Subject: [PATCH V4 1/2] of: Add generic device tree DMA helpers

From: [email-masked] (Jon Hunter)
Date: Fri, 14 Sep 2012 08:27:16 -0500

Hi Arnd,

On 09/14/2012 04:43 AM, Arnd Bergmann wrote:
> On Thursday 13 September 2012, Jon Hunter wrote:
>> This is based upon the work by Benoit Cousson [1] and Nicolas Ferre [2]
>> to add some basic helpers to retrieve a DMA controller device_node and the
>> DMA request/channel information.
> 
> I think we're getting very close now, I only have a few small comments left
> that should all be uncontroversial.
> 
>> +
>> +Client drivers should specify the DMA property using a phandle to the controller
>> +followed by DMA controller specific data.
>> +
>> +Required property:
>> +- dmas:			List of one or more DMA specifiers, each consisting of
>> +			- A phandle pointing to DMA controller node
>> +			- A single integer cell containing DMA controller
>> +			  specific information. This typically contains a dma
>> +			  request line number or a channel number, but can
>> +			  contain any data that is used required for configuring
>> +			  a channel.
>> +- dma-names: 		Contains one identifier string for each dma specifier in
>> +			the dmas property. The specific strings that can be used
>> +			are defined in the binding of the DMA client device.
> 
> I think here we need to clarify that listing the same name multiple times implies
> having multiple alternatives for the same channel.

Ok, however, the way it works right now is that we will use the first
specifier that matches the name. So if there are multiple with the same
name that would imply that someone will need call the
xxx_request_slave_channel() multiple times to extract these. Is that ok?

>> +Example:
>> +
>> +- One DMA write channel, one DMA read/write channel:
>> +
>> +	i2c1: i2c at 1 {
>> +		...
>> +		dmas = <&sdma 2		/* read channel */
>> +			&sdma 3>;	/* write channel */
>> +		dma-names = "rx", "tx"
>> +		...
>> +	};
> 
> And please add an example documenting this case.

Ok.

>> +/**
>> + * of_dma_find_channel - Find a DMA channel by name
>> + * @np:		device node to look for DMA channels
>> + * @name:	name of desired channel
>> + * @dma_spec:	pointer to DMA specifier as found in the device tree
>> + *
>> + * Find a DMA channel by the name. Returns 0 on success or appropriate
>> + * errno value on error.
>> + */
>> +static int of_dma_find_channel(struct device_node *np, char *name,
>> +			       struct of_phandle_args *dma_spec)
>> +{
>> +	int count, i;
>> +	const char *s;
>> +
>> +	count = of_property_count_strings(np, "dma-names");
>> +	if (count < 0)
>> +		return count;
>> +
>> +	for (i = 0; i < count; i++) {
>> +		of_property_read_string_index(np, "dma-names", i, &s);
>> +
>> +		if (strcmp(name, s))
>> +			continue;
>> +
>> +		return of_parse_phandle_with_args(np, "dmas", "#dma-cells",
>> +							i, dma_spec);
>> +	}
>> +
>> +	return -ENODEV;
>> +}
> 
> I think we should at least keep trying the other channels with the same name when 
> of_parse_phandle_with_args fails. We might want to do something smarter in
> the long run, e.g. to spread channel allocations across the avaialable controllers.

Ok, can add this.

>> +/**
>> + * of_dma_request_slave_channel - Get the DMA slave channel
>> + * @np:		device node to get DMA request from
>> + * @name:	name of desired channel
>> + *
>> + * Returns pointer to appropriate dma channel on success or NULL on error.
>> + */
>> +struct dma_chan *of_dma_request_slave_channel(struct device_node *np,
>> +					      char *name)
>> +{
>> ...
>> +}
>> +EXPORT_SYMBOL_GPL(of_dma_request_slave_channel);
> 
> I think this no longer needs to be exported, with patch 2 on top.

Right, I was in two minds but I can remove this.

>> +/**
>> + * of_dma_simple_xlate - Simple DMA engine translation function
>> + * @dma_spec:	pointer to DMA specifier as found in the device tree
>> + * @of_dma:	pointer to DMA controller data
>> + *
>> + * A simple translation function for devices that use a 32-bit value for the
>> + * filter_param when calling the DMA engine dma_request_channel() function.
>> + * Note that this translation function requires that #dma-cells is equal to 1
>> + * and the argument of the dma specifier is the 32-bit filter_param. Returns
>> + * pointer to appropriate dma channel on success or NULL on error.
>> + */
>> +struct dma_chan *of_dma_simple_xlate(struct of_phandle_args *dma_spec,
>> +						struct of_dma *ofdma)
>> +{
>> +	int count = dma_spec->args_count;
>> +	struct of_dma_filter_info *info = ofdma->of_dma_data;
>> +
>> +	if (!info || !info->filter_fn)
>> +		return NULL;
>> +
>> +	if (count != 1)
>> +		return NULL;
>> +
>> +	return dma_request_channel(info->dma_cap, info->filter_fn,
>> +			&dma_spec->args[0]);
>> +}
> 
> But this one does.

Good catch!

Thanks
Jon