linux-kernel - Re: [PATCH 08/11] MTD: m25p80: Add option to limit SPI transfer size.

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Date:	Wed, 15 Jul 2015 13:52:27 +0200
From:	Marek Vasut <marex@...x.de>
To:	Michal Suchanek <hramrach@...il.com>
Cc:	Richard Cochran <richardcochran@...il.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Mark Rutland <mark.rutland@....com>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	MTD Maling List <linux-mtd@...ts.infradead.org>,
	Alison Chaiken <alison_chaiken@...tor.com>,
	"Bean Huo 霍斌斌 (beanhuo)" 
	<beanhuo@...ron.com>,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>,
	Russell King <linux@....linux.org.uk>,
	Vinod Koul <vinod.koul@...el.com>,
	Rafał Miłecki <zajec5@...il.com>,
	Kukjin Kim <kgene@...nel.org>,
	Ben Hutchings <ben@...adent.org.uk>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Mark Brown <broonie@...nel.org>,
	Dan Williams <dan.j.williams@...el.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"grmoore@...era.com" <grmoore@...era.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-spi" <linux-spi@...r.kernel.org>,
	Huang Shijie <b32955@...escale.com>,
	Rob Herring <robh+dt@...nel.org>,
	Han Xu <han.xu@...escale.com>,
	Knut Wohlrab <knut.wohlrab@...bosch.com>,
	dmaengine <dmaengine@...r.kernel.org>,
	Brian Norris <computersforpeace@...il.com>,
	David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH 08/11] MTD: m25p80: Add option to limit SPI transfer size.

On Wednesday, July 15, 2015 at 11:45:07 AM, Michal Suchanek wrote:
> On 4 June 2015 at 19:15, Richard Cochran <richardcochran@...il.com> wrote:
> > On Thu, Jun 04, 2015 at 10:31:45AM +0200, Michal Suchanek wrote:
> >> You might want to try to run the bus at 60MHz or 80MHz and then the
> >> values would probably again be different.
> >> 
> >> The first two values are set in DT so the logical place for setting
> >> the third is also in DT.
> >> 
> >> Otherwise you would need some in-kernel table of these settings.
> > 
> > Or a formula.
> 
> This formula probably needs to take into account
> 
>  - the unknown reason for the pl330 to fail transfer

Shouldn't that be fixed at the PL330 level ? This looks like fixing
a problem at the wrong place :)

>  - the device transfer speed and transfer phase as set in DT
>  - possibly device-specific latency and board-specific trace design
> and assembly tolerances

If the design is broken, then cap the speed as for normal SPI device.

> Seriously, until I have at least a vague idea why the transfer fails I
> am not comfortable pulling some formula out of thin air and pretending
> I have a working patch.
> 
> On the other hand, a parameter you can set in the DT and which comes
> with a suggested value which can be tuned depending on the system
> seems more viable.

The problem is, if you add a new DT binding, you'd have to support it
forever, no matter how bad idea that binding turned out to be.

Best regards,
Marek Vasut
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