Date:	Fri, 29 Nov 2013 11:53:49 +0000
From:	Lee Jones <lee.jones@...aro.org>
To:	Huang Shijie <b32955@...escale.com>
Cc:	Brian Norris <computersforpeace@...il.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	dwmw2@...radead.org, linux-mtd@...ts.infradead.org,
	angus.clark@...com, linus.walleij@...aro.org,
	Mark Brown <broonie@...nel.org>, linux-spi@...r.kernel.org
Subject: Re: [PATCH 00/23] mtd: st_spi_fsm: Add new device

> thanks for your explanations.
> >Most controllers just take an OPCODE and pass it on to the controller
> >and have done with it. The issue that you're attempting to rectify is
> >that the m25p80 expects every controller to be an SPI controller
> >registered to the SPI framework, but as we both know that's not always
> >practical as the SPI framework doesn't allow all configuration
> >information to be passed back to the controller driver. Our issue is
> >not the same. We are required to send entire 'message sequences', to
> >the controller rather than just opcodes. The JEDEC message sequence
> >can be seen below. Bear in mind that this is also one of the more
> >simple message sequences. Some of them even vary depending on which
> >chip is present.
> Frankly speaking, my quadspi driver's code is just like Jones's code.
> Yes, a big "switch".

That sounds awful.

It begs the question, what are you saving/improving by using this
framework then?

> The opcode is just like an index to trigger the proper operation.
> That's why i add this hook @->read_reg(). (the hook acts as the ioctl)

As I've said before, the framework you're suggesting is likely to aid
a Controller which shares a great deal of functionality with the
m25p80 where the back-end controller driver doesn't use the SPI
Subsystem. That won't apply to some Serial Flash Controller drivers,
ours included.

The idea of providing a framework like this is to reduce unnecessary
duplication of code by consolidating shared routines into a central
point. mp25p80 is a good place for that, as a great many Controllers
share the majority of its code. As the only commonality we have with
the m25p80 is a small piece of the JEDEC extraction and a sub-section
of the supported device table (which A. our Controller doesn't support
all of the devices and C. we've extended the table to make it more
useful for our use-case); what you're suggesting would not only
create a larger code-base, but execution would also take more cycles,
which to me sounds completely counter-intuitive. It's lose-lose!

Using the m25p80 as a pass-through, then creating an OPCODE based
switch statement to then go and do the _real_ work sound absurd.

> If we do not use this hooks, we should add more hooks such as
> @->read_id, @->read_sr, @->read_cr...
> 
> That's make the interface not graceful enough.

I agree.

> I read the your patch implementing the read_id:
> 
> http://lists.infradead.org/pipermail/linux-mtd/2013-November/050221.html
> 
> it's more readable. But i think Jones's stfsm_read_reg() is workable too.

It's 'workable', but not efficient. I wrote that example to show you
how bad it would look and to show that is was not a good idea, not
as a good example to be followed.

> If you do not like the read_reg() hook, do you have any better idea?

I don't have a better idea. I think your framework will work just fine
for some controllers. I just don't think bending ours to use it (by
basically adding _extra_ code, and subsequently extra cycles) is the
best way to go.

Kind regards,
Lee

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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