linux-kernel - Re: [PATCH v11 3/3] DMA: Freescale: update driver to support 8-channel DMA engine

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Date:	Tue, 12 Nov 2013 15:05:43 +0800
From:	Hongbo Zhang <hongbo.zhang@...escale.com>
To:	Dan Williams <dan.j.williams@...el.com>
CC:	"Koul, Vinod" <vinod.koul@...el.com>, <rob.herring@...xeda.com>,
	<pawel.moll@....com>, <mark.rutland@....com>,
	Stephen Warren <swarren@...dotorg.org>,
	<ian.campbell@...rix.com>, <devicetree@...r.kernel.org>,
	linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	<dmaengine@...r.kernel.org>
Subject: Re: [PATCH v11 3/3] DMA: Freescale: update driver to support 8-channel
 DMA engine

On 11/12/2013 08:09 AM, Dan Williams wrote:
> On Mon, Nov 11, 2013 at 1:12 AM, Hongbo Zhang
> <hongbo.zhang@...escale.com> wrote:
>>>>>> diff --git a/drivers/dma/fsldma.c b/drivers/dma/fsldma.c
>>>>>> index 49e8fbd..16a9a48 100644
>>>>>> --- a/drivers/dma/fsldma.c
>>>>>> +++ b/drivers/dma/fsldma.c
>>>>>> @@ -1261,7 +1261,9 @@ static int fsl_dma_chan_probe(struct
>>>>>> fsldma_device
>>>>>> *fdev,
>>>>>>         WARN_ON(fdev->feature != chan->feature);
>>>>>>           chan->dev = fdev->dev;
>>>>>> -    chan->id = ((res.start - 0x100) & 0xfff) >> 7;
>>>>>> +    chan->id = (res.start & 0xfff) < 0x300 ?
>>>>>> +           ((res.start - 0x100) & 0xfff) >> 7 :
>>>>>> +           ((res.start - 0x200) & 0xfff) >> 7;
>>>>>>         if (chan->id >= FSL_DMA_MAX_CHANS_PER_DEVICE) {
>>> Isn't it a bit fragile to have this based on the resource address?
>>> Can't device tree tell you the channel id directly by an index into
>>> the "dma0: dma@...300" node?
>>
>> Yes, both this way and putting a "cell-index" into device tree work.
>> This won't be fragile, because the resource address should always be defined
>> correctly, otherwise even if we can tell a channel id by "cell-index" but
>> with wrong resource address, nothing will work.
>> This piece of code only doesn't seem as neat as using "cell-index", but we
>> prefer the style that let the device tree describes as true as what hardware
>> really has. This doesn't mean "cell-index" isn't acceptable, if it is
>> necessary and unavoidable, we can send another patch to add it, but
>> currently there is no need and we don't have to do this.
>>
> I'm pointing it out because we just had a bug fix to another driver
> motivated by the fact that resource addresses may move from one
> implementation to another, whereas the cell index provided by device
> tree is static.  Just a note, no need to fix it now.

Get it, and will remember it, thank you Dan.
> --
> Dan
>



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