Message-ID: <CAD=FV=Xf4Hm5tCaaW3QG-ZSQGovh6RKH=1Og1K-yxj+jKyGsmA@mail.gmail.com>
Date:	Wed, 20 Aug 2014 08:55:09 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	Thierry Reding <thierry.reding@...il.com>
Cc:	Heiko Stuebner <heiko@...ech.de>,
	Caesar Wang <caesar.wang@...k-chips.com>,
	Sonny Rao <sonnyrao@...omium.org>,
	Olof Johansson <olof@...om.net>,
	Eddie Cai <eddie.cai@...k-chips.com>,
	Russell King <linux@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] ARM: rockchip: rk3288: Switch to use the proper PWM IP

Thierry,

On Wed, Aug 20, 2014 at 8:38 AM, Thierry Reding
<thierry.reding@...il.com> wrote:
> On Wed, Aug 20, 2014 at 08:20:53AM -0700, Doug Anderson wrote:
>> Thierry,
>>
>> On Tue, Aug 19, 2014 at 11:08 PM, Thierry Reding
>> <thierry.reding@...il.com> wrote:
>> > On Tue, Aug 19, 2014 at 08:18:54AM -0700, Doug Anderson wrote:
>> >> Thierry,
>> >>
>> >> On Tue, Aug 19, 2014 at 12:10 AM, Thierry Reding
>> >> <thierry.reding@...il.com> wrote:
>> >> > On Mon, Aug 18, 2014 at 10:09:06AM -0700, Doug Anderson wrote:
>> >> >> The rk3288 SoC has an option to switch all of the PWMs in the system
>> >> >> between the old IP block and the new IP block.  The new IP block is
>> >> >> working and tested and the suggested PWM to use, so setup the SoC to
>> >> >> use it and then we can pretend that the other IP block doesn't exist.
>> >
>> > A few more questions as to how this actually works. Does it mean there
>> > are two physically separate blocks (with different physical addresses)
>> > to control the same PWM? And this register simply causes some of the
>> > pins to be routed to one or another? As far as I recall there are a
>> > number of instances of the PWM block, so the above would need to count
>> > for all of them. Or are there separate bits for each of them?
>>
>> All I have is the TRM (technical reference manual) which doesn't give
>> me much more info than I've provided you.  But I can answer some of
>> your questoins:
>>
>> 1. If there are two physically separate blocks then the "old" block is
>> not documented in my TRM.
>>
>> 1a) It's entirely possible it's located at some memory address that is
>> marked "Reserved" in the TRM, but I have no idea.
>>
>> 1b) It's entirely possible that the old IP block and the new IP block
>> are supposed to be "compatible" but that the old block is broken and
>> thus isn't behaving properly.
>>
>> 1c) It's entirely possible that the old IP block and the new IP block
>> are located at the same physical addresses but somehow work
>> differently.  If so, the old IP block isn't documented.
>>
>>
>> 2. As per the patch description, there is a single bit that controls
>> all of the PWMs.  My guess is that there's actually a single IP block
>> that implements all 4 PWMs.
>
> Looking at the register offsets in the device tree that seems likely. At
> least PWMs 0 and 1 as well as 2 and 3 seem like they could be in the
> same IP block. Their placement in the register map is somewhat strange:
>
>         pwm0: pwm@...30000 {
>                 ...
>                 reg = <0x20030000 0x10>;
>                 ...
>                 clocks = <&cru PCLK_PWM01>;
>                 ...
>         };
>
>         pwm1: pwm@...30010 {
>                 ...
>                 reg = <0x20030010 0x10>;
>                 ...
>                 clocks = <&cru PCLK_PWM01>;
>                 ...
>         };
>
>         ...
>
>         pwm2: pwm@...50020 {
>                 ...
>                 reg = <0x20050020 0x10>;
>                 ...
>                 clocks = <&cru PCLK_PWM23>;
>                 ...
>         };
>
>         pwm3: pwm@...50030 {
>                 ...
>                 reg = <0x20050030 0x10>;
>                 ...
>                 clocks = <&cru PCLK_PWM23>;
>                 ...
>         };

Ah, you're looking at "rk3xxx.dtsi".  That doesn't apply to rk3288
(the downsides of trying to guess ahead of time what SoC vendors will
name new models).

