[PATCH 0/4] dw_dmac: few cleanups to the driver

[PATCH 0/4] dw_dmac: few cleanups to the driver

[Andy Shevchenko]

There are few cleanups to the driver which already acked and reviewed.
I decide to split last series to two parts. This is first part.

Heikki Krogerus (4):
  dmaengine: dw_dmac: use helper macro module_platform_driver()
  dmaengine: dw_dmac: add module alias
  dmaengine: dw_dmac: remove CLK dependency
  dmaengine: dw_dmac: amend description and indentation

 drivers/dma/Kconfig   |    1 -
 drivers/dma/dw_dmac.c |   20 ++++++--------------
 2 files changed, 6 insertions(+), 15 deletions(-)

-- 
1.7.10.4

[PATCH 1/4] dmaengine: dw_dmac: use helper macro module_platform_driver()

[Andy Shevchenko]

From: Heikki Krogerus <heikki.krogerus@linux.intel.com>

Since v3.2 we have nice macro to define the platform driver's init and exit
calls. This patch simplifies the dw_dmac driver by using that macro.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
---
 drivers/dma/dw_dmac.c |   13 ++-----------
 1 file changed, 2 insertions(+), 11 deletions(-)

diff --git a/drivers/dma/dw_dmac.c b/drivers/dma/dw_dmac.c
index c4b0eb3..0b88ced 100644
--- a/drivers/dma/dw_dmac.c
+++ b/drivers/dma/dw_dmac.c
@@ -1700,6 +1700,7 @@ MODULE_DEVICE_TABLE(of, dw_dma_id_table);
 #endif
 
 static struct platform_driver dw_driver = {
+	.probe		= dw_probe,
 	.remove		= __devexit_p(dw_remove),
 	.shutdown	= dw_shutdown,
 	.driver = {
@@ -1709,17 +1710,7 @@ static struct platform_driver dw_driver = {
 	},
 };
 
-static int __init dw_init(void)
-{
-	return platform_driver_probe(&dw_driver, dw_probe);
-}
-subsys_initcall(dw_init);
-
-static void __exit dw_exit(void)
-{
-	platform_driver_unregister(&dw_driver);
-}
-module_exit(dw_exit);
+module_platform_driver(dw_driver);
 
 MODULE_LICENSE("GPL v2");
 MODULE_DESCRIPTION("Synopsys DesignWare DMA Controller driver");
-- 
1.7.10.4

[PATCH 2/4] dmaengine: dw_dmac: add module alias

[Andy Shevchenko]

From: Heikki Krogerus <heikki.krogerus@linux.intel.com>

It's good to have a quasistatic name for the platform driver.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
---
 drivers/dma/dw_dmac.c |    1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/dma/dw_dmac.c b/drivers/dma/dw_dmac.c
index 0b88ced..bbb2a82 100644
--- a/drivers/dma/dw_dmac.c
+++ b/drivers/dma/dw_dmac.c
@@ -1712,6 +1712,7 @@ static struct platform_driver dw_driver = {
 
 module_platform_driver(dw_driver);
 
+MODULE_ALIAS("platform:dw_dmac");
 MODULE_LICENSE("GPL v2");
 MODULE_DESCRIPTION("Synopsys DesignWare DMA Controller driver");
 MODULE_AUTHOR("Haavard Skinnemoen (Atmel)");
-- 
1.7.10.4

[PATCH 3/4] dmaengine: dw_dmac: remove CLK dependency

[Andy Shevchenko]

From: Heikki Krogerus <heikki.krogerus@linux.intel.com>

This driver could be used on different platforms. Thus, the HAVE_CLK dependency
is dropped away.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
---
 drivers/dma/Kconfig |    1 -
 1 file changed, 1 deletion(-)

diff --git a/drivers/dma/Kconfig b/drivers/dma/Kconfig
index 677cd6e..df32537 100644
--- a/drivers/dma/Kconfig
+++ b/drivers/dma/Kconfig
@@ -83,7 +83,6 @@ config INTEL_IOP_ADMA
 
 config DW_DMAC
 	tristate "Synopsys DesignWare AHB DMA support"
-	depends on HAVE_CLK
 	select DMA_ENGINE
 	default y if CPU_AT32AP7000
 	help
-- 
1.7.10.4

