Date:	Fri, 20 Nov 2015 12:25:06 +0200
From:	Peter Ujfalusi <peter.ujfalusi@...com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Vinod Koul <vinod.koul@...el.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Tony Lindgren <tony@...mide.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Dan Williams <dan.j.williams@...el.com>,
	<dmaengine@...r.kernel.org>,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	Linux MMC List <linux-mmc@...r.kernel.org>,
	<linux-crypto@...r.kernel.org>,
	linux-spi <linux-spi@...r.kernel.org>,
	Linux Media Mailing List <linux-media@...r.kernel.org>,
	ALSA Development Mailing List <alsa-devel@...a-project.org>
Subject: Re: [PATCH 02/13] dmaengine: Introduce
 dma_request_slave_channel_compat_reason()

On 11/19/2015 01:25 PM, Arnd Bergmann wrote:
>> If we have two main APIs, one to request slave channels and one to get any
>> channel with given capability
>> dma_request_slave_channel(NULL, NULL, &mask, fn, fn_param); /* Legacy slave */
>> dma_request_slave_channel(dev, name, NULL, NULL, NULL); /* DT/ACPI, current
>>                                                            slave */
>> dma_request_slave_channel(dev, name, &mask, fn, fn_param); /* current compat*/
>>
>> This way we can omit the mask also in cases when the client only want to get
>> DMA_SLAVE, we can just build up the mask within the function. If the mask is
>> provided we would copy the bits from the provided mask, so for example if you
>> want to have DMA_SLAVE+DMA_CYCLIC, the driver only needs to pass DMA_CYCLIC,
>> the DMA_SLAVE is going to be set anyways.
> 
> I think it's more logical here to have mask=NULL mean that we want DMA_SLAVE,
> but otherwise pass the full mask as DMA_SLAVE|DMA_CYCLIC etc.

Yep, could be, while I would write the core part to set the DMA_SLAVE
unconditionally anyways. If the API say it is dma_request_slavechan() it is
expected to get channel which is capable of DMA_SLAVE.

>> dma_request_channel(mask); /* memcpy. etc, non slave mostly */
>>
>> Not sure how to name this as reusing existing (good, descriptive) function
>> names would mean changes all over the kernel to start off this.
>>
>> Not used and
>> request_dma_channel(); /* as _irq/_mem_region/_resource, etc */
>> request_dma();
>> dma_channel_request();
> 
> dma_request_slavechan();
> dma_request_slave();
> dma_request_mask();

Let me think aloud here a bit...
1. To request slave channel which will return you the channel your device is
bind via DT/ACPI or the platform map table you propose later:

dma_request_chan(struct device *dev, const char *name);

2. To request a channel (any channel) matching with the capabilities the
driver needs, like memcpy, memset, etc:

#define dma_request_chan_by_mask(mask) __dma_request_chan_by_mask(&(mask))
__dma_request_chan_by_mask(const dma_cap_mask_t *mask);

I think the dma_request_chan() does not need mask to be passed, since via this
we request a specific channel which has been defined/set by DT/ACPI or the
lookup map. We could add a mask parameter which could be used to sanity check
the channel we got against the capabilities the driver needs from the channel.
We currently do this in the drivers where the author wanted to make sure that
the channel is capable of what it should be capable.

So two API to request channel:
struct dma_chan *dma_request_chan(struct device *dev, const char *name);
struct dma_chan *dma_request_chan_by_mask(const dma_cap_mask_t *mask);

Both will return with the valid channel pointer or in case of failure with
ERR_PTR().

We need to go through the code in regards to return codes also to have sane
mapping.

> 
>> All in all, not sure which way would be better...
> 
> I think I would prefer the simplest API to have only the dev+name
> arguments, as we tend to move that way for all platforms anyway, and it
> seems silly to have all drivers pass three NULL arguments all the time.
> At the moment, there are 139 references to dma_request_slave_channel_*
> in the kernel, and only 46 of them are dma_request_slave_channel_compat.
> Out of those 46, a couple can already be converted back to use
> dma_request_slave_channel() because the platform now only supports
> devicetree based boots and will not go back to platform data.
> 
> How about something like
> 
> extern struct dma_chan *
> __dma_request_chan(struct device *dev, const char *name,
> 		    const dma_cap_mask_t *mask, dma_filter_fn fn, void *fn_param);
> 
> static inline struct dma_chan *
> dma_request_slavechan(struct device *dev, const char *name)
> {
> 	return __dma_request_chan(dev, name, NULL, NULL, NULL);
> }
> 
> static inline struct dma_chan *
> dma_request_chan(const dma_cap_mask_t *mask)
> {
> 	return __dma_request_chan(NULL, NULL, mask, NULL, NULL);
> }
> 
> That way the vast majority of drivers can use one of the two nice interfaces
> and the rest can be converted to use __dma_request_chan().
> 
> On a related topic, we had in the past considered providing a way for
> platform code to register a lookup table of some sort, to associate
> a device/name pair with a configuration. That would let us use the
> simplified dma_request_slavechan(dev, name) pair everywhere. We could
> use the same method that we have for clk_register_clkdevs() or
> pinctrl_register_map().
> 
> Something like either
> 
> static struct dma_chan_map myplatform_dma_map[] = {
> 	{ .devname = "omap-aes0", .slave = "tx", .filter = omap_dma_filter_fn, .arg = (void *)65, },
> 	{ .devname = "omap-aes0", .slave = "rx", .filter = omap_dma_filter_fn, .arg = (void *)66, },
> };
> 
> or
> 
> static struct dma_chan_map myplatform_dma_map[] = {
> 	{ .devname = "omap-aes0", .slave = "tx", .master = "omap-dma-engine0", .req = 65, },
> 	{ .devname = "omap-aes0", .slave = "rx", .master = "omap-dma-engine0", .req = 66, },

sa11x0-dma expects the fn_param as string :o

> };

Basically we are deprecating the use of IORESOURCE_DMA?

For legacy the filter function is pretty much needed to handle the differences
between the platforms as not all of them does the filtering in a same way. So
the first type of map would be feasible IMHO.

> we could even allow a combination of the two, so the simple case just specifies
> master and req number, which requires changes to the dmaengine driver, but we could
> also do a mass-conversion to the .filter/.arg variant.

This will get rid of the need for the fn and fn_param parameters when
requesting dma channel, but it will not get rid of the exported function from
the dma engine drivers since in arch code we need to have visibility to the
filter_fn.

-- 
Péter
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