linux-kernel - Re: [PATCH 02/10] mtd: st_spi_fsm: Supply all register address and bit logic defines

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Date:	Mon, 18 Nov 2013 14:24:47 +0000
From:	Lee Jones <lee.jones@...aro.org>
To:	Mark Brown <broonie@...nel.org>
Cc:	Linus Walleij <linus.walleij@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Woodhouse <dwmw2@...radead.org>,
	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
	angus.clark@...com
Subject: Re: [PATCH 02/10] mtd: st_spi_fsm: Supply all register address and
 bit logic defines

On Mon, 18 Nov 2013, Mark Brown wrote:

> On Mon, Nov 18, 2013 at 09:32:29AM +0000, Lee Jones wrote:
> 
> > I've actually travelled down the route of separating the SPI
> > Controller parts to drivers/spi. It's possible to do that and perhaps
> > we could then use the generic m25p80 Serial Flash driver as the
> > back-end, but it would be incredibly complicated and would mean we'd
> > need to duplicate almost all of the m25p80 driver into the SPI
> > Controller. The Falcon SPI driver tried to do something similar, but
> > now looks broken due to some incompatible changes in m25p80. We also
> > want to avoid putting ourselves in that position of fragility.
> 
> What I've said to people doing similar drivers before is that it seems
> like there should be an abstraction added in the MTD framework for SPI
> flash controllers like this is that if there is genunie flash-specific
> stuff going on then the mp25p80 driver ought to be split so that the
> code that understands what commands to send to the flash chip is split
> out from the code that actually sends those commands to the chip.  The
> existing SPI support would then be a function driver for this.  This
> would mean we don't need to support the flash chips multiple times.

Actually, there isn't much duplication. We reuse a subset of the
device table, but even that is extended for our use-case. The majority
of the code is setting up the register configs for every given
operation we issue on the controller. There are some parts which
'could' be bent in such a way that they could be abstracted, but not
much.

For example, we have thought about inserting a layer which handles the
type of communication that'll be utilised i.e. true SPI, or our
bespoke FSM implementation for instance. This would enable us to issue
serial_flash_write(), serial_flash_write_then_read(), ... in the m25p80
driver and not care which protocol is used. However, in reality this
won't really save a great deal of code - not in our case at least.

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
Follow Linaro: Facebook | Twitter | Blog
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