Subject: [PATCH V2] dma: tegra: register as an OF DMA controller

From: [email-masked] (Stephen Warren)
Date: Tue, 03 Dec 2013 10:59:54 -0700

On 11/29/2013 07:17 AM, Thierry Reding wrote:
> On Mon, Nov 25, 2013 at 02:53:36PM -0700, Stephen Warren wrote: 
> [...]
>> memcpy(&tdc->dma_sconfig, sconfig, sizeof(*sconfig)); +	if
>> (!tdc->slave_id) +		tdc->slave_id = sconfig->slave_id; 
>> tdc->config_init = true;
> 
> This could use some blank lines to unclutter it a bit.

To be honest, I feel the opposite; random blank lines sprinkled in the
middle of related code make the code structure harder to follow.

>> @@ -942,7 +947,7 @@ static struct dma_async_tx_descriptor
>> *tegra_dma_prep_slave_sg( ahb_seq |=
>> TEGRA_APBDMA_AHBSEQ_BUS_WIDTH_32;
>> 
>> csr |= TEGRA_APBDMA_CSR_ONCE | TEGRA_APBDMA_CSR_FLOW; -	csr |=
>> tdc->dma_sconfig.slave_id << TEGRA_APBDMA_CSR_REQ_SEL_SHIFT; +
>> csr |= tdc->slave_id << TEGRA_APBDMA_CSR_REQ_SEL_SHIFT;
> 
> Perhaps I'm missing something, but couldn't we reuse the .slave_id
> field of struct dma_slave_config? It seems like it might be
> overwritten by the DMA engine core or users when they call
> dmaengine_slave_config().

The slave ID seems channel-specific to me, and hence should be managed
at the channel level. struct dma_slave_config is the client-specified
runtime properties. As you mention, I also worry about client drivers
trampling over the dma_slave_config data, so storing it where they
can't doesn't seem like a good idea.

> But wouldn't it be better to have the core take care of all the
> slave ID management, so we don't have to jump through hoops? Or
> perhaps the concept isn't general enough to map well to other
> drivers.

It's a HW specific detail, I think. At least, the representation of
the data in a DT binding is, and hence so is the parsing of the data
from DT. The rest of the code that's in the driver re: slave ID is
trivial enough it doesn't seem worth sharing. And in fact, once this
series is done, we can even remove the conditional assignments to
slave_id and require that it be provided by DT and never from
dma_slave_config, which simplifies it even further.

>> static int tegra_dma_probe(struct platform_device *pdev) { struct
>> resource	*res; @@ -1383,10 +1402,22 @@ static int
>> tegra_dma_probe(struct platform_device *pdev) goto err_irq; }
>> 
>> +	tdma->xlate_info.device = &tdma->dma_dev; +
>> tdma->xlate_info.post_alloc = tegra_dma_of_xlate_post_alloc; +
>> ret = of_dma_controller_register(pdev->dev.of_node, +
>> of_dma_slave_xlate, &tdma->xlate_info); +	if (ret < 0) { +
>> dev_err(&pdev->dev, +			"Tegra20 APB DMA OF registration failed
>> %d\n", ret); +		goto err_unregister_dma_dev; +	}
> 
> Would it be useful to move this into the core and have it register
> the OF parts transparently to the driver? That's of course nothing
> that should be done in this patch.

That'd probably be possible, yes, given a few extra fields in struct
dma_device, e.g. for the of_xlate function pointer. As you say, I
won't address it in this patch though.