Re: [PATCHv3 2/7] clk: ti: add clkdev get helper

From: Tero Kristo <t-kristo@ti.com>
To: Russell King - ARM Linux <linux@armlinux.org.uk>
Cc: <linux-omap@vger.kernel.org>, <linux-clk@vger.kernel.org>,
	<tony@atomide.com>, <paul@pwsan.com>, <sboyd@codeaurora.org>,
	<mturquette@baylibre.com>, <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCHv3 2/7] clk: ti: add clkdev get helper
Date: Tue, 12 Jul 2016 18:49:15 +0300	[thread overview]
Message-ID: <5785117B.4050907@ti.com> (raw)
In-Reply-To: <20160712153418.GF5783@n2100.arm.linux.org.uk>

On 12/07/16 18:34, Russell King - ARM Linux wrote:
> On Tue, Jul 12, 2016 at 06:18:01PM +0300, Tero Kristo wrote:
>> On 12/07/16 13:22, Russell King - ARM Linux wrote:
>>> On Thu, Jun 30, 2016 at 05:13:33PM +0300, Tero Kristo wrote:
>>>>   /**
>>>> + * ti_clk_get - lookup a TI clock handle
>>>> + * @dev_id: device to lookup clock for
>>>> + * @con_id: connection ID to find
>>>> + *
>>>> + * Searches for a TI clock handle based on the DT node name.
>>>> + * Returns the pointer to the clock handle, or ERR_PTR in failure.
>>>> + */
>>>> +static struct clk *ti_clk_get(const char *dev_id, const char *con_id)
>>>> +{
>>>> +	struct of_phandle_args clkspec;
>>>> +	struct device_node *node;
>>>> +	struct clk *clk;
>>>> +
>>>> +	/* Only check for cases of type clk_get_sys(NULL, "xyz") */
>>>> +	if (dev_id || !con_id)
>>>> +		return ERR_PTR(-ENOENT);
>>>> +
>>>> +	if (of_have_populated_dt()) {
>>>> +		node = of_find_node_by_name(NULL, con_id);
>>>> +		clkspec.np = node;
>>>> +		clk = of_clk_get_from_provider(&clkspec);
>>>> +
>>>> +		if (!IS_ERR(clk))
>>>> +			return clk;
>>>> +	}
>>>> +
>>>> +	return ERR_PTR(-ENOENT);
>>>> +}
>>>
>>> I _really_ don't like this.  This takes us back to the totally broken
>>> idea, that's already been proven to be greatly harmful in OMAP land,
>>> that the "con_id" is not a _DEVICE SPECIFIC CONNECTION NAME_ but is a
>>> CLOCK NAME.  The connection ID is supposed to be a static string in
>>> the requestor of the clock, not some random string picked out from
>>> DT.
>>
>> What is the harm in that actually? I have most likely missed some discussion
>> but please educate me.
>
> The problem is that we end up with drivers that we end up needing platform
> information to know what clocks they should get, and what "connection names"
> to use to get their clocks.
>
> We've been there before with OMAP, pre-DT times, and I fixed it up by
> creating clkdev and converting much of OMAP over to clkdev, doing
> everything the right way.  Now, we're heading around the same mistake
> that I already solved several years ago.
>
> It's _well_ documented that the _connection id_ is a connection id and
> not a clock name.  It's been documented that way ever since the clk API
> was first published, and I'm on record for having brought up this point
> _many_ times.

Yeah I know this is documented this way, the problem is we have plenty 
of code around which doesn't really use it this way. >.<

>
> So, when I see something reintroducing ways to persist with this crap,
> I'm just not going to allow it.  This is 2016 after all, it's a full
> _ten_ years after this API was created.

Ok fair enough, lets try to figure out something else. I was pretty much 
worried before posting this patch/approach that you will shoot it down, 
thus was requesting your feedback specifically. :P

>
>> The problem is, we are already using this convention
>> all over the place in the kernel (not only OMAPs), and trying to clean up
>> the kernel requires some intermediate steps being taken, otherwise we end up
>> doing all the clean-ups in a single massive patch I fear (change all the
>> hwmod data + clock data + any implementations related to these at the same
>> time.)
>
> This isn't an intermediate step if you're having to put stuff into DT
> in order to support it.  Anything you throw into DT needs to be supported
> for years by the kernel, since we have the requirement that newer kernels
> are bootable with older DT files without regression.  That effectively
> means we'll never be able to remove this, and we'll end up fighting other
> people abusing the hook.
>
>> That is true, it will end up adding new DT nodes. However, as OMAP already
>> is doing this extensively, I wonder what would be the alternative. Maybe
>> move all the clock data back to kernel space to increase the size of
>> multi-v7 builds?
>
> Why can't you use:
>
> 	clocks = <&clk_provider fck_index>, <&clk_provider ick_index>, ...;
> 	clock-names = "fck", "ick", ...;
>
> when describing the hardware?  If you don't have a struct device, but
> only a struct device_node, you can use of_clk_get(), which allows you
> to omit the connection ID and look up by index in the clocks property.

That can be done. However, this means the individual clock provider must 
implement all the clocks in the kernel, adding loads of static data. And 
the data is different for every TI SoC we have out there in the 
wild..... aaand, we can't load the clock data as modules either due to 
early boot time dependencies. This is a royal mess we have here in our 
hands I am trying to clean-up somehow. I think I have posted several 
different series along the years trying to get rid of the hwmod static 
boot time dependency, but it always seems to get shot down. See this 
approach from last year for example:

http://comments.gmane.org/gmane.linux.ports.arm.omap/125555

-Tero