[PATCH 4/4] dmaengine: dw_dmac: amend description and indentation

[Andy Shevchenko]

From: Heikki Krogerus <heikki.krogerus@linux.intel.com>

The driver will be used as a core part for various implementations of the
DesignWare DMA device. The patch adjusts description on the top and corrects
paragraph indentation in few places across the code.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
---
 drivers/dma/dw_dmac.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/dma/dw_dmac.c b/drivers/dma/dw_dmac.c
index bbb2a82..236ccb5 100644
--- a/drivers/dma/dw_dmac.c
+++ b/drivers/dma/dw_dmac.c
@@ -1,6 +1,5 @@
 /*
- * Driver for the Synopsys DesignWare DMA Controller (aka DMACA on
- * AVR32 systems.)
+ * Core driver for the Synopsys DesignWare DMA Controller
  *
  * Copyright (C) 2007-2008 Atmel Corporation
  * Copyright (C) 2010-2011 ST Microelectronics
@@ -9,6 +8,7 @@
  * it under the terms of the GNU General Public License version 2 as
  * published by the Free Software Foundation.
  */
+
 #include <linux/bitops.h>
 #include <linux/clk.h>
 #include <linux/delay.h>
@@ -222,7 +222,6 @@ static inline void dwc_dump_chan_regs(struct dw_dma_chan *dwc)
 		channel_readl(dwc, CTL_LO));
 }
 
-
 static inline void dwc_chan_disable(struct dw_dma *dw, struct dw_dma_chan *dwc)
 {
 	channel_clear_bit(dw, CH_EN, dwc->mask);
@@ -1679,6 +1678,7 @@ static int dw_resume_noirq(struct device *dev)
 
 	clk_prepare_enable(dw->clk);
 	dma_writel(dw, CFG, DW_CFG_DMA_EN);
+
 	return 0;
 }
 
-- 
1.7.10.4

Re: [PATCH 0/4] dw_dmac: few cleanups to the driver

[viresh kumar]

On Tue, Oct 2, 2012 at 5:11 PM, Andy Shevchenko
<andriy.shevchenko@linux.intel.com> wrote:
> There are few cleanups to the driver which already acked and reviewed.
> I decide to split last series to two parts. This is first part.

Looks good.

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>

Re: [PATCH 2/4] dmaengine: dw_dmac: add module alias

[Andy Shevchenko]

On Tue, 2012-10-02 at 14:41 +0300, Andy Shevchenko wrote: 
> From: Heikki Krogerus <heikki.krogerus@linux.intel.com>
> 
> It's good to have a quasistatic name for the platform driver.
Please, do not push this one.
It makes no sense now. Better we have it after a split.

> 
> Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> Reviewed-by: Felipe Balbi <balbi@ti.com>
> Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
> ---
>  drivers/dma/dw_dmac.c |    1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/dma/dw_dmac.c b/drivers/dma/dw_dmac.c
> index 0b88ced..bbb2a82 100644
> --- a/drivers/dma/dw_dmac.c
> +++ b/drivers/dma/dw_dmac.c
> @@ -1712,6 +1712,7 @@ static struct platform_driver dw_driver = {
>  
>  module_platform_driver(dw_driver);
>  
> +MODULE_ALIAS("platform:dw_dmac");
>  MODULE_LICENSE("GPL v2");
>  MODULE_DESCRIPTION("Synopsys DesignWare DMA Controller driver");
>  MODULE_AUTHOR("Haavard Skinnemoen (Atmel)");

-- 
Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Intel Finland Oy

Re: [PATCH 1/4] dmaengine: dw_dmac: use helper macro module_platform_driver()

[Andy Shevchenko]

On Tue, 2012-10-02 at 14:41 +0300, Andy Shevchenko wrote: 
> From: Heikki Krogerus <heikki.krogerus@linux.intel.com>
> 
> Since v3.2 we have nice macro to define the platform driver's init and exit
> calls. This patch simplifies the dw_dmac driver by using that macro.

