Messages in this thread |  | | Subject | Re: [PATCH v3 5/5] reset: simple: read back to make sure changes are applied | From | Alexandru Gagniuc <> | Date | Wed, 16 Aug 2017 14:00:59 -0700 |
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On 08/16/2017 02:47 AM, Philipp Zabel wrote: > Read back the register after setting or clearing a reset bit to make > sure that the changes are applied to the reset controller hardware.
> Theoretically, this avoids the write to stay stuck in a store buffer
Is there hardware where this has been observed to happen, or is this purely theoretical? It would be nice to have a "this is needed on hardware XYZ because ABC, and doesn't affect other hardware" comment in the source.
> during the delay of an assert-delay-deassert sequence, and makes sure > that the reset really is asserted for the specified duration. > > Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de> > --- > drivers/reset/reset-simple.c | 7 +++++-- > 1 file changed, 5 insertions(+), 2 deletions(-) > > diff --git a/drivers/reset/reset-simple.c b/drivers/reset/reset-simple.c > index 13e7d5559acc9..d98a7e7d802d1 100644 > --- a/drivers/reset/reset-simple.c > +++ b/drivers/reset/reset-simple.c > @@ -39,17 +39,20 @@ static int reset_simple_set(struct reset_controller_dev *rcdev, > int reg_width = sizeof(u32); > int bank = id / (reg_width * BITS_PER_BYTE); > int offset = id % (reg_width * BITS_PER_BYTE); > + void __iomem *addr = data->membase + (bank * reg_width); > unsigned long flags; > u32 reg; > > spin_lock_irqsave(&data->lock, flags); > > - reg = readl(data->membase + (bank * reg_width)); > + reg = readl(addr); > if (assert ^ data->active_low) > reg |= BIT(offset); > else > reg &= ~BIT(offset); > - writel(reg, data->membase + (bank * reg_width)); > + writel(reg, addr); > + /* Read back to make sure the write doesn't linger in a store buffer */ > + readl(addr);
You're not using the returned value to check that the reset was actually set. This seems a very arbitrary readback workaround, which gives no indication if it actually succeeded or not.
Also the set() is now asymmetrical to clear(). In cases when releasing reset on a HW block that is about to have IO performed on it, one would want to make sure the reset is actually deasserted before doing any IO.
Alex
> > spin_unlock_irqrestore(&data->lock, flags); > >
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