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

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Message-ID: <CACDNBm3WJ_U22uqKuVssV4ntATTHnz_8=pq9_qa33S+eKreKSA@mail.gmail.com>
Date:	Mon, 3 Jun 2013 11:22:13 +0800
From:	Xiang Wang <wangxfdu@...il.com>
To:	Andy Shevchenko <andy.shevchenko@...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

2013/5/31 Andy Shevchenko <andy.shevchenko@...il.com>:
> 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.
Hi, Andy
Your comment makes sense. But if the user is allowed to query the
residue value in real-time, we cannot just return a saved value to
him.
Why I use a saved value (chan->bytes_residue)?
In current mmp pdma driver, a phy channel will be freed after the
transmission finishes (chan->phy is set to NULL). So we cannot get the
physical channel information after we call DMA_TERMINATE_ALL or DMA
finishes itself.
That is to say, when the use queries the channel information at these
points, the chan->phy is usually NULL.
>
>>                 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.
Hi, Andy
There is one exception from my point of view. When we are receiving
data from peripheral devices, we usually start a DMA transaction with
a target length of 4K for example. When a timed-out event occurs in
peripheral device, it will notify DMA controller and DMA controller
will send out a End of Receive interrupt (Marvell specific?).
In such situation, DMA status is also DMA_SUCCESS. But the residual
bytes is not 0 and the user must query it.
Thanks!
>
> --
> With Best Regards,
> Andy Shevchenko
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