In rk3288 they have the same clocks.  See patch #3 in this series.


> The clocks would also indicate that there are actually two blocks. I
> seem to remember a discussion about whether to handle them as a single
> block or two/four, but I can't seem to find a reference to it. Maybe I'm
> confusing it with another driver.

At this point it seems like the choice has already been made to handle
them as separate PWMs.  I can change this choice if you want...


>> >> >> This code could go lots of other places, but we've put it here.  Why?
>> >> >> - Pushing it to the bootloader just makes the code harder to update in
>> >> >>   the field.  If we later find a bug in the new IP block and want to
>> >> >>   change our mind about what to use we want it to be easy to update.
>> >
>> > Depending on how this muxing works you won't be able to change your mind
>> > anyway. If the IP blocks are different then the device tree will
>> > effectively make the decision for you. So if you really want to be safe
>> > you'd need to have code in the kernel that parses the device tree and
>> > checks that all PWM instances are of the new type, then set this
>> > register accordingly.
>>
>> Since there is no documentation about how you would instantiate the
>> "old" type in the TRM and no good reason I can think of why someone
>> would want to do this, it doesn't seem super fruitful.
>
> Okay, so if it's not at all documented and never used then yes, we'd
> better just ignore it.

Heiko just pointed me at the base address for the other block.
There's nothing in the rk3288 TRM about it, but we can see the base
address.  We could probably guess that it behaves the same as the
older PWM if we need to.  I'm still not convinced there's a good
reason for someone to use it.


>> >> >> diff --git a/arch/arm/mach-rockchip/rockchip.c b/arch/arm/mach-rockchip/rockchip.c
>> >> >> index 8ab9e0e..99133b9 100644
>> >> >> --- a/arch/arm/mach-rockchip/rockchip.c
>> >> >> +++ b/arch/arm/mach-rockchip/rockchip.c
>> >> >> @@ -24,6 +24,24 @@
>> >> >>  #include <asm/hardware/cache-l2x0.h>
>> >> >>  #include "core.h"
>> >> >>
>> >> >> +static void __init rk3288_init_machine(void)
>> >> >> +{
>> >> >> +     void *grf = ioremap(0xff770000, 0x10000);
>> >> >
>> >> > This region of memory is part of the "grf" "syscon" device (according to
>> >> > arch/arm/boot/dts/rk3288.dtsi) so the register should be accessed from
>> >> > that driver. It looks as if no such driver currently exists, but given
>> >> > the existence of the device tree node it's fair to assume that one will
>> >> > eventually be merged.
>> >>
>> >> The "grf" syscon device is the "general register file".  It's a
>> >> collection of totally random registers stuffed together in one address
>> >> space.  Sometimes a single 32-bit register has things you need to
>> >> tweak for completely different subsystems.
>> >>
>> >> Most drivers referene the syscon using this in dts:
>> >>   rockchip,grf = <&grf>;
>> >>
>> >> Then the drivers do:
>> >>   grf = syscon_regmap_lookup_by_phandle(np, "rockchip,grf");
>> >>
>> >>
>> >> See the Rockchip i2c, pinctrl, or clock drivers for examples.
>> >
>> > That's one way to do it. But if it's really just a one-time thing, then
>> > you could easily perform the register write from the syscon driver where
>> > the memory is already parsed from device tree and mapped. That way you
>> > don't have to hardcode the physical address in some other random piece
>> > of code and map the memory again.
>>
>> Well, except that we're using the general "syscon" driver.  I could
>> create a whole new driver that "subclasses" this syscon driver I
>> suppose.
>
> Ah, I wasn't aware that there was even something like a generic syscon
> driver. But yes, subclassing it sounds like a reasonable thing to do.

I will do that if need be, but it's not my favorite.  I will let
others chime in.
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