Date:	Tue, 12 Apr 2016 12:52:14 +0300
From:	Peter Ujfalusi <peter.ujfalusi@...com>
To:	Tony Lindgren <tony@...mide.com>
CC:	Paul Walmsley <paul@...an.com>, <jarkko.nikula@...mer.com>,
	<t-kristo@...com>, <linux-omap@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>, <devicetree@...r.kernel.org>
Subject: Re: [PATCH v2 0/3] ARM: OMAP3: Fix McBSP2/3 hwmod setup for sidetone

Tony,

On 04/12/16 00:28, Tony Lindgren wrote:
>>> Then for the long term solution using
>>> PM runtime to block gating of the clock while sidetone is active is
>>> the way to go it seems.
>>
>> Hrm, I think one of the main issue is that with pm_runtime we can not block
>> the clock gating, this is why legacy code uses enable_st_clock(), which will
>> call omap2_clk_deny_idle() or omap2_clk_allow_idle().
> 
> I see. I think Tero wanted to export omap2_clk_allow_idle() and
> omap2_clk_deny_idle() for drivers to use. That should get discussed in
> the linux-clk list, probably best to use the pdata callbacks until
> the clock idling issue has been discussed.

It is already exported, used by the arch/arm/mach-omap2/mcbsp.c file.

> 
>>>> The ST does not have clocks coming from PRCM level, it only uses the McBSP
>>>> iclk when it is enabled (the McBSP block of the McBSP). As far as pm_runtime
>>>> goes I think the ST module should not use it. We can not tell hwmod to
>>>> enable/disable the McBSP2/3 iclk when we pm_runtime for the ST. It does not
>>>> help at all. We can have nop action for the ST when pm_runtime is used, but
>>>> then why would we have it?
>>>
>>> Using PM runtime in the sidetone driver should just work as long as the
>>> sidetone device driver depends on the McBSP driver before it gets probed.
>>> The clock framework handles things for the mcbsp ick with the usecount.
>>
>> Yes, that is not the problem. The problem is that when McBSP is used w/o
>> sidetone the iclk can and should be autogated, but if the sidetone is enabled
>> then the iclk must not autogate and this needs to be prevented in PRCM level.
>> Note also that while the McBSP is running we must be able to enable/disable
>> the sidetone any time w/o affecting the McBSP operation.
> 
> OK I see.
> 
>>> And doing pm_runtime_get() in the sidetone driver will do what the legacy
>>> enable_st_clock() does currently.
>>
>> it can't do that as we do not have way to deny/enable just the autoidle for a
>> given clock.
> 
> Yup. So I suggest the pdata callbacs for the omap2_clk_allow_idle() and
> omap2_clk_deny_idle() for now.

Why not to remove the callback for legacy also and handle it in the driver? It
is less ugly in my opinion.
Going via the pdata callback is just going to cement the current setup.

>>>>> So having two separate drivers might make things a lot simpler.
>>>>
>>>> Not really. It will make things way more complicated imho. How to handle
>>>> legacy boot as we still have that supported?
>>>
>>> Hey both the legacy driver and DT driver are really just platform devices
>>> and drivers. And passing both dts and platform data can be done just
>>> fine, no?
>>
>> Sure, but McBSP2-ST needs to bind with McBSP2 driver instance and the
>> McBSP3-ST should bind with McBSP3 instance. In legacy mode we can store the
>> McBSP pdev pointers in a array and use the devid or get the McBSP id from the
>> device name. While with DT boot we must have phandle pointing to/from ST
>> from/to McBSP node to be able to figure out who is who.
>>
>>>> When the McBSP driver is loaded we must know if it has sidetone or not so
>>>> we can create the needed audio controls, sysfs entries. The sysfs and
>>>> kcontrol registration could be moved out to the new ST driver, true.
>>>
>>> Yeah during the probe, the sidetone driver must register with the McBSP
>>> driver to tell it's there.
>>
>> When McBSP driver probes, it registers itself to ASoC core and it needs to
>> know at that point if we need to prepare for ST or not. So probably the McBSP
>> driver needs to register to ST driver?
> 
> OK yes if that makes more sense.

I have drafted out something which would be needed if we separate the McBSP-ST
from the McBSP driver. It is not pretty...

In the new omap3-mcbsp-st.h:

struct omap3_mcbspst;

struct omap_st_to_mcbsp_data {
	bool (*is_enabled)(struct omap_st_to_mcbsp_data *st_data);
	bool (*enable)(struct omap_st_to_mcbsp_data *st_data);
	bool (*disable)(struct omap_st_to_mcbsp_data *st_data);
	struct omap3_mcbspst *st_priv;
};

In the current omap-mcbsp.h:

#include <omap3-mcbsp-st.h>
...
struct omap_mcbsp_to_st_data {
	bool (*is_enabled)(struct omap_mcbsp_to_st_data *mcbsp_data);
	bool (*iclk_idle)(struct omap_mcbsp_to_st_data *mcbsp_data, bool allow);
	bool (*enable)(struct omap_mcbsp_to_st_data *mcbsp_data);
	bool (*disable)(struct omap_mcbsp_to_st_data *mcbsp_data);
	struct omap_mcbsp *mcbsp_priv;
};

#ifdef CONFIG_SND_SOC_OMAP3_MCBSPST
struct omap_mcbsp_to_st_data *omap_mcbsp_st_register(
						struct platform_device *pdev, /* McBSP pdev! probably? */
						struct omap_st_to_mcbsp_data *st_data);
int omap_mcbsp_st_unregister(struct omap_st_to_mcbsp_data *st_data);
#else
static inline int omap_mcbsp_st_register(struct platform_device *pdev,
					 struct omap_st_to_mcbsp_data *st_data)
{
	return -ENODEV;
}
static inline int omap_mcbsp_st_unregister(struct omap_st_to_mcbsp_data *st_data)
{
	return 0;
}
#endif

Since the ST would be separate driver, it should create the needed ALSA
controls as well, probably I need to pass something else here and there.
But, in this setup it would be possible to remove the ST driver while the
McBSP and the sound card is up, which means we must be able to remove
kcontrols runtime, probably there is a way, but not sure about this.

There will be issues like this we have not prepared for I'm sure if we do
dramatic change to the simple implementation we have right now.

>>> I guess no need to pass anything in the
>>> dts or platform_data for that.
>>>
>>>> I actually started with two separate drivers approach first, but decided that
>>>> it does not worth the effort (legacy boot support, pm_runtime/hwmod hassle,
>>>> platform data, callback API design, etc).
>>>> I know, it is not rocket science but it is king of shoot out of cannon into
>>>> sparrows.
>>>> I'll think about it for a little while ;)
>>>
>>> Well what we've seen so far is that any kind of non-standard solution
>>> will always be a pain to maintain in the long run :)
>>
>> The current implementation (one driver to handle McBSP and the ST) is there
>> ever since OMAP3 was introduced afaik. Changing a working (was working) design
>> to something which has not been tested will for sure going to open issues we
>> have not prepared for.
>> I would avoid the rewrite of a proven driver architecture if it is not broken.
> 
> Well probably the best thing to do is the use of platform callback
> for now until we know how it can be done incrementally :)

I have reasonably clean patches (6 of them) on top of this three which would
remove the arch code for the iclk handling and implements it in the mcbsp
driver w/o changing the architecture of the McBSP driver itself. Both DT and
legacy boot works. The only part I was not happy about the one where I looked
up the mcbsp2/3_ick, but I think I have found much cleaner way to do it
(meaning that the code will not look hackish at all).
If you want to see, I can make this change and I can send the whole thing as
RFC and continue the discussion around that?

-- 
Péter

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