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

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Message-ID: <aOODt2lj--GOZkhU@bogus>
Date: Mon, 6 Oct 2025 09:54:15 +0100
From: Sudeep Holla <sudeep.holla@....com>
To: Jassi Brar <jassisinghbrar@...il.com>,
	Adam Young <admiyo@...eremail.onmicrosoft.com>
Cc: Adam Young <admiyo@...amperecomputing.com>, netdev@...r.kernel.org,
	Sudeep Holla <sudeep.holla@....com>, 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 net-next v29 1/3] mailbox: add callback function for rx
 buffer allocation

On Sun, Oct 05, 2025 at 06:34:51PM -0500, Jassi Brar wrote:
> 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.
> 

Exactly, this is what I have been telling.

Adam,

Please share the code that you have attempted with this approach and the
problems you have faced.

-- 
Regards,
Sudeep