Re: [PATCH] mfd: ti_am335x_tscadc: fix spin lock and reg_cache

[Lee Jones <lee.jones@linaro.org>]

On Tue, 22 Oct 2013, Sebastian Andrzej Siewior wrote:

> 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@gmail.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

Okay. So what does this mean?

> 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 don't have any h/w to actually test this, so you two are going to
have to fudge this out by yourselves.

> 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().

Not sure I get this point.

> 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.

I see the patch, but let's sort this out first, before I apply it.

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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