linux-kernel - Re: [patch 4/4] MFD: Change mfd platform device usage to wrapper platform

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Date:	Wed, 9 Jul 2008 12:50:45 +0100
From:	Ben Dooks <ben-linux@...ff.org>
To:	Dmitry <dbaryshkov@...il.com>
Cc:	Ben Dooks <ben-linux@...ff.org>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.arm.linux.org.uk
Subject: Re: [patch 4/4] MFD: Change mfd platform device usage to wrapper platform_device

On Wed, Jul 09, 2008 at 03:31:04PM +0400, Dmitry wrote:
> 2008/7/9 Ben Dooks <ben-linux@...ff.org>:
> > On Wed, Jul 09, 2008 at 03:15:47PM +0400, Dmitry wrote:
> >> 2008/7/9 Ben Dooks <ben-linux@...ff.org>:
> >> > This patch changes the mfd core behaviour to wrapper the platform_device
> >> > it creates in an struct mfd_device which contains the information
> >> > about the cell that was created.
> >> >
> >> > 1) The creation of the resource list and then passing it to the
> >> >   platform_device_add_resources() causes the allocation of a
> >> >   large array on the stack as well as copying the source data
> >> >   twice (it is copied from the mfd_cell to the temporary array
> >> >   and then copied into the newly allocated array)
> >> >
> >> > 2) We can wrapper the platform_device into an mfd_device and use
> >> >   that to do the platform_device and resource allocation in one
> >> >   go to reduce the failiure.
> >> >
> >> > Note, is there actually any reason to pass the sub devices any
> >> > information about the cell they are created from? The mfd core
> >> > already makes the appropriate resource adjustments and anything
> >> > else like clocks should be exported by the clock drivers?
> >> >
> >> > Signed-off-by: Ben Dooks <ben-linux@...ff.org>
> >>
> >>
> >> NAK.
> >> 0) It was discussed yesterday on the list and the decision was to go
> >> in a different way.
> >>   I've provided a bit cleaner patch with the same idea, but then we
> >> decided to go in a bit different way.
> >> 1) I prefer patch by Mike Rapoport which is more clear and goes in a
> >> more correct way.
> >
> > How "more correct", whilst the patch by Mike makes the platform data
> > be passed from the cell, there is no longer any way to get from the
> > platform device to the mfd_cell...
> 
> Basically we have two choises for the subdevice driver:
> 1) it doesn't know about cells at all (e.g. generic-bl, IIRC). Then we are safe
>    to loose that "cell" information
> 2) If it does use cell information (to get access to hooks), we pass it
>     via platform_data pointer in the mfd_cell and we are ok with it.

Erm, that is complete non-answer. The driver model and various other
parts of the kernel are littered with examples of embedding one
structure within another to gain an C++ like object inheritance.

I've supplied an reasonable example of doing this to create an mfd_cell
device from an platform_device without creating an large amount of code
and improving the efficiency and code-lineage in the process. I do not
see how this isn't "correct" or in any way breaing the current linux
model of doing things.
 
> 
> > The current driver is being inefficent in the way it creates resources
> > on the stack and then calls a routine that does an kalloc/memcpy on
> > the resources.
> 
> I don't see any inefficiency ATM.
> 
> >> 2) Please examine the tmio-nand driver (was here on the list and on
> >> linux-mtd). It uses the mfd_cell
> >>    to call hooks from the "host" driver (tc6393xb, more to be added soon).
> >
> > The one posted in [1] does not call these hooks at-all, can ou please
> > explain why these hooks are needed in addition to the ones already
> > available in the platform device driver?
> >
> > [1] http://lists.infradead.org/pipermail/linux-mtd/2008-June/022137.html
> 
> +
> +static int tmio_hw_init(struct platform_device *dev, struct tmio_nand *tmio)
> +{
> +	struct mfd_cell *cell = mfd_get_cell(dev);
> +	const struct resource *nfcr = NULL;
> +	unsigned long base;
> +	int i;
> +
> +	for (i = 0; i < cell->num_resources; i++)
> +		if (!strcmp((cell->resources+i)->name, TMIO_NAND_CONTROL))
> +			nfcr = &cell->resources[i];
> +
> +	if (nfcr == NULL)
> +		return -ENOMEM;
> +
> +	if (cell->enable) {
> +		int rc = cell->enable(dev);
> +		if (rc)
> +			return rc;
> +	}
> 
> That cell->enable() is necessary to set up the host (in the tc6393xb
> case to enable buffers)
> to enable access to the nand.

So, the enable/disable calls might be useful, however is there any
reason this could not be handled by the clock framework? The suspend/resume
entries where not used, and I belive should not be in here.

As noted before, mfd_get_cell() got dropped by [2]

[2] http://lists.arm.linux.org.uk/lurker/message/20080708.153450.bb33046d.en.html

-- 
Ben

Q:      What's a light-year?
A:      One-third less calories than a regular year.

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