Re: [PATCH v4 08/11] mtd: brcmnand: add BCM63138 support

[Brian Norris <computersforpeace@gmail.com>]

On Wed, May 13, 2015 at 10:02:49PM +0200, Arnd Bergmann wrote:
> On Wednesday 13 May 2015 12:45:21 Brian Norris wrote:
> > I could still avoid one pointer chase and one extra memory allocation by
> > embedding 'struct brcmnand_soc' in a 'struct bcm63138_nand_soc'. e.g.:
> > 
> > struct bcm63138_nand_soc {
> > 	void __iomem *base;
> > 	struct brcmnand_soc soc;
> > };
> > 
> > static bool bcm63138_nand_intc_ack(struct brcmnand_soc *soc)
> > {
> > 	struct bcm63138_nand_soc *priv;
> > 	priv = container_of(soc, struct bcm63138_nand_soc, soc);
> > 
> > 	...
> > }
> > 
> > static int bcm63138_nand_probe(...)
> > {
> > 	struct bcm63138_nand_soc *priv;
> > 
> > 	priv = devm_kzalloc(dev, sizeof(*priv), GFP_KERNEL);
> > 	...
> > 	return brcmnand_probe(pdev, &priv->soc);
> > }
> 
> That would make struct brcmnand_soc an empty structure, right?

No, it still contains the function pointers for our callbacks, which is
the entire point. I guess it's more of a 'nand_soc_ops' structure than a
'nand_soc' pointer now though.

> I think that's fine though, at least it avoids passing void pointers
> and it avoids one of the two allocations you do.
> 
> There is another variation of this model, which some drivers use:
> 
> static int bcm63138_nand_probe(...)
> {
> 	struct bcm63138_nand_soc *priv;
> 	struct brcmnand_controller *controller;
> 
> 	controller = brcmnand_controller_alloc(dev, sizeof (*priv));
> 
> 	priv = brcmnand_controller_priv(controller);
> 
> 	...
> 
> 	return brcmnand_register(controller);
> }
> 
> struct brcmnand_controller *brcmnand_controller_alloc(struct device *pdev, size_t extra)
> {
> 	struct brcmnand_controller *p = dev_kzalloc(dev, sizeof(*p) + extra);
> 
> 	...
> 
> 	return p;
> }
> 
> void *brcmnand_controller_priv(brcmnand_controller *p)
> {
> 	/* extra data follows at the next byte after the controller structure */
> 	return p + 1;
> }

Ah, so this allows the driver to still be agnostic about the contents of
brcmnand_controller.

> Some subsystem maintainers prefer this model over the other one, up to you.

I'll probably stick with mine. But thanks for the suggestion. I'll keep
it in mind. I was actually thinking of imitating this model for other
larger portions of drivers/mtd/nand/, to aid in bounding what drivers
are expected to do vs. allowing the core subsystem to handle things.

Brian