linux-kernel - Re: [PATCH 1/5] spi: introduce mmap read support for spi flash devices

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Date:	Wed, 16 Sep 2015 18:16:55 +0530
From:	Jagan Teki <jteki@...nedev.com>
To:	Mark Brown <broonie@...nel.org>
Cc:	devicetree@...r.kernel.org, hramrach@...il.com,
	Russell King <linux@....linux.org.uk>,
	Vignesh R <vigneshr@...com>, Tony Lindgren <tony@...mide.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-spi@...r.kernel.org, linux-omap@...r.kernel.org,
	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
	Benoit Cousson <bcousson@...libre.com>,
	Brian Norris <computersforpeace@...il.com>,
	David Woodhouse <dwmw2@...radead.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/5] spi: introduce mmap read support for spi flash devices

On 15 September 2015 at 00:05, Mark Brown <broonie@...nel.org> wrote:
> On Fri, Sep 04, 2015 at 04:55:33PM +0530, Jagan Teki wrote:
>> On 4 September 2015 at 13:59, Vignesh R <vigneshr@...com> wrote:
>
>> > + * @spi_mtd_mmap_read: some spi-controller hardwares provide memory
>> > + *                     mapped interface to communicate with mtd flashes.
>> > + *                     For this, spi  controller needs to know flash
>> > + *                     memory settings like read command to use, dummy
>> > + *                     bytes and address width. Once these settings are
>> > + *                     populated in hardware registers, any read
>> > + *                     accesses to flash's memory map region(as defined
>> > + *                     by SoC) through memcpy or mem-to-mem DMA copy
>> > + *                     will be handled by controller hardware. The
>> > + *                     hardware will automatically generate spi signals
>> > + *                     required to read data from flash and present it
>> > + *                     to CPU or DMA. SPI master drivers can use this
>> > + *                     callback to implement memory mapped read
>> > + *                     interface. Flash driver (like m25p80) requests
>> > + *                     memory mapped read via this method. The interface
>> > + *                     should  only be used mtd flashes and cannot be
>> > + *                     used with other spi devices.
>
> This comment is *way* too verbose - probably you just need up to the
> "Once" here.
>
>> > +       int (*spi_mtd_mmap_read)(struct  spi_device *spi,
>> > +                                loff_t from, size_t len, size_t *retlen,
>> > +                                u_char *buf, u8 read_opcode,
>> > +                                u8 addr_width, u8 dummy_bytes);
>
>> This looks un-manageable to know spi core or master knows what are the
>> command or opcode used by spi-nor flash and also I think mmap support
>> seems to be flash related or any justification this is spi bus
>> master/controller feature?
>
> There seem to be a reasonable number of SPI controllers out there which
> have as an extension the ability to do memory mapped reads but are
> otherwise perfectly normal SPI controllers and which rely on that for
> everything except reads.

This is true, but exposing direct spi-nor arguments or features like
read_opcode, addr_width or dummy_byte to spi layer isn't fine? because
for spi layer there is a spi to flash transition mtd m25p80.c is there
to handle is it?

thanks!
-- 
Jagan | openedev.
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