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Date:	Mon, 14 Jul 2014 12:06:32 +0300
From:	Mikko Perttunen <mperttunen@...dia.com>
To:	Thierry Reding <thierry.reding@...il.com>
CC:	Mikko Perttunen <mikko.perttunen@...si.fi>,
	Peter De Schrijver <pdeschrijver@...dia.com>,
	Prashant Gaikwad <pgaikwad@...dia.com>,
	"mturquette@...aro.org" <mturquette@...aro.org>,
	"swarren@...dotorg.org" <swarren@...dotorg.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH 5/8] of: Add Tegra124 EMC bindings

On 14/07/14 11:15, Thierry Reding wrote:
> * PGP Signed by an unknown key
>
> On Mon, Jul 14, 2014 at 10:55:51AM +0300, Mikko Perttunen wrote:
>> On 11/07/14 19:01, Mikko Perttunen wrote:
>>> On 07/11/2014 05:51 PM, Thierry Reding wrote:
>>>> On Fri, Jul 11, 2014 at 05:18:30PM +0300, Mikko Perttunen wrote:
>>>>> ...
>>>> ...
>>>
>>> In this case, all the registers that will be written are such that the
>>> MC driver will never need to write them. They are shadowed registers,
>>> meaning that all writes are stored and are only effective starting from
>>> the next time the EMC rate change state machine is activated, so writing
>>> them from anywhere except than the EMC driver would be pointless.
>>>
>>> I can find two users of these registers in downstream:
>>> 1) mc.c saves and loads them on suspend/restore (I don't know why, that
>>> shouldn't do anything. They will be overridden anyway during the next
>>> EMC rate change).
>>> 2) tegra12x_la.c reads MC_EMEM_ARB_MISC0 during a core_initcall to
>>> calculate a value which it then writes to a register that is also
>>> shadowed and that is part of downstream burst registers so that doesn't
>>> do anything either.
>>>
>>> The reason I implemented two ways to specify the MC register area was
>>> that this could be merged before an MC driver and retain
>>> backwards-compatibility after the MC driver arrives.
>>>
>>> If this is not acceptable, we can certainly wait for the MC driver to be
>>> merged first. (Although with the general rate of things, I hope I won't
>>> be back at school at that point..) I assume that this is blocked on the
>>> IOMMU bindings discussion? In that case, there are several options: the
>>> MC driver could have its own tables for each EMC rate or we could just
>>> make the EMC tables global (i.e. not under the EMC node). In any case,
>>> the MC driver would need to implement a function that would just write
>>> these values but be guaranteed to not do anything else, since that could
>>> cause nasty things during the EMC rate change sequence.
>>
>> Having taken another look at the code, I don't think the MC driver could do
>> anything that bad. There are also two other places where the EMC driver
>> needs to read MC registers: Inside the sequence, it reads a register but
>> discards its contents. According to comments, this acts as a memory barrier,
>> probably for the preceding step that writes into MC memory. If the register
>> writes are moved to the MC driver, it could also handle that. In another
>> place it reads the number of RAM modules from a MC register. The MC driver
>> could export this as another function.
>
> Exporting this functionality from the MC driver is the right thing to do
> in my opinion.

Ok, let's do that then. Do you think I could make a bare-bones MC driver 
to support the EMC driver before your MC driver with IOMMU/LA is ready? 
Can the MC device tree node be stabilized yet? Of course, if things go 
well, that concern might turn out to be unnecessary.

Thanks, Mikko.

>
>> That said, I still suspect it might not be worth it to divide this between
>> two drivers.
>
> If we don't separate, then we make it easy for people to break things in
> the future. Both drivers may not be accessing the same registers now,
> but if there's no barrier in place to actively enforce this split, then
> it's only a matter of time before it gets broken. A typical way to
> ensure this is done properly is via request_resource() (called by the
> devm_ioremap_resource() function). That will fail if two drivers try to
> use the same memory region, and with good reason.
>
> Thierry
>
> * Unknown Key
> * 0x7F3EB3A1
>
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