Re: [PATCH v6 2/2] memory: atmel-ebi: add DT bindings documentation - Jean-Jacques Hiblot

From: Jean-Jacques Hiblot <jjhiblot@traphandler.com>
To: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	devicetree <devicetree@vger.kernel.org>,
	Jean-Jacques Hiblot <jjhiblot@traphandler.com>,
	Pawel Moll <pawel.moll@arm.com>, Arnd Bergmann <arnd@arndb.de>,
	Ian Campbell <ijc+devicetree@hellion.org.uk>,
	Nicolas Ferre <nicolas.ferre@atmel.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Rob Herring <robh+dt@kernel.org>,
	Alexandre Belloni <alexandre.belloni@free-electrons.com>,
	Kumar Gala <galak@codeaurora.org>,
	Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v6 2/2] memory: atmel-ebi: add DT bindings documentation
Date: Thu, 28 Apr 2016 14:54:39 +0200	[thread overview]
Message-ID: <CACh+v5ObRAf=7D3tbQdAzef+QRVObF_9eHJEreXwxVyrV8THcA@mail.gmail.com> (raw)
In-Reply-To: <20160428144607.191a29f2@bbrezillon>

2016-04-28 14:46 GMT+02:00 Boris Brezillon <boris.brezillon@free-electrons.com>:
> On Thu, 28 Apr 2016 14:18:25 +0200
> Jean-Jacques Hiblot <jjhiblot@traphandler.com> wrote:
>> >> > +
>> >> > +- atmel,ncs-rd-setup-ns
>> >> > +- atmel,nrd-setup-ns
>> >> > +- atmel,ncs-wr-setup-ns
>> >> > +- atmel,nwe-setup-ns
>> >> > +- atmel,ncs-rd-pulse-ns
>> >> > +- atmel,nrd-pulse-ns
>> >> > +- atmel,ncs-wr-pulse-ns
>> >> > +- atmel,nwe-pulse-ns
>> >> > +- atmel,nwe-cycle-ns
>> >> > +- atmel,nrd-cycle-ns
>> >> > +- atmel,tdf-ns
>> >>
>> >> One thought about the configuration in 'ns' unit: Some devices may
>> >> have requirements expressed in clock cycles (I'm thinking of FPGA
>> >> here). At  a fixed frequency one can always convert manually from 'ns'
>> >> to 'clocks' but it's a bit tedious and prone to rounding errors. And
>> >> It 'll  break when the EBI frequency is changed
>> >
>> > If you don't mind, I'd like to first get this version accepted, and
>> > we'll extend it with timings expressed in clock cycles afterward.
>> >
>> > BTW, could you describe a real use case where timings should be
>> > expressed in clock cycles? I mean, usually the devices have some timing
>> > constraints (tXX_min = Y ns), and I don't see why it would differ for
>> > FPGA interfaces, but I'm clearly not an FPGA expert.
>>
>> I'm not either, I only toyed with FPGA. That's just what experienced
>> FPGA designer told me.
>> I guess that it boils down to: FPGA are more suited for a synchronous
>> design than an asynchronous one.
>
> The thing is, all the timings are based on the master clock, and,
> AFAICS, this clk signal is not exposed, so you're basing your clk-cycle
while EBI itself is asynchronous, the clk can be exposed through one
of the PCK. I've seen this in real projects.
> based timings on something that can change depending on how the
> bootstrap/bootloader decided to configure the master clk.
>
> One option would be to define one of the timing as the reference,
> define this one in nanosecond, and define the other ones as multiple of
> the reference timing. But I'm not sure it's easier to do that than
> defining all the timings directly in nanoseconds.
>
> --
> Boris Brezillon, Free Electrons
> Embedded Linux and Kernel engineering
> http://free-electrons.com
>
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