linux-kernel - Re: [RFC PATCH 1/2] spi: Add support for Zynq QSPI controller

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Date:	Thu, 10 Jul 2014 13:25:53 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Harini Katakam <harinikatakamlinux@...il.com>
Cc:	Mark Brown <broonie@...nel.org>,
	Grant Likely <grant.likely@...aro.org>,
	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>,
	linux-spi <linux-spi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Brian Norris <computersforpeace@...il.com>,
	Marek Vašut <marex@...x.de>,
	Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>,
	Geert Uytterhoeven <geert+renesas@...ux-m68k.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Jingoo Han <jg1.han@...sung.com>,
	Sourav Poddar <sourav.poddar@...com>,
	"michals@...inx.com" <michals@...inx.com>,
	Punnaiah Choudary Kalluri <punnaia@...inx.com>
Subject: Re: [RFC PATCH 1/2] spi: Add support for Zynq QSPI controller

Hi Harini,

On Thu, Jul 10, 2014 at 12:33 PM, Harini Katakam
<harinikatakamlinux@...il.com> wrote:
> On Thu, Jul 10, 2014 at 3:12 PM, Geert Uytterhoeven
> <geert@...ux-m68k.org> wrote:
>> On Thu, Jul 10, 2014 at 11:31 AM, Harini Katakam
>> <harinikatakamlinux@...il.com> wrote:
>>>>> +       master->mode_bits = SPI_CPOL | SPI_CPHA | SPI_RX_DUAL | SPI_RX_QUAD |
>>>>> +                           SPI_TX_DUAL | SPI_TX_QUAD;
>>>>
>>>> Your driver advertises Dual/Quad SPI Transfer capabilities, but it doesn't
>>>> check spi_transfer.[tr]x_nbits? How can it determine when to enable Dual/Quad?
>>>
>>> Here the driver is just giving information that the controller support it.
>>> The MTD layer enables dual/quad based on what the flash supports; quad
>>> being the first priority
>>> I understand that the spi core reads rx, tx-bus-width property and
>>> master support flags and
>>> performs the necessary checks.
>>
>> That's correct: as long as the rx, tx-bus-width  properties do not indicate a
>> Dual or Quad wiring, it won't be used.
>>
>> However, based on schematics, someone may set the rx, tx-bus-width properties
>> to 4, which is correct, as DT describes the hardware. But this will fail to
>> work.
>> So I think it's safer not to announce Dual/Quad support in the driver until
>> the actual driver support is there.
>
> OK. Correct me if I'm wrong but announcing this support in master->flags is
> just to say the controller supports it - Like Punnaiah mentioned in the other
> mail, nothing specific needs to be done from the controller driver to enable
> dual/quad support. This is at the SOC/IP level.
> I agree it might or might not be supported at board-level.

IC. So this is not a generic SPI controller, but a controller meant for QSPI
FLASHes? I.e. if you would connect a different device, the controller may
unexpectedly use Dual or Quad mode if it sees a byte fly by that looks like
a Quad SPI FLASH read command?

> But that's based on the user's hardware. Should master->flags
> really take this into consideration?

You mean master->mode_bits?

> BTW, I dint see master->mode_bits being used anywhere at the moment.

It is used to match SPI controller and slave features, cfr. spi_setup() in
drivers/spi/spi.c.

If Dual/Quad is supported, the bits should be set. Else spi_setup() will
clear the bits in the SPI slave's mode field, disabling Dual/Quad transfers.

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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