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

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Message-ID: <53C3CD06.8070907@nvidia.com>
Date:	Mon, 14 Jul 2014 15:28:54 +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 14:10, Thierry Reding wrote:
> * PGP Signed by an unknown key
>
> On Mon, Jul 14, 2014 at 01:54:36PM +0300, Mikko Perttunen wrote:
>> On 14/07/14 13:29, Thierry Reding wrote:
>>>> Old 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.
>
> I think I prefer 1. Using an explicit call into the MC driver allow us
> to precisely determine the moment in time when the registers should be
> updated. The pre-change notifier, as I understand it, doesn't give us
> that. Also, the notifier doesn't give us a way to determine success or
> failure of the MC call.

Indeed. I'll go with this.

>
>>>> 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.
>
> Hehe, well, we don't want any of that upstream. =) If it can be computed
> at runtime, then let's compute it. Also, if it's encoded in platform
> data or DTS, then there's no way it can be adjusted based on use-case.
> For example if you have a device with two display outputs (an internal
> panel and HDMI for example) but you never have HDMI plugged in, then
> there's no reason why you would want to program the latency allowance
> for the second display controller. If you provide the values in a static
> frequency/register value table, then you need to account for any
> possible scenario up front, irrespective of what (if any) HDMI monitor
> is attached.

Yeah, I guess the values in downstream must be designed for the worst 
case :P the strange thing is that downstream also has an API for drivers 
to specify their requirements. I guess the clients that have hardcoded 
values and that use the API don't overlap. But I definitely agree that 
on upstream we can have something nicer.

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