linux-kernel - Re: [PATCH] dma: mmp_pdma: support for getting residual bytes

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Message-ID: <CAHp75Veb+FK=5u_HBUp+KNT4__GjbSg1T-kq1tGebDq2pEJOBA@mail.gmail.com>
Date:	Fri, 31 May 2013 11:34:36 +0300
From:	Andy Shevchenko <andy.shevchenko@...il.com>
To:	Xiang Wang <wangxfdu@...il.com>
Cc:	Dan Williams <djbw@...com>, Vinod Koul <vinod.koul@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	cxie4@...vell.com, Xiang Wang <wangx@...vell.com>
Subject: Re: [PATCH] dma: mmp_pdma: support for getting residual bytes

On Fri, May 31, 2013 at 11:21 AM, Xiang Wang <wangxfdu@...il.com> wrote:
> In some of our drivers (e.g. UART) we may stop a running DMA
> before it finishes. So we need to know how many bytes have
> been transferred.

Couple of comments below.

> --- a/drivers/dma/mmp_pdma.c
> +++ b/drivers/dma/mmp_pdma.c

> @@ -589,7 +638,13 @@ static int mmp_pdma_control(struct dma_chan *dchan, enum dma_ctrl_cmd cmd,
>                 mmp_pdma_free_desc_list(chan, &chan->chain_pending);
>                 mmp_pdma_free_desc_list(chan, &chan->chain_running);
>                 spin_unlock_irqrestore(&chan->desc_lock, flags);
> -               chan->idle = true;
> +               chan->status = DMA_SUCCESS;
> +               chan->bytes_residue = 0;
> +               break;
> +       case DMA_PAUSE:
> +               disable_chan(chan->phy);
> +               chan->status = DMA_PAUSED;
> +               chan->bytes_residue = mmp_pdma_get_bytes_residue(chan);

Does it mean user has to do DMA_PAUSE first to get more or less
accurate residue?
Logically that sound correct, but in general we may allow user to get
approximate residue value of on going transfer.

>                 break;
>         case DMA_SLAVE_CONFIG:
>                 if (cfg->direction == DMA_DEV_TO_MEM) {
> @@ -637,7 +692,8 @@ static enum dma_status mmp_pdma_tx_status(struct dma_chan *dchan,
>         unsigned long flags;
>
>         spin_lock_irqsave(&chan->desc_lock, flags);
> -       ret = dma_cookie_status(dchan, cookie, txstate);
> +       ret = chan->status;
> +       dma_set_residue(txstate, chan->bytes_residue);
>         spin_unlock_irqrestore(&chan->desc_lock, flags);

Besides my patch which removes this spinlock I think the workflow
should be something like

status = dma_cookie_status()
if status == DMA_SUCCESS or !txstate:
return status

dma_set_residue()
return status

Because there is no reason to return residue of successfully finished
transfer. It should be 0.

--
With Best Regards,
Andy Shevchenko
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