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

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Date:   Thu, 24 Jan 2019 06:57:25 -0800
From:   Dalon L Westergreen <dalon.westergreen@...ux.intel.com>
To:     Atsushi Nemoto <atsushi.nemoto@...d.co.jp>
Cc:     thor.thayer@...ux.intel.com, netdev@...r.kernel.org,
        tomonori.sakita@...d.co.jp
Subject: Re: [PATCH] net: altera_tse: fix msgdma_tx_completion on non-zero
 fill_level case

On Thu, 2019-01-24 at 14:05 +0900, Atsushi Nemoto wrote:
> On Wed, 23 Jan 2019 14:47:06 -0800, Dalon L Westergreen <
> dalon.westergreen@...ux.intel.com> wrote:
> > > >   	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?
> 
> Thank you for review.
> 
> I agree with you.  It would be OK returning a possibly off-by-one
> value unless it is not an negative value.
> 
> Then, how about this instead?
> 

This works for me. thanks.

> --- a/drivers/net/ethernet/altera/altera_msgdma.c
> +++ b/drivers/net/ethernet/altera/altera_msgdma.c
> @@ -145,7 +145,8 @@ 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 = max_t(int,
> +			      priv->tx_prod - priv->tx_cons - inuse - 1, 0);
>  	} else {
>  		/* Check for buffered last packet */
>  		status = csrrd32(priv->tx_dma_csr, msgdma_csroffs(status));