Actually we can't do this. It will break initialization of some other
drivers.


> 
> Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> Reviewed-by: Felipe Balbi <balbi@ti.com>
> Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
> ---
>  drivers/dma/dw_dmac.c |   13 ++-----------
>  1 file changed, 2 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/dma/dw_dmac.c b/drivers/dma/dw_dmac.c
> index c4b0eb3..0b88ced 100644
> --- a/drivers/dma/dw_dmac.c
> +++ b/drivers/dma/dw_dmac.c
> @@ -1700,6 +1700,7 @@ MODULE_DEVICE_TABLE(of, dw_dma_id_table);
>  #endif
>  
>  static struct platform_driver dw_driver = {
> +	.probe		= dw_probe,
>  	.remove		= __devexit_p(dw_remove),
>  	.shutdown	= dw_shutdown,
>  	.driver = {
> @@ -1709,17 +1710,7 @@ static struct platform_driver dw_driver = {
>  	},
>  };
>  
> -static int __init dw_init(void)
> -{
> -	return platform_driver_probe(&dw_driver, dw_probe);
> -}
> -subsys_initcall(dw_init);
> -
> -static void __exit dw_exit(void)
> -{
> -	platform_driver_unregister(&dw_driver);
> -}
> -module_exit(dw_exit);
> +module_platform_driver(dw_driver);
>  
>  MODULE_LICENSE("GPL v2");
>  MODULE_DESCRIPTION("Synopsys DesignWare DMA Controller driver");

-- 
Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Intel Finland Oy

Re: [PATCH 1/4] dmaengine: dw_dmac: use helper macro module_platform_driver()

[viresh kumar]

On Wed, Oct 10, 2012 at 2:34 PM, Andy Shevchenko
<andriy.shevchenko@linux.intel.com> wrote:
> On Tue, 2012-10-02 at 14:41 +0300, Andy Shevchenko wrote:
>> From: Heikki Krogerus <heikki.krogerus@linux.intel.com>
>>
>> Since v3.2 we have nice macro to define the platform driver's init and exit
>> calls. This patch simplifies the dw_dmac driver by using that macro.
>
> Actually we can't do this. It will break initialization of some other
> drivers.

why?

Re: [PATCH 1/4] dmaengine: dw_dmac: use helper macro module_platform_driver()

[Andy Shevchenko]

On Wed, Oct 10, 2012 at 12:08 PM, viresh kumar <viresh.kumar@linaro.org> wrote:
> On Wed, Oct 10, 2012 at 2:34 PM, Andy Shevchenko
> <andriy.shevchenko@linux.intel.com> wrote:
>> On Tue, 2012-10-02 at 14:41 +0300, Andy Shevchenko wrote:
>>> From: Heikki Krogerus <heikki.krogerus@linux.intel.com>
>>>
>>> Since v3.2 we have nice macro to define the platform driver's init and exit
>>> calls. This patch simplifies the dw_dmac driver by using that macro.
>>
>> Actually we can't do this. It will break initialization of some other
>> drivers.
>
> why?

We have spi, i2c and hsuart devices connected to the DMA controller.
In case we would like to use DMA we have to have the dw_dmac loaded
before them. Currently we have spi driver on subsys_initcall level,
and Mika, who is developing it, will change to module_init_call level.
However, it will just hide the potential issue. He also tried to use
deferred module loading, but we don't know if it's good solution or
not, and that solution requires something to stop deferring at some
moment.

Might be we missed something and there is a better solution.

