linux-kernel - Re: [PATCH 0/4] add minimal bcm2835-sdram driver

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Date:	Thu, 12 May 2016 17:55:47 +0200 (CEST)
From:	Stefan Wahren <stefan.wahren@...e.com>
To:	Martin Sperl <kernel@...tin.sperl.org>
Cc:	Lee Jones <lee@...nel.org>, Eric Anholt <eric@...olt.net>,
	Pawel Moll <pawel.moll@....com>,
	Rob Herring <robh+dt@...nel.org>, linux-kernel@...r.kernel.org,
	Russell King <linux@....linux.org.uk>,
	Stephen Warren <swarren@...dotorg.org>,
	linux-rpi-kernel@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org,
	Mark Rutland <mark.rutland@....com>, devicetree@...r.kernel.org
Subject: Re: [PATCH 0/4] add minimal bcm2835-sdram driver

Hi,

> Martin Sperl <kernel@...tin.sperl.org> hat am 12. Mai 2016 um 17:28
> geschrieben:
> 
> 
> 
> > On 12.05.2016, at 16:50, Stefan Wahren <stefan.wahren@...e.com> wrote:
> > 
> > Hi Martin,
> > 
> >> kernel@...tin.sperl.org hat am 12. Mai 2016 um 14:38 geschrieben:
> >> 
> >> 
> >> From: Martin Sperl <kernel@...tin.sperl.org>
> >> 
> >> As the sdram clock is a critical clock to the system
> >> the minimal bcm2835-sdram driver claims (and enables)
> >> this clock and also exposes the corresponding sdram
> >> registers via debugfs.
> > 
> > sounds like this driver should fix an clock handling issue. Unfortunately
> > this
> > isn't a solution in case the driver is disabled.
> Unfortunately there is no way around this - the driver has
> to be enabled so that the sdram clock or the parent pll,
> which typically is plld_core, never gets disabled.
> 
> The only other option would be marking the clock as critical
> for those legacy drivers.

i would prefer this option. Since this would be more a fix we could get this
faster in, it's more clear why we are doing that and less to review.

Did i miss a drawback?

Stefan

> 
> See also the discussions around the clock register for the
> sdram clock, where we have the “normal” clock and some
> pll as well - even if the “normal” clock is disabled, 
> then clearing the sdram register (or the parent) freezes
> the system.
> 
> > 
> > Does the GPU firmware handle the SDRAM controller or is it initialized by
> > bootcode?
> > 
> 
> AFAIK it is enabled in bootcode.bin and - as of now - the firmware
> updates refresh cycles when SOC temperatures change.
> FW checks every 30 seconds unless there is a key set in config.txt,
> which - supposedly - produces a slight impact every 30 seconds to
> the system.
> 
> Martin