RE: [PATCH] irqchip: stm32: don't set rising configuration registers at init

[Fabien DESSENNE]

> -----Original Message-----
> From: Marc Zyngier <marc.zyng...@arm.com>
> Sent: jeudi 7 mars 2019 18:46
> To: Fabien DESSENNE <fabien.desse...@st.com>; Thomas Gleixner
> <t...@linutronix.de>; Jason Cooper <ja...@lakedaemon.net>; Maxime Coquelin
> <mcoquelin.st...@gmail.com>; Alexandre TORGUE
> <alexandre.tor...@st.com>; linux-kernel@vger.kernel.org; linux-stm32@st-md-
> mailman.stormreply.com; linux-arm-ker...@lists.infradead.org
> Cc: Benjamin GAIGNARD <benjamin.gaign...@st.com>
> Subject: Re: [PATCH] irqchip: stm32: don't set rising configuration registers 
> at init
> 
> On 07/03/2019 17:24, Fabien DESSENNE wrote:
> > Hi
> >
> >
> >> -----Original Message-----
> >> From: Marc Zyngier <marc.zyng...@arm.com>
> >> Sent: jeudi 7 mars 2019 17:40
> >> To: Fabien DESSENNE <fabien.desse...@st.com>; Thomas Gleixner
> >> <t...@linutronix.de>; Jason Cooper <ja...@lakedaemon.net>; Maxime
> >> Coquelin <mcoquelin.st...@gmail.com>; Alexandre TORGUE
> >> <alexandre.tor...@st.com>; linux-kernel@vger.kernel.org;
> >> linux-stm32@st-md- mailman.stormreply.com;
> >> linux-arm-ker...@lists.infradead.org
> >> Cc: Benjamin GAIGNARD <benjamin.gaign...@st.com>
> >> Subject: Re: [PATCH] irqchip: stm32: don't set rising configuration
> >> registers at init
> >>
> >> On 07/03/2019 16:15, Fabien Dessenne wrote:
> >>> The rising configuration status register (rtsr) is not banked.
> >>> As it is shared with the co-processor, it should not be written at
> >>> probe time, else the co-processor configuration will be lost.
> >>>
> >>> Signed-off-by: Fabien Dessenne <fabien.desse...@st.com>
> >>
> >> Fixes:?
> >>
> >>> ---
> >>>  drivers/irqchip/irq-stm32-exti.c | 5 -----
> >>>  1 file changed, 5 deletions(-)
> >>>
> >>> diff --git a/drivers/irqchip/irq-stm32-exti.c
> >>> b/drivers/irqchip/irq-stm32-exti.c
> >>> index 6edfd4b..ff8a84f 100644
> >>> --- a/drivers/irqchip/irq-stm32-exti.c
> >>> +++ b/drivers/irqchip/irq-stm32-exti.c
> >>> @@ -716,7 +716,6 @@ stm32_exti_chip_data
> >>> *stm32_exti_chip_init(struct
> >> stm32_exti_host_data *h_data,
> >>>   const struct stm32_exti_bank *stm32_bank;
> >>>   struct stm32_exti_chip_data *chip_data;
> >>>   void __iomem *base = h_data->base;
> >>> - u32 irqs_mask;
> >>>
> >>>   stm32_bank = h_data->drv_data->exti_banks[bank_idx];
> >>>   chip_data = &h_data->chips_data[bank_idx]; @@ -725,10 +724,6 @@
> >>> stm32_exti_chip_data *stm32_exti_chip_init(struct
> >>> stm32_exti_host_data *h_data,
> >>>
> >>>   raw_spin_lock_init(&chip_data->rlock);
> >>>
> >>> - /* Determine number of irqs supported */
> >>> - writel_relaxed(~0UL, base + stm32_bank->rtsr_ofst);
> >>> - irqs_mask = readl_relaxed(base + stm32_bank->rtsr_ofst);
> >>> -
> >>
> >> And I guess you don't need to find out the number of supported IRQs?
> >
> > That's correct, this informed is useless : irqs_mask is never used (it
> > used to be output in a log for debug purpose.and the log has been
> > removed)
> >
> >
> >>
> >> Also, a handful of lines down, you're writing again to the same
> >> register. Why isn't that a problem?
> >
> > It's obviously a problem : another patch is missing, I am going to add it 
> > in v2.
> > Thanks for pointing this out!
> 
> You are also happily writing to that register in other places via 
> stm32_exti_set_bit
> and co. All that is done without any cooperation with the coprocessor 
> (whatever
> that is...), so I really wonder if it all works by magic or luck...

There is certainly some magic and luck! But there is a bit more : the access to 
both rtsr and ftsr regs are controlled with a call to stm32_exti_hwspin_lock() 
which uses an HWSpinlock shared with the coprocessor.
The other registers are not accessed by the coprocessor, hence are not 
hwspinlock-protected.


> 
> Thanks,
> 
>       M.
> --
> Jazz is not dead. It just smells funny...