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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 > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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