linux-kernel - Re: [PATCH 5/8] of: Add Tegra124 EMC bindings

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Message-ID: <53C3B6EC.9090904@nvidia.com>
Date:	Mon, 14 Jul 2014 13:54:36 +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 13:29, Thierry Reding wrote:
> * PGP Signed by an unknown key
 > ...
>> Yes, this sounds sensible. I'll make such a patch. I suppose having another
>> timings table in the MC node with just the rate and mc-burst-data would
>> separate the concerns best. It occurs to me that we could also write the
>> regs in the pre-rate change notifier, but this would turn the dependency
>> around and would mean that the regs are not written when entering backup
>> rates. The latter shouldn't be a problem but the reversed dependency would,
>> so I guess a custom function is the way to go, and we need to add at least
>> one anyway.
>
> It sounds like maybe moving enough code and data into the MC driver to
> handle frequency changes would be a good move. From the above it sounds
> like all the MC driver needs to know is that a frequency change is about
> to happen and what the new frequency is.
>
> In addition to exposing things like number of DRAM banks, etc.
>

Yes, so there are two ways to do this:
1) EMC calls tegra_mc_emem_update(freq) at the correct time
2) MC has an optional clock phandle to the EMC clock and registers a 
pre-change notifier.

Both work, but the dependency is reversed. In both cases, the other 
driver is also optional. In the first case, the EMC driver can give a 
warning if the call fails. (As mentioned, if the MC_EMEM updates don't 
happen, things still work but potentially with a hefty perf loss.)
TBH, I haven't yet decided which one is better. If you have an opinion,
I'll go with it.

>> The downstream kernel also overwrites most LA registers during EMC rate
>> change without regard for the driver-set values, and we might have to read
>> those values from the device tree too. Upstream can do this in rate change
>> notifiers if needed. I'll look into this a bit more.
>
> As I understand it, the latency allowance should be specified in terms
> of the maximum amount of time that requests are delayed, so that the
> proper values for the LA registers can be recomputed on an EMC rate
> change.

That's how I understand it too, but in downstream, the LA register 
values are hardcoded into EMC tables in platform data/DTS that are just 
written into the LA registers as-is during rate change.

>
> Thierry
>
> * Unknown Key
> * 0x7F3EB3A1
>
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