Subject: [PATCH RFC] of: add a basic memory driver

From: [email-masked] (Emilio López)
Date: Wed, 11 Sep 2013 06:34:01 -0300

Hi Maxime,

El 11/09/13 04:54, Maxime Ripard escribi?:
> Hi Emilio,
>
> On Tue, Sep 10, 2013 at 10:43:01PM -0300, Emilio L?pez wrote:
>> This driver's only job is to claim and ensure that the necessary clock
>> for memory operation on a DT-enabled machine remains enabled.
>>
>> Signed-off-by: Emilio L?pez <[email-masked]>
>> ---
>>
>> Hi,
>>
>> I am currently facing an issue with the clock setup: a critical but
>> unclaimed clock gets disabled as a side effect of disabling one of its
>> children. The clock setup looks something like this:
>>
>>        PLL
>>         |
>>   ------------
>>   |          |
>> DDR       Others
>>              |
>>            periph
>>
>> The PLL clock is marked with the CLK_IGNORE_UNUSED flag, so on a normal
>> boot it remains on, even after the unused clocks cleanup code runs. The
>> problem occurs when someone enables "periph" and then, later on, disables
>> it: the framework starts disabling clocks upwards on the tree,
>> eventually switching the PLL off (and that kills the machine, as the memory
>> clock is shut down).
>
> That looks like a bug in the clock framework. I'd expect it to at least
> behave in the same way when disabling the unused clocks at late startup
> and when going up disabling some clocks' parent later on.

Yes, I kind of expected the same, and the flag description seems to 
imply so too:

     #define CLK_IGNORE_UNUSED	BIT(3) /* do not gate even if unused */

>> There's two possible solutions I can think of:
>>   1) add some extra checks on the framework to not turn off clocks marked
>>      with such a flag on the non-explicit case (ie, when I'm disabling
>>      some other clock)
>>
>>   2) create an actual user of the DDR clock, that way it won't get
>>      disabled simply because it's being used.
>>
>> I considered 1) and implemented it, but the result was not pretty.
>
> What was not pretty about it?

It required adding an extra parameter to __clk_disable/__clk_unprepare 
to keep track of the call's explicitness, and ignore the 
disable/unprepare callback on the implicit case (when 
__clk_disable/__clk_unprepare is called recursively) if the flag is set. 
This also means adding a wrapping function to at least __clk_unprepare, 
so as to to not break callers outside of the clk framework. Overall it 
felt too hacky for something that could be properly handled by the 
generic code if it had at least 1 user.

I would like to hear Mike's thoughts on this; maybe CLK_IGNORE_UNUSED is 
not what we think it should be.

>> This patch is my take on 2). Please let me know what you think; all
>> feedback is welcome :)
>>
>> Cheers,
>>
>> Emilio
>>
>>   drivers/of/Kconfig     |  6 ++++++
>>   drivers/of/Makefile    |  1 +
>>   drivers/of/of_memory.c | 30 ++++++++++++++++++++++++++++++
>>   3 files changed, 37 insertions(+)
>>   create mode 100644 drivers/of/of_memory.c
>>
>> diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
>> index 9d2009a..f6c5e20 100644
>> --- a/drivers/of/Kconfig
>> +++ b/drivers/of/Kconfig
>> @@ -80,4 +80,10 @@ config OF_RESERVED_MEM
>>   	help
>>   	  Initialization code for DMA reserved memory
>>
>> +config OF_MEMORY
>> +	depends on COMMON_CLK
>> +	def_bool y
>> +	help
>> +	  Simple memory initialization
>> +
>>   endmenu # OF
>> diff --git a/drivers/of/Makefile b/drivers/of/Makefile
>> index ed9660a..15f0167 100644
>> --- a/drivers/of/Makefile
>> +++ b/drivers/of/Makefile
>> @@ -10,3 +10,4 @@ obj-$(CONFIG_OF_PCI)	+= of_pci.o
>>   obj-$(CONFIG_OF_PCI_IRQ)  += of_pci_irq.o
>>   obj-$(CONFIG_OF_MTD)	+= of_mtd.o
>>   obj-$(CONFIG_OF_RESERVED_MEM) += of_reserved_mem.o
>> +obj-$(CONFIG_OF_MEMORY) += of_memory.o
>> diff --git a/drivers/of/of_memory.c b/drivers/of/of_memory.c
>> new file mode 100644
>> index 0000000..a833f7a
>> --- /dev/null
>> +++ b/drivers/of/of_memory.c
>> @@ -0,0 +1,30 @@
>> +/*
>> + * Simple memory driver
>> + */
>> +
>> +#include <linux/of.h>
>> +#include <linux/clk.h>
>> +
>> +static int __init of_memory_enable(void)
>> +{
>> +	struct device_node *np;
>> +	struct clk *clk;
>> +
>> +	np = of_find_node_by_path("/memory");
>> +	if (!np) {
>> +		pr_err("no /memory on DT!\n");
>> +		return 0;
>> +	}
>> +
>> +	clk = of_clk_get(np, 0);
>> +	if (!IS_ERR(clk)) {
>> +		clk_prepare_enable(clk);
>> +		clk_put(clk);
>> +	}
>> +
>> +	of_node_put(np);
>> +
>> +	return 0;
>> +}
>> +
>> +device_initcall(of_memory_enable);
>
> I like this idea as well. But imho, both 1 and 2 should be done. 2) is
> only about memory devices, while 1) is much more generic.
>
> And fwiw, the Marvell Armada 370 is also in this case of having a
> gatable clock for the DDR that could potentially be disabled (but is
> not, since it has no other users than the DDR itself, and as such, no
> one ever calls clk_disable on it).

Nice to know, thanks for the information :)

Cheers,

Emilio