linux-kernel - Re: [RFC] Virtual methods for devices and generalized GPIO support using it

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Date:	Thu, 29 Mar 2007 02:52:15 +0300
From:	Paul Sokolovsky <pmiscml@...il.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	linux-kernel@...r.kernel.org, kernel-discuss@...dhelds.org,
	Anton Vorontsov <cbou@...l.ru>
Subject: Re: [RFC] Virtual methods for devices and generalized GPIO support using it

Hello H.,

Wednesday, March 28, 2007, 7:32:57 PM, you wrote:

> Paul Sokolovsky wrote:
>> 
>> In this respect, VTABLE(), METHOD() macros serve the same purpose as 
>> container_of() and list_for_each() - they are besides offering (more) 
>> convenient syntax, also carry important annotattion and educational
>> messages, like "it's ok, and encouraged to embed one structure into 
>> another - use it!" or "list manipulation is a trivial operation for kernel,
>> and we want you to treat it as such and use in standard, easily 
>> distinguishable way".
>> 

> You realize, right, that the Linux kernel already have a much cleaner 
> way to do vtables in the kernel, without this kind of macro crappage? 
> It's called an _ops table, and is used in a patternized way:

foo->x_ops->func(foo, ...);

> ... all over the kernel.  We like it that way.

  Sure! I wrote it's nothing really new. And I hope it's clear why
those macros appeared in the first place: with the type of structures
the device virtual methods are intended to be used, there're always
pretty comprehensive member selection and typecasting is required. In
this regard, there were 3 choices:

1. Use long but explicit expressions, like

((struct dev_pdata*)pdev.dev->platform_device)->x_ops->func(dev)

2. Use temporary variables:

struct dev_pdata *tmp = (struct dev_pdata*)pdev.dev->platform_device;
tmp->x_ops->func(dev);

3. Introduce macros which would hide guts and would provide syntax
more resembling usual function call (especially for folks who remember
that preprocessor is unalienable part of C ;-) ).


     As I also noted in the original mail, macros are also nice device
for in-place annotation - to emphasize the fact that this is not just
a mundane case of pointer manipulation, but paradigmatic thing.


     By this criteria I happened to choose macros syntax. But it's still
merely a syntax, and I don't pledge for it. If there's more movement
towards using explicit low-level forms like 1) or 2) instead of
introducing new syntactic pattern, then macro syntax can be considered
to have fulfilled its introductory role and can be dropped.




>         -hpa



-- 
Best regards,
 Paul                            mailto:pmiscml@...il.com

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