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH 1/4] dmaengine: dw_dmac: use helper macro module_platform_driver()

[Viresh Kumar]

On 10 October 2012 14:51, Andy Shevchenko <andy.shevchenko@gmail.com> wrote:
> We have spi, i2c and hsuart devices connected to the DMA controller.
> In case we would like to use DMA we have to have the dw_dmac loaded
> before them. Currently we have spi driver on subsys_initcall level,
> and Mika, who is developing it, will change to module_init_call level.
> However, it will just hide the potential issue. He also tried to use
> deferred module loading, but we don't know if it's good solution or
> not, and that solution requires something to stop deferring at some
> moment.
>
> Might be we missed something and there is a better solution.

My eyes aren't 6/6 now. I need specs now.
How could i miss that you just removed the very intentional subsys_init()
call. My bad.

I take my Acked-by: back :)

--
viresh

Re: [PATCH 1/4] dmaengine: dw_dmac: use helper macro module_platform_driver()

[Felipe Balbi]

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On Wed, Oct 10, 2012 at 12:21:04PM +0300, Andy Shevchenko wrote:
> On Wed, Oct 10, 2012 at 12:08 PM, viresh kumar <viresh.kumar@linaro.org> wrote:
> > On Wed, Oct 10, 2012 at 2:34 PM, Andy Shevchenko
> > <andriy.shevchenko@linux.intel.com> wrote:
> >> On Tue, 2012-10-02 at 14:41 +0300, Andy Shevchenko wrote:
> >>> From: Heikki Krogerus <heikki.krogerus@linux.intel.com>
> >>>
> >>> Since v3.2 we have nice macro to define the platform driver's init and exit
> >>> calls. This patch simplifies the dw_dmac driver by using that macro.
> >>
> >> Actually we can't do this. It will break initialization of some other
> >> drivers.
> >
> > why?
> 
> We have spi, i2c and hsuart devices connected to the DMA controller.
> In case we would like to use DMA we have to have the dw_dmac loaded
> before them. Currently we have spi driver on subsys_initcall level,
> and Mika, who is developing it, will change to module_init_call level.
> However, it will just hide the potential issue. He also tried to use
> deferred module loading, but we don't know if it's good solution or
> not, and that solution requires something to stop deferring at some
> moment.
> 
> Might be we missed something and there is a better solution.

if they can only work with DMA, they should return -EPROBE_DEFER so
their probe() function can be called after DMA driver has finished
probing.

-- 
balbi

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Re: [PATCH 1/4] dmaengine: dw_dmac: use helper macro module_platform_driver()

[Andy Shevchenko]

On Wed, Oct 10, 2012 at 3:40 PM, Felipe Balbi <balbi@ti.com> wrote:
> On Wed, Oct 10, 2012 at 12:21:04PM +0300, Andy Shevchenko wrote:
>> On Wed, Oct 10, 2012 at 12:08 PM, viresh kumar <viresh.kumar@linaro.org> wrote:
>> > On Wed, Oct 10, 2012 at 2:34 PM, Andy Shevchenko
>> > <andriy.shevchenko@linux.intel.com> wrote:
>> >> On Tue, 2012-10-02 at 14:41 +0300, Andy Shevchenko wrote:
>> >>> From: Heikki Krogerus <heikki.krogerus@linux.intel.com>
>> >>>
>> >>> Since v3.2 we have nice macro to define the platform driver's init and exit
>> >>> calls. This patch simplifies the dw_dmac driver by using that macro.
>> >>
>> >> Actually we can't do this. It will break initialization of some other
>> >> drivers.
>> >
>> > why?
>>
>> We have spi, i2c and hsuart devices connected to the DMA controller.
>> In case we would like to use DMA we have to have the dw_dmac loaded
>> before them. Currently we have spi driver on subsys_initcall level,
>> and Mika, who is developing it, will change to module_init_call level.
>> However, it will just hide the potential issue. He also tried to use
>> deferred module loading, but we don't know if it's good solution or
>> not, and that solution requires something to stop deferring at some
>> moment.
>>
>> Might be we missed something and there is a better solution.
>
> if they can only work with DMA, they should return -EPROBE_DEFER so
> their probe() function can be called after DMA driver has finished
> probing.

