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

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Message-ID: <CAHp75Vdh0Z1GPD0qNHZKdoLLhScWyLgsaNN8tsZkPB6eBDVd4g@mail.gmail.com>
Date:	Sun, 9 Jun 2013 22:43:38 +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, Jun 7, 2013 at 6:02 AM, Xiang Wang <wangxfdu@...il.com> wrote:
> 2013/6/6 Andy Shevchenko <andy.shevchenko@...il.com>:
>> On Thu, Jun 6, 2013 at 6:09 AM, Xiang Wang <wangxfdu@...il.com> wrote:

[]

>>> 2. DMA controller handles trailing bytes for us.
>>> This is the case I mentioned in my previous email. "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"
>>> I think we should know how many residual bytes in this case even the
>>> DMA status is DMA_SUCCESS.
>>
>> I'm still not convinced that you have implement such algorithm. Anyway,
>> if I got it correctly you have something like "Receive FIFO Occupancy
>> Register (FOR)" in UART. When you got timeout interrupt, you may get
>> residue and value from FOR, sum them, program DMA to transfer the
>> trailing bytes.
>> What did I miss?
> This case is another working mode of peripheral device (e.g. UART). If
> we configure UART to work in this mode, the hardware(DMA controller
> and UART controller) will deal with the trailing bytes for us.
> The workflow is likely to be:
> a. When timeout occurs in UART, it notifies DMA controller to move the
> trailing bytes in UART FIFO. (software transparent)
> b. After DMA finishes moving all bytes in UART FIFO, it will send out
> an End Of Receive(EOR) interrupt.
> So in this case, software sees an EOR interrupt from DMA controller
> instead of an Timeout Interrupt from UART controller.

Thanks for explanation.

It's quite good your hw could manage such cases, though I thing it's
wrong to have residue great than 0 and DMA_SUCCESS as a status in the
driver.

So, if Vinod, Dan, or any experienced developer is okay with it, I'm okay too.

--
With Best Regards,
Andy Shevchenko
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