linux-kernel - Re: [PATCHv9] dmaengine: Add support for BCM2835

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Message-ID: <CABDT8WyUVbPu60qwMbAxKS=2m2akskybZQS3-+rehuUD+3oBXQ@mail.gmail.com>
Date:	Sun, 5 Jan 2014 16:06:39 +0100
From:	Florian Meier <florian.meier@...lo.de>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Stephen Warren <swarren@...dotorg.org>,
	Vinod Koul <vinod.koul@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	devicetree <devicetree@...r.kernel.org>,
	"alsa-devel@...a-project.org" <alsa-devel@...a-project.org>,
	Lars-Peter Clausen <lars@...afoo.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Mark Brown <broonie@...nel.org>,
	linux-rpi-kernel <linux-rpi-kernel@...ts.infradead.org>,
	dmaengine <dmaengine@...r.kernel.org>,
	Stephen Warren <swarren@...dia.com>
Subject: Re: [PATCHv9] dmaengine: Add support for BCM2835

On 05.01.2014 15:06, Arnd Bergmann wrote:
> On Saturday 04 January 2014, Florian Meier wrote:
>> On 02.01.2014 19:03, Arnd Bergmann wrote:
>>> On Thursday 02 January 2014 18:49:23 Florian Meier wrote:
>>>> Add support for DMA controller of BCM2835 as used in the Raspberry Pi.
>>>> Currently it only supports cyclic DMA.
>>>
>>> Looks very nice. Just a few details I noticed:
>>>
>>>> +#if defined(CONFIG_OF)
>>>> +static const struct of_device_id bcm2835_dma_of_match[] = {
>>>> + { .compatible = "brcm,bcm2835-dma", },
>>>> + {},
>>>> +};
>>>> +MODULE_DEVICE_TABLE(of, bcm2835_dma_of_match);
>>>> +#endif
>>>
>>> I doubt we are going to see non-DT versions of this driver, so the #ifdef
>>> can just get removed here.
>>
>> As already explained in previous versions of this patch thread, there is
>> a non-DT version in a downstream kernel and the more I make this patch
>> incompatible with non-DT, the harder it gets to upstream the remaining
>> stuff. I hope this is not something that blocks this driver from getting
>> accepted.
>
> In this case, that's certainly not an excuse unless you are worried about
> 400 bytes of .rodata in one driver and there are no side-effects of having
> the device ID listed when it's not used.

You are right! I forgot that having these lines doesn't prevent non-DT
initialization. I will change that.

> Even if it made a real difference
> to another project, you'd need a much better excuse -- if you are worried
> about compatibility between mainline and some downstream kernel, how about
> changing that downstream kernel to use DT?

That is the future plan. Although, the current state is that there is
a downstream kernel with lots of devicetree incompatible drivers and
an upstream kernel with some missing important drivers. Therefore,
since I want my drivers being tested by the community, they have to be
non-DT compatible (or at least it should be easy to bring changes from
the non-DT version to the DT version and vice versa).
Anyway, in this case this introduces no problem as you have mentioned before.

>>> [...]
>>> This can now be simplified using the dma_get_any_slave_channel() interface
>>> taht Stephen Warren introduced.
>>> [...]
>>> dma_set_mask_and_coherent()
>>
>> Sigh, the API is developing faster than I can keep track with updating
>> this patch. I hope some day I will be faster....
>> When Russell told me about the second one before, it hoped that I can
>> avoid merging different trees on my own, but it seems that you want me
>> to do that ;-)
>
> The dma_get_any_slave_channel() change is probably my fault. I suggested
> both the initial dma_get_slave_channel() API and this one because the
> original approach turned out too complicated. If dma_set_mask_and_coherent().
>
> I don't think you have to merge other trees, to get both APIs, they should
> already be part of the dma-slave tree that your patch would get merged
> into. If not, we can probably come up with a different solution. The
> dma_set_mask_and_coherent() suggestion is not as important as the
> dma_get_any_slave_channel() one, if you have to choose between them.

Both changes are in the slave-dma tree, but I need patches from the
bcm2835 tree and the asoc tree, too. Although, it shouldn't be too
complicated to merge them, I hope.

>>>> + if (pdev->dev.of_node) {
>>>> + /* Device-tree DMA controller registration */
>>>> + rc = of_dma_controller_register(pdev->dev.of_node,
>>>> + bcm2835_dma_xlate, od);
>>>> + if (rc) {
>>>> + dev_err(&pdev->dev, "Failed to register DMA controller\n");
>>>> + goto err_no_dma;
>>>> + }
>>>> + }
>>>
>>> If pdev->dev.of_node isn't set, you didn't get here, so the if() can be removed.
>>
>> Why not? (In the case of non-DT initialization)
>
> The code I quoted is right after these lines:
>
> +       /* Request DMA channel mask from device tree */
> +       if (of_property_read_u32(pdev->dev.of_node,
> +                       "brcm,dma-channel-mask",
> +                       &chans_available)) {
> +               dev_err(&pdev->dev, "Failed to get channel mask\n");
> +               rc = -EINVAL;
> +               goto err_no_dma;
> +       }
>
> If DT is disabled or not used, you return -EINVAL here.

Stupid me, thank you!

Greetings,
Florian
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