: [PATCH v23 1/2] mailbox/pcc: support mailbox management of the
 shared buffer

On 9/4/25 07:00, Sudeep Holla wrote:
> On Mon, Jul 14, 2025 at 08:10:07PM -0400,admiyo@...amperecomputing.com wrote:
>> From: Adam Young<admiyo@...amperecomputing.com>
>>
>> Define a new, optional, callback that allows the driver to
>> specify how the return data buffer is allocated.  If that callback
>> is set,  mailbox/pcc.c is now responsible for reading from and
>> writing to the PCC shared buffer.
>>
>> This also allows for proper checks of the Commnand complete flag
>> between the PCC sender and receiver.
>>
>> For Type 4 channels, initialize the command complete flag prior
>> to accepting messages.
>>
>> Since the mailbox does not know what memory allocation scheme
>> to use for response messages, the client now has an optional
>> callback that allows it to allocate the buffer for a response
>> message.
>>
>> When an outbound message is written to the buffer, the mailbox
>> checks for the flag indicating the client wants an tx complete
>> notification via IRQ.  Upon receipt of the interrupt It will
>> pair it with the outgoing message. The expected use is to
>> free the kernel memory buffer for the previous outgoing message.
>>
> I know this is merged. Based on the discussions here, I may send a revert
> to this as I don't think it is correct.

Have you decided what to do?  The MCTP over PCC driver depends on the 
behavior in this patch. If you do revert, I will need a path forward.

Based on other code review feed back, I need to make an additional 
change:  the rx_alloc callback function needs to be atomically set, and 
thus needs to move to the mailbox API.  There it will pair with the 
prepare transaction function.  It is a small change, but I expect some 
feedback from the mailbox maintainers.

I know all of the other drivers that use the PCC mailbox currently do 
direct management of the shared buffer.  I suspect that is the biggest 
change that is causing you concern.  Are you OK with maintaining a 
mailbox-managed path to buffer management as well?  I think it will be 
beneficial to other drivers in the long run.