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

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Date:	Mon, 11 Nov 2013 16:09:53 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Hongbo Zhang <hongbo.zhang@...escale.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 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.

--
Dan
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