They could work either with DMA or via PIO mode.
How does the driver know when to stop to return -EPROBE_DEFER?

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH 1/4] dmaengine: dw_dmac: use helper macro module_platform_driver()

[Felipe Balbi]

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Hi,

On Wed, Oct 10, 2012 at 03:52:40PM +0300, Andy Shevchenko wrote:
> On Wed, Oct 10, 2012 at 3:40 PM, Felipe Balbi <balbi@ti.com> wrote:
> > On Wed, Oct 10, 2012 at 12:21:04PM +0300, Andy Shevchenko wrote:
> >> On Wed, Oct 10, 2012 at 12:08 PM, viresh kumar <viresh.kumar@linaro.org> wrote:
> >> > On Wed, Oct 10, 2012 at 2:34 PM, Andy Shevchenko
> >> > <andriy.shevchenko@linux.intel.com> wrote:
> >> >> On Tue, 2012-10-02 at 14:41 +0300, Andy Shevchenko wrote:
> >> >>> From: Heikki Krogerus <heikki.krogerus@linux.intel.com>
> >> >>>
> >> >>> Since v3.2 we have nice macro to define the platform driver's init and exit
> >> >>> calls. This patch simplifies the dw_dmac driver by using that macro.
> >> >>
> >> >> Actually we can't do this. It will break initialization of some other
> >> >> drivers.
> >> >
> >> > why?
> >>
> >> We have spi, i2c and hsuart devices connected to the DMA controller.
> >> In case we would like to use DMA we have to have the dw_dmac loaded
> >> before them. Currently we have spi driver on subsys_initcall level,
> >> and Mika, who is developing it, will change to module_init_call level.
> >> However, it will just hide the potential issue. He also tried to use
> >> deferred module loading, but we don't know if it's good solution or
> >> not, and that solution requires something to stop deferring at some
> >> moment.
> >>
> >> Might be we missed something and there is a better solution.
> >
> > if they can only work with DMA, they should return -EPROBE_DEFER so
> > their probe() function can be called after DMA driver has finished
> > probing.
> 
> They could work either with DMA or via PIO mode.
> How does the driver know when to stop to return -EPROBE_DEFER?

Why would you even allow to work as PIO-only ? Who would even want to
use the driver as PIO only ?

In any case, you can add a Kconfig choice like WHATEVER_PIO_ONLY and
only return -EPROBE_DEFER ifndef WHATEVER_PIO_ONLY.

-- 
balbi

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Re: [PATCH 1/4] dmaengine: dw_dmac: use helper macro module_platform_driver()

[Felipe Balbi]

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Hi,

On Wed, Oct 10, 2012 at 04:42:00PM +0300, Felipe Balbi wrote:
> On Wed, Oct 10, 2012 at 03:52:40PM +0300, Andy Shevchenko wrote:
> > On Wed, Oct 10, 2012 at 3:40 PM, Felipe Balbi <balbi@ti.com> wrote:
> > > On Wed, Oct 10, 2012 at 12:21:04PM +0300, Andy Shevchenko wrote:
> > >> On Wed, Oct 10, 2012 at 12:08 PM, viresh kumar <viresh.kumar@linaro.org> wrote:
> > >> > On Wed, Oct 10, 2012 at 2:34 PM, Andy Shevchenko
> > >> > <andriy.shevchenko@linux.intel.com> wrote:
> > >> >> On Tue, 2012-10-02 at 14:41 +0300, Andy Shevchenko wrote:
> > >> >>> From: Heikki Krogerus <heikki.krogerus@linux.intel.com>
> > >> >>>
> > >> >>> Since v3.2 we have nice macro to define the platform driver's init and exit
> > >> >>> calls. This patch simplifies the dw_dmac driver by using that macro.
> > >> >>
> > >> >> Actually we can't do this. It will break initialization of some other
> > >> >> drivers.
> > >> >
> > >> > why?
> > >>
> > >> We have spi, i2c and hsuart devices connected to the DMA controller.
> > >> In case we would like to use DMA we have to have the dw_dmac loaded
> > >> before them. Currently we have spi driver on subsys_initcall level,
> > >> and Mika, who is developing it, will change to module_init_call level.
> > >> However, it will just hide the potential issue. He also tried to use
> > >> deferred module loading, but we don't know if it's good solution or
> > >> not, and that solution requires something to stop deferring at some
> > >> moment.
> > >>
> > >> Might be we missed something and there is a better solution.
> > >
> > > if they can only work with DMA, they should return -EPROBE_DEFER so
> > > their probe() function can be called after DMA driver has finished
> > > probing.
> > 
> > They could work either with DMA or via PIO mode.
> > How does the driver know when to stop to return -EPROBE_DEFER?
> 
> Why would you even allow to work as PIO-only ? Who would even want to
> use the driver as PIO only ?
> 
> In any case, you can add a Kconfig choice like WHATEVER_PIO_ONLY and
> only return -EPROBE_DEFER ifndef WHATEVER_PIO_ONLY.

