netdev - RE: [net-next v4 06/15] iecm: Implement mailbox functionality

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Date:   Thu, 23 Jul 2020 00:45:55 +0000
From:   "Brady, Alan" <alan.brady@...el.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        "Nguyen, Anthony L" <anthony.l.nguyen@...el.com>
CC:     "davem@...emloft.net" <davem@...emloft.net>,
        "Michael, Alice" <alice.michael@...el.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "nhorman@...hat.com" <nhorman@...hat.com>,
        "sassmann@...hat.com" <sassmann@...hat.com>,
        "Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
        "Burra, Phani R" <phani.r.burra@...el.com>,
        "Hay, Joshua A" <joshua.a.hay@...el.com>,
        "Chittim, Madhu" <madhu.chittim@...el.com>,
        "Linga, Pavan Kumar" <pavan.kumar.linga@...el.com>,
        "Skidmore, Donald C" <donald.c.skidmore@...el.com>,
        "Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
        "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
Subject: RE: [net-next v4 06/15] iecm: Implement mailbox functionality

> -----Original Message-----
> From: Jakub Kicinski <kuba@...nel.org>
> Sent: Tuesday, July 21, 2020 11:05 AM
> To: Nguyen, Anthony L <anthony.l.nguyen@...el.com>
> Cc: davem@...emloft.net; Michael, Alice <alice.michael@...el.com>;
> netdev@...r.kernel.org; nhorman@...hat.com; sassmann@...hat.com;
> Kirsher, Jeffrey T <jeffrey.t.kirsher@...el.com>; Brady, Alan
> <alan.brady@...el.com>; Burra, Phani R <phani.r.burra@...el.com>; Hay,
> Joshua A <joshua.a.hay@...el.com>; Chittim, Madhu
> <madhu.chittim@...el.com>; Linga, Pavan Kumar
> <pavan.kumar.linga@...el.com>; Skidmore, Donald C
> <donald.c.skidmore@...el.com>; Brandeburg, Jesse
> <jesse.brandeburg@...el.com>; Samudrala, Sridhar
> <sridhar.samudrala@...el.com>
> Subject: Re: [net-next v4 06/15] iecm: Implement mailbox functionality
> 
> On Mon, 20 Jul 2020 17:38:01 -0700 Tony Nguyen wrote:
> > +		(cq->next_to_use)++;
> > +		if (cq->next_to_use == cq->ring_size)
> > +			cq->next_to_use = 0;
> > +	}
> > +
> > +	/* Force memory write to complete before letting hardware
> > +	 * know that there are new descriptors to fetch.
> > +	 */
> > +	iecm_wmb();
> 
> dma_wmb() would probably be sufficient here?
> 
> > +	wr32(hw, cq->reg.tail, cq->next_to_use);

We will investigate and see if dma_wmb is sufficient.  We do have a test for triggering the weak-ordered issue so we should be able to tell for certain.  The reasoning makes sense dma_wmb is sufficient but we want to test to be sure.

Alan