Re: [PATCH v2 00/32] driver core: Constify API device_find_child() and adapt for various existing usages

[James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>]

On Tue, 2024-12-03 at 22:56 +0800, Zijun Hu wrote:
> On 2024/12/3 22:07, Thomas Weißschuh wrote:
> > On 2024-12-03 08:58:26-0500, James Bottomley wrote:
> > > On Tue, 2024-12-03 at 21:02 +0800, Zijun Hu wrote:
> > > > On 2024/12/3 20:41, Greg Kroah-Hartman wrote:
> > > > > On Tue, Dec 03, 2024 at 08:23:45PM +0800, Zijun Hu wrote:
> > > [...]
> > > > > > or squash such patch series into a single patch ?
> > > > > > 
> > > > > > various subsystem maintainers may not like squashing way.
> > > > > 
> > > > > Agreed, so look into either doing it in a bisectable way if
> > > > > at all possible.  As I don't see a full series here, I can't
> > > > > suggest how it needs to happen :(
> > > > > 
> > > > 
> > > > let me send you a full series later and discuss how to solve
> > > > this issue.
> > > 
> > > It's only slightly more complex than what we normally do: modify
> > > all instances and then change the API.  In this case you have an
> > > additional problem because the prototype "const void *" will
> > > cause a mismatch if a function has "void *".  The easiest way to
> > > solve this is probably to make device_find_child a macro that
> > > coerces its function argument to having a non const "void *" and
> > > then passes off to the real function.  If you do that in the
> > > first patch, then you can constify all the consumers and finally
> > > remove the macro coercion in the last patch.
> > 
> > Casting function pointers like that should be detected and trapped
> > by control flow integrity checking (KCFI).
> > 
> > Another possibility would be to use a macro and _Generic to
> > dispatch to two different backing functions. See __BIN_ATTR() in
> > include/linux/sysfs.h for an inspiration.

That's way over complicated for this conversion: done properly there
should be no need for _Generic() compile time type matching at all.

> this way may fix building error issue but does not achieve our
> purpose. our purpose is that there are only constified
> device_find_child().
> 
> 
> > This also enables an incremental migration.
> 
> change the API prototype from:
> device_find_child(..., void *data_0, int (*match)(struct device *dev,
> void *data));
> 
> to:
> device_find_child(..., const void *data_0, int (*match)(struct device
> *dev, const void *data));
> 
> For @data_0,  void * -> const void * is okay.
> but for @match, the problem is function pointer type incompatibility.
> 
> there are two solutions base on discussions.
> 
> 1) squashing likewise Greg mentioned.
>    Do all of the "prep work" first, and then
>    do the const change at the very end, all at once.
> 
> 2)  as changing platform_driver's remove() prototype.
> Commit: e70140ba0d2b ("Get rid of 'remove_new' relic from platform
> driver struct")
> 
>  introduce extra device_find_child_new() which is constified  -> use
> *_new() replace ALL device_find_child() instances one by one -> 
> remove device_find_child() -> rename *_new() to device_find_child()
> once.

Why bother with the last step, which churns the entire code base again?
Why not call the new function device_find_child_const() and simply keep
it (it's descriptive of its function).  That way you can have a patch
series without merging and at the end simply remove the old function.

Regards,

James