yet another comment is that any user of this dma engine will have to
pass the filter function as argument to request_channel, which means
Modules.dep will make sure for dw_dmac to probe before its users.

What am I missing here ?

-- 
balbi

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Re: [PATCH 1/4] dmaengine: dw_dmac: use helper macro module_platform_driver()

[Mika Westerberg]

On Wed, Oct 10, 2012 at 04:42:00PM +0300, Felipe Balbi wrote:
> Hi,
> 
> On Wed, Oct 10, 2012 at 03:52:40PM +0300, Andy Shevchenko wrote:
> > On Wed, Oct 10, 2012 at 3:40 PM, Felipe Balbi <balbi@ti.com> wrote:
> > > On Wed, Oct 10, 2012 at 12:21:04PM +0300, Andy Shevchenko wrote:
> > >> On Wed, Oct 10, 2012 at 12:08 PM, viresh kumar <viresh.kumar@linaro.org> wrote:
> > >> > On Wed, Oct 10, 2012 at 2:34 PM, Andy Shevchenko
> > >> > <andriy.shevchenko@linux.intel.com> wrote:
> > >> >> On Tue, 2012-10-02 at 14:41 +0300, Andy Shevchenko wrote:
> > >> >>> From: Heikki Krogerus <heikki.krogerus@linux.intel.com>
> > >> >>>
> > >> >>> Since v3.2 we have nice macro to define the platform driver's init and exit
> > >> >>> calls. This patch simplifies the dw_dmac driver by using that macro.
> > >> >>
> > >> >> Actually we can't do this. It will break initialization of some other
> > >> >> drivers.
> > >> >
> > >> > why?
> > >>
> > >> We have spi, i2c and hsuart devices connected to the DMA controller.
> > >> In case we would like to use DMA we have to have the dw_dmac loaded
> > >> before them. Currently we have spi driver on subsys_initcall level,
> > >> and Mika, who is developing it, will change to module_init_call level.
> > >> However, it will just hide the potential issue. He also tried to use
> > >> deferred module loading, but we don't know if it's good solution or
> > >> not, and that solution requires something to stop deferring at some
> > >> moment.
> > >>
> > >> Might be we missed something and there is a better solution.
> > >
> > > if they can only work with DMA, they should return -EPROBE_DEFER so
> > > their probe() function can be called after DMA driver has finished
> > > probing.
> > 
> > They could work either with DMA or via PIO mode.
> > How does the driver know when to stop to return -EPROBE_DEFER?
> 
> Why would you even allow to work as PIO-only ? Who would even want to
> use the driver as PIO only ?

Think about SPI or I2C, if we don't have DMA available we are still able to
use the driver (and the bus) instead of just failing.

> In any case, you can add a Kconfig choice like WHATEVER_PIO_ONLY and
> only return -EPROBE_DEFER ifndef WHATEVER_PIO_ONLY.

Why would we add more Kconfig options for things that can be checked
runtime? Distro makers need to select that option anyway so it doesn't gain
anything, except confuses users.