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

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Date:	Thu, 4 Jun 2015 08:42:59 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Michal Suchanek <hramrach@...il.com>
Cc:	Rob Herring <robh+dt@...nel.org>, Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Russell King <linux@....linux.org.uk>,
	Kukjin Kim <kgene@...nel.org>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	Vinod Koul <vinod.koul@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Brian Norris <computersforpeace@...il.com>,
	Han Xu <han.xu@...escale.com>, Mark Brown <broonie@...nel.org>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Marek Vasut <marex@...x.de>,
	Rafał Miłecki <zajec5@...il.com>,
	Alison Chaiken <alison_chaiken@...tor.com>,
	Huang Shijie <b32955@...escale.com>,
	Ben Hutchings <ben@...adent.org.uk>,
	Knut Wohlrab <knut.wohlrab@...bosch.com>,
	Bean Huo 霍斌斌 (beanhuo) 
	<beanhuo@...ron.com>, "grmoore@...era.com" <grmoore@...era.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>, dmaengine@...r.kernel.org,
	MTD Maling List <linux-mtd@...ts.infradead.org>,
	linux-spi <linux-spi@...r.kernel.org>
Subject: Re: [PATCH 08/11] MTD: m25p80: Add option to limit SPI transfer size.

On Wed, Jun 3, 2015 at 11:26 PM, Michal Suchanek <hramrach@...il.com> wrote:
> On sunxi the SPI controller currently does not have DMA support and fails
> any transfer larger than 63 bytes.

This is a driver limitation, not a hardware limitation.

> On Exynos the pl330 DMA controller fails any transfer larger than 64kb
> when using slower speed like 40MHz and any transfer larger than 128bytes
> when running at 133MHz.

This may be a driver bug.

> The best thing is that in both cases the controller can just lock up and
> never finish potentially leaving the hardware in unusable state.
>
> So it is required that the m25p80 driver actively prevents doing
> transfers that are too large for the current driver state on a
> particular piece of hardware.

OK.

> Signed-off-by: Michal Suchanek <hramrach@...il.com>
>
> --
>
> Update commit message and documentation text.
> ---
>  .../devicetree/bindings/mtd/jedec,spi-nor.txt      |  6 ++++++
>  drivers/mtd/devices/m25p80.c                       | 24 ++++++++++++++++++++--
>  2 files changed, 28 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt b/Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt
> index 2bee681..4e63ae8 100644
> --- a/Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt
> +++ b/Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt
> @@ -19,6 +19,12 @@ Optional properties:
>                     all chips and support for it can not be detected at runtime.
>                     Refer to your chips' datasheet to check if this is supported
>                     by your chip.
> +- linux,max_tx_len : With some SPI controller drivers possible transfer size is
> +                     limited. This may be hardware or driver bug.
> +                     Transfer data in chunks no larger than this value.
> +                     Using this option may severely degrade performance and
> +                     possibly flash memory life when max_tx_len is smaller than
> +                     flash page size (typically 256 bytes)

DT describes the hardware, not buggy drivers.

So IMHO this doesn't belong in DT, but it can be a field in struct spi_master.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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