linux-kernel - Re: [PATCH] mfd: ti_am335x_tscadc: fix spin lock and reg

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Date:	Tue, 22 Oct 2013 15:15:24 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Lee Jones <lee.jones@...aro.org>
CC:	Zubair Lutfullah <zubair.lutfullah@...il.com>,
	sameo@...ux.intel.com, linux-kernel@...r.kernel.org,
	gregkh@...uxfoundation.org
Subject: Re: [PATCH] mfd: ti_am335x_tscadc: fix spin lock and reg_cache

On 08/07/2013 10:40 AM, Lee Jones wrote:
> On Mon, 05 Aug 2013, Zubair Lutfullah wrote:
> 
>> Reg_cache variable is used to lock step enable register
>> from being accessed and written by both TSC and ADC
>> at the same time.
>> However, it isn't updated anywhere in the code at all.
>>
>> If both TSC and ADC are used, eventually 1FFFF is always
>> written enabling all 16 steps uselessly causing a mess.
>>
>> Patch fixes it by correcting the locks and updates the
>> variable by reading the step enable register
>>
>> Signed-off-by: Zubair Lutfullah <zubair.lutfullah@...il.com>
>> ---
>>  drivers/mfd/ti_am335x_tscadc.c |    4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> Better that it comes from somewhere.

I don't understand. All three functions are used before the patch has
been applied:

$ git grep -l am335x_tsc_se_set
drivers/iio/adc/ti_am335x_adc.c
drivers/input/touchscreen/ti_am335x_tsc.c
drivers/mfd/ti_am335x_tscadc.c

$ git grep -l am335x_tsc_se_clr
drivers/iio/adc/ti_am335x_adc.c
drivers/input/touchscreen/ti_am335x_tsc.c
drivers/mfd/ti_am335x_tscadc.c

$ git grep -l am335x_tsc_se_update
drivers/iio/adc/ti_am335x_adc.c
drivers/input/touchscreen/ti_am335x_tsc.c
drivers/mfd/ti_am335x_tscadc.c
include/linux/mfd/ti_am335x_tscadc.h

It has been initialized to 0 by time the mfd part was loaded and
updated via …_set() from both parts (TSC & ADC). The lock ensured that
we never lose or add bits due to a race. So I don't understand why we
end up with 0x1FFFF.
Could some please explain to me how this can happen?

I added reg_se_cache to cache the content of REG_SE once and
synchronize it among TSC & ADC access. REG_SE is set to 0 by the HW
after "work" has been done. So you need to know the old value or TSC may
disable ADC and the other way around.

In tree (staging-next) I see that reg_se_cache ended being pointless.
am335x_tsc_se_update() is no longer used from TSC or ADC. Only the
_set() and _clr() functions are used which (both) read back the content
of the REG_SE register before calling am335x_tsc_se_update().

That makes me think that we might cut of one part by accident. On the
other hand Zubair said that he tested using ADC & TSC at the same time
and it worked. So I have to double check if the HW really resets the
content back to zero or not; maybe there is another explanation :)

One thing that is an issue is that now the _set() function is using the
lock without disabling interrupts and is called from non-IRQ
(tiadc_read_raw()) and IRQ (titsc_irq()) context which might lead to
deadlock. I'm going to send a patch for this.

> Applied, thanks.

Sebastian
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