netdev - Re: [PATCH] Revert "mailbox/pcc: support mailbox management of the shared buffer"

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Message-ID: <20251016-swift-of-unmatched-luck-dfdebe@sudeepholla>
Date: Thu, 16 Oct 2025 13:50:46 +0100
From: Sudeep Holla <sudeep.holla@....com>
To: Jassi Brar <jassisinghbrar@...il.com>,
	Adam Young <admiyo@...amperecomputing.com>
Cc: <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Sudeep Holla <sudeep.holla@....com>
Subject: Re: [PATCH] Revert "mailbox/pcc: support mailbox management of the
 shared buffer"

On Fri, Sep 26, 2025 at 04:33:11PM +0100, Sudeep Holla wrote:
> This reverts commit 5378bdf6a611a32500fccf13d14156f219bb0c85.
> 
> Commit 5378bdf6a611 ("mailbox/pcc: support mailbox management of the shared buffer")
> attempted to introduce generic helpers for managing the PCC shared memory,
> but it largely duplicates functionality already provided by the mailbox
> core and leaves gaps:
> 
> 1. TX preparation: The mailbox framework already supports this via
>   ->tx_prepare callback for mailbox clients. The patch adds
>   pcc_write_to_buffer() and expects clients to toggle pchan->chan.manage_writes,
>   but no drivers set manage_writes, so pcc_write_to_buffer() has no users.
> 
> 2. RX handling: Data reception is already delivered through
>    mbox_chan_received_data() and client ->rx_callback. The patch adds an
>    optional pchan->chan.rx_alloc, which again has no users and duplicates
>    the existing path.
> 
> 3. Completion handling: While adding last_tx_done is directionally useful,
>    the implementation only covers Type 3/4 and fails to handle the absence
>    of a command_complete register, so it is incomplete for other types.
> 
> Given the duplication and incomplete coverage, revert this change. Any new
> requirements should be addressed in focused follow-ups rather than bundling
> multiple behavioral changes together.
> 

The discussion on this revert stopped and I am not sure if we agreed to
revert it or not.

> Signed-off-by: Sudeep Holla <sudeep.holla@....com>
> ---
>  drivers/mailbox/pcc.c | 102 ++----------------------------------------
>  include/acpi/pcc.h    |  29 ------------
>  2 files changed, 4 insertions(+), 127 deletions(-)
> 
> diff --git a/drivers/mailbox/pcc.c b/drivers/mailbox/pcc.c
> index 0a00719b2482..f6714c233f5a 100644
> --- a/drivers/mailbox/pcc.c
> +++ b/drivers/mailbox/pcc.c
> @@ -306,22 +306,6 @@ static void pcc_chan_acknowledge(struct pcc_chan_info *pchan)
>  		pcc_chan_reg_read_modify_write(&pchan->db);
>  }
>  
> -static void *write_response(struct pcc_chan_info *pchan)
> -{
> -	struct pcc_header pcc_header;
> -	void *buffer;
> -	int data_len;
> -
> -	memcpy_fromio(&pcc_header, pchan->chan.shmem,
> -		      sizeof(pcc_header));
> -	data_len = pcc_header.length - sizeof(u32) + sizeof(struct pcc_header);
> -
> -	buffer = pchan->chan.rx_alloc(pchan->chan.mchan->cl, data_len);
> -	if (buffer != NULL)
> -		memcpy_fromio(buffer, pchan->chan.shmem, data_len);
> -	return buffer;
> -}
> -
>  /**
>   * pcc_mbox_irq - PCC mailbox interrupt handler
>   * @irq:	interrupt number
> @@ -333,8 +317,6 @@ static irqreturn_t pcc_mbox_irq(int irq, void *p)
>  {
>  	struct pcc_chan_info *pchan;
>  	struct mbox_chan *chan = p;
> -	struct pcc_header *pcc_header = chan->active_req;
> -	void *handle = NULL;
>  
>  	pchan = chan->con_priv;
>  
> @@ -358,17 +340,7 @@ static irqreturn_t pcc_mbox_irq(int irq, void *p)
>  	 * required to avoid any possible race in updatation of this flag.
>  	 */
>  	pchan->chan_in_use = false;
> -
> -	if (pchan->chan.rx_alloc)
> -		handle = write_response(pchan);
> -
> -	if (chan->active_req) {
> -		pcc_header = chan->active_req;
> -		if (pcc_header->flags & PCC_CMD_COMPLETION_NOTIFY)
> -			mbox_chan_txdone(chan, 0);

The above change in the original patch has introduced race on my platform
where the mbox_chan_txdone() kicks the Tx while the response to the
command is read in the below mbox_chan_received_data().

So I am going to repost/resend this revert as a fix now.

-- 
Regards,
Sudeep