linux-kernel - Re: [PATCH net-next v29 1/3] mailbox: add callback function for rx buffer allocation

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Message-ID: <CABb+yY3T6LdPoGysNAyNr_EgCAcq2Vxz3V1ReDgF_fGYcqRrbw@mail.gmail.com>
Date: Sun, 5 Oct 2025 18:34:51 -0500
From: Jassi Brar <jassisinghbrar@...il.com>
To: Adam Young <admiyo@...eremail.onmicrosoft.com>
Cc: Adam Young <admiyo@...amperecomputing.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>, 
	Sudeep Holla <sudeep.holla@....com>, Jonathan Cameron <Jonathan.Cameron@...wei.com>, 
	Huisong Li <lihuisong@...wei.com>
Subject: Re: [PATCH net-next v29 1/3] mailbox: add callback function for rx
 buffer allocation

On Sun, Oct 5, 2025 at 12:13 AM Adam Young
<admiyo@...eremail.onmicrosoft.com> wrote:
>
> Jassi, this one needs your attention specifically.
>
> Do you have an issue with adding this callback?  I think it will add an
> important ability to the receive path for the mailbox API: letting the
> client driver specify how to allocate the memory that the message is
> coming in.  For general purpose mechanisms like PCC, this is essential:
> the mailbox cannot know all of the different formats that the drivers
> are going to require.  For example, the same system might have MPAM
> (Memory Protection) and MCTP (Network Protocol) driven by the same PCC
> Mailbox.
>
Looking at the existing code, I am not even sure if rx_alloc() is needed at all.

Let me explain...
1) write_response, via rx_alloc, is basically asking the client to
allocate a buffer of length parsed from the pcc header in shmem.
2) write_response is called from isr and even before the
mbox_chan_received_data() call.

Why can't you get rid of write_response() and simply call
    mbox_chan_received_data(chan, pchan->chan.shmem)
for the client to allocate and memcpy_fromio itself?
Ideally, the client should have the buffer pre-allocated and only have
to copy the data into it, but even if not it will still not be worse
than what you currently have.

-jassi