linux-kernel - Re: [PATCH 0/3] dmaengine: Stear users towards dma_request_slave

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Date:   Tue, 4 Feb 2020 10:15:36 +0200
From:   Peter Ujfalusi <peter.ujfalusi@...com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
CC:     John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Vinod Koul <vkoul@...nel.org>,
        dmaengine <dmaengine@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Linux-sh list <linux-sh@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>
Subject: Re: [PATCH 0/3] dmaengine: Stear users towards
 dma_request_slave_chan()

Hi Geert,

On 04/02/2020 10.01, Geert Uytterhoeven wrote:
> Hi Peter,
> 
> On Tue, Feb 4, 2020 at 7:52 AM Peter Ujfalusi <peter.ujfalusi@...com> wrote:
>> On 03/02/2020 22.34, Geert Uytterhoeven wrote:
>>> On Mon, Feb 3, 2020 at 9:21 PM John Paul Adrian Glaubitz
>>> <glaubitz@...sik.fu-berlin.de> wrote:
>>>> On 2/3/20 2:32 PM, Geert Uytterhoeven wrote:
>>>>> Both rspi and sh-msiof have users on legacy SH (i.e. without DT):
>>>>
>>>> FWIW, there is a patch set by Yoshinori Sato to add device tree support
>>>> for classical SuperH hardware. It was never merged, unfortunately :(.
>>>
>>> True.
>>>
>>>>> Anyone who cares for DMA on SuperH?
>>>>
>>>> What is DMA used for on SuperH? Wouldn't dropping it cut support for
>>>> essential hardware features?
>>>
>>> It may make a few things slower.
>>
>> I would not drop DMA support but I would suggest to add dma_slave_map
>> for non DT boot so the _compat() can be dropped.
> 
> Which is similar in spirit to gpiod_lookup and clk_register_clkdev(),
> right?

Yes, it is similar:

/* OMAP730, OMAP850 */
static const struct dma_slave_map omap7xx_sdma_map[] = {
	{ "omap-mcbsp.1", "tx", SDMA_FILTER_PARAM(8) },
	{ "omap-mcbsp.1", "rx", SDMA_FILTER_PARAM(9) },
	{ "omap-mcbsp.2", "tx", SDMA_FILTER_PARAM(10) },
	{ "omap-mcbsp.2", "rx", SDMA_FILTER_PARAM(11) },
	{ "mmci-omap.0", "tx", SDMA_FILTER_PARAM(21) },
	{ "mmci-omap.0", "rx", SDMA_FILTER_PARAM(22) },
	{ "omap_udc", "rx0", SDMA_FILTER_PARAM(26) },
	{ "omap_udc", "rx1", SDMA_FILTER_PARAM(27) },
	{ "omap_udc", "rx2", SDMA_FILTER_PARAM(28) },
	{ "omap_udc", "tx0", SDMA_FILTER_PARAM(29) },
	{ "omap_udc", "tx1", SDMA_FILTER_PARAM(30) },
	{ "omap_udc", "tx2", SDMA_FILTER_PARAM(31) },
};

"device name", "channel name", "parameter for filter"

The in the DMA driver (omap-dma.c):
	od->ddev.filter.map = od->plat->slave_map;
	od->ddev.filter.mapcnt = od->plat->slavecnt;
	od->ddev.filter.fn = omap_dma_filter_fn;

When things are converted the filter function no longer needs to be
exported, it is local to the DMA driver.

> 
>> Imho on lower spec SoC (and I believe SuperH is) the DMA makes big
>> difference offloading data movement from the CPU.
> 
> Assumed it is actually used...

Right, imho (again) we should not decide if given SoC needs it or not.
It is up to the drivers to use it or not, but with the dma_slave_map
there is no difference between DT or legacy boot handling towards DMA.

> Gr{oetje,eeting}s,
> 
>                         Geert
> 

- Péter

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