netdev - Re: [PATCH] net: altera_tse: fix msgdma_tx_completion on non-zero fill

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Date:   Wed, 23 Jan 2019 14:47:06 -0800
From:   Dalon L Westergreen <dalon.westergreen@...ux.intel.com>
To:     thor.thayer@...ux.intel.com,
        Atsushi Nemoto <atsushi.nemoto@...d.co.jp>,
        netdev@...r.kernel.org
Cc:     Tomonori Sakita <tomonori.sakita@...d.co.jp>
Subject: Re: [PATCH] net: altera_tse: fix msgdma_tx_completion on non-zero
 fill_level case

On Tue, 2019-01-22 at 11:20 -0600, Thor Thayer wrote:
> On 1/21/19 2:29 AM, Atsushi Nemoto wrote:
> > From: Tomonori Sakita <tomonori.sakita@...d.co.jp>
> > 
> > If fill_level was not zero and status was not BUSY,
> > result of "tx_prod - tx_cons - inuse" might be zero.
> > Subtracting 1 unconditionally results invalid negative return value
> > on this case.
> > The subtraction is not needed if fill_level was not zero.
> > 
> > Signed-off-by: Tomonori Sakita <tomonori.sakita@...d.co.jp>
> > Signed-off-by: Atsushi Nemoto <atsushi.nemoto@...d.co.jp>
> > ---
> >   drivers/net/ethernet/altera/altera_msgdma.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/net/ethernet/altera/altera_msgdma.c
> > b/drivers/net/ethernet/altera/altera_msgdma.c
> > index 0fb986b..3df73d3 100644
> > --- a/drivers/net/ethernet/altera/altera_msgdma.c
> > +++ b/drivers/net/ethernet/altera/altera_msgdma.c
> > @@ -145,7 +145,7 @@ u32 msgdma_tx_completions(struct altera_tse_private
> > *priv)
> >   			& 0xffff;
> >   
> >   	if (inuse) { /* Tx FIFO is not empty */
> > -		ready = priv->tx_prod - priv->tx_cons - inuse - 1;
> > +		ready = priv->tx_prod - priv->tx_cons - inuse;
dont think my last email went through..

I am not sure about this.  This register indicates the number of entries
still to be processed by the dma.  the -1 is intended to represent the
decriptor currently being processed.  If ready is 
priv->tx_prod - priv->tx_cons - inuse couldn't you end up processing 1
too many packets?  IE: ready is 1 greater then the actual completed
packets?

I do agree that we should not be returning a negative value, but i dont
think i agree removing the -1 is the answer.  perhaps just check that ready
is greater than 0?

--dalon

> >   	} else {
> >   		/* Check for buffered last packet */
> >   		status = csrrd32(priv->tx_dma_csr, msgdma_csroffs(status));
> > 
> 
> I'm adding Dalon who has done a lot of work on msgdma. I'd like to get 
> his comments.
> 
> Thanks,
> 
> Thor
>