linux-kernel - Re: [PATCH 1/3] mfd: pm8921: Expose pm8xxx_read_irq

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Date:	Wed, 9 Jul 2014 07:13:18 -0700
From:	Bjorn Andersson <bjorn@...o.se>
To:	Linus Walleij <linus.walleij@...aro.org>
Cc:	Bjorn Andersson <bjorn.andersson@...ymobile.com>,
	Lee Jones <lee.jones@...aro.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH 1/3] mfd: pm8921: Expose pm8xxx_read_irq_status

On Wed, Jul 9, 2014 at 12:59 AM, Linus Walleij <linus.walleij@...aro.org> wrote:
> On Tue, Jul 8, 2014 at 3:26 AM, Bjorn Andersson
> <bjorn.andersson@...ymobile.com> wrote:
>
>> Most status bits, e.g. for GPIO and MPP input, is retrieved by reading
>> the interrupt status registers, so this needs to be exposed to clients.
>>
>> Signed-off-by: Bjorn Andersson <bjorn.andersson@...ymobile.com>
>
> Hm do you mean you read the input *values* from the interrupt status
> registers?
>

Due to the limited address space (I presume), many of the status bits
on the pm8xxx are not exposed in any other place than through a banked
register in the interrupt "block". So we have to read the interrupt
status in order to get information related to things like gpio status
or if a battery is present for charging.

> What madness in that case.... :-)
>

Totally!

> Anyway, since the driver is based on regmap, can't the children just
> get a regmap * somehow and then just go read the same register
> instead of having to add a special function for it?
>

That we have, and we have access to that part of the ssbi address
space. Unfortunately, like everything else in these pmics, things are
banked. So we need first a bank selector and then a read; so it's racy
with the interrupt handler code doing the same thing.

> When I look at it it seems like it's doing regmap strangely or
> something, like it's one write then one read operation to get
> the register(s) and isn't that all supposed to be hidden behind
> regmap so you don't need the local lock chip->pm_irq_lock?
>

I guess one could have exposed a regmap that instead of exposing the
ssbi address space presented a logical view of the pm8xxx registers;
something like the bitplanes on Amiga. I prefer having a ssbi regmap
for simplicity (and sanity) but then we need to have this read in the
irq driver.

Regards,
Bjorn
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