linux-kernel - Re: [PATCH] devres: Simple code optimization

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Date: Thu, 27 Jun 2024 23:13:14 +0800
From: quic_zijuhu <quic_zijuhu@...cinc.com>
To: Greg KH <gregkh@...uxfoundation.org>
CC: <rafael@...nel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] devres: Simple code optimization

On 6/27/2024 10:35 PM, Greg KH wrote:
> On Thu, Jun 27, 2024 at 10:29:43PM +0800, quic_zijuhu wrote:
>> On 6/27/2024 9:54 PM, Greg KH wrote:
>>> On Thu, Jun 27, 2024 at 09:47:16PM +0800, Zijun Hu wrote:
>>>> Initialize an uninitialized struct member for devres_open_group()
>>>> and simplify devm_percpu_match() implementation.
>>>
>>> Huge hint, when you say "and" or "also" in a patch, it's a good idea to
>>> split it up into different commits, right?
>>>
>> you are right.
>> i would like to split this change into two changes within a patchset
>> even if this change is *very* simple.
>>>>
>>>> Signed-off-by: Zijun Hu <quic_zijuhu@...cinc.com>
>>>> ---
>>>> This change is intend to replace below one:
>>>> https://lore.kernel.org/lkml/1718629765-32720-1-git-send-email-quic_zijuhu@quicinc.com/#t
>>>
>>> Why?  SHouldn't this be v2 instead?
>>>
>> this change has different title and maybe be identified as different
>> patch, so i send it as v1.
>>>>  drivers/base/devres.c | 5 +++--
>>>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/base/devres.c b/drivers/base/devres.c
>>>> index 3df0025d12aa..5b1d498e83ab 100644
>>>> --- a/drivers/base/devres.c
>>>> +++ b/drivers/base/devres.c
>>>> @@ -567,6 +567,7 @@ void * devres_open_group(struct device *dev, void *id, gfp_t gfp)
>>>>  	grp->id = grp;
>>>>  	if (id)
>>>>  		grp->id = id;
>>>> +	grp->color = 0;
>>>>  
>>>>  	spin_lock_irqsave(&dev->devres_lock, flags);
>>>>  	add_dr(dev, &grp->node[0]);
>>>> @@ -1172,9 +1173,9 @@ static void devm_percpu_release(struct device *dev, void *pdata)
>>>>  
>>>>  static int devm_percpu_match(struct device *dev, void *data, void *p)
>>>>  {
>>>> -	struct devres *devr = container_of(data, struct devres, data);
>>>> +	void __percpu *ptr = *(void __percpu **)data;
>>>>  
>>>> -	return *(void **)devr->data == p;
>>>> +	return ptr == (void __percpu *)p;
>>>
>>> What exactly is being "optimized" here?
>>>
>> 1) remove redundant container_of() and devr->data operations
>>    pointer parameter @data already is address of devr->data.
> 
> But do we really know that ahead of time?  If so, how, just by virtue of
> this being the first field?  If so, then no, keep the container_of.
> 
yes. the 2nd parameter for match() must be devr->data by below reasons:
1) devres.c only call match() by this way match(dev, dr->data, match_data).
2) all implements of match() don't do such redundant operations to get
dr->data. such as devm_action_match()/devm_pages_match()/....
3) API user should only know address devr->data and known nothing about
devres internal struct devres. so they should not write their match() by
involving the struct.

for below match() type definition, the 1st parameter @dev have already
have fixed meaning.
typedef int (*dr_match_t)(struct device *dev, void *res, void *match_data);

suppose your 3rd question have typo error.
>> 2) compare with right data type
>>     original type of @p is void __percpu * returned by
>> __devm_alloc_percpu().
> 
> It's pointer math, no need for types, right?
> 
yes, it is more simpler for no need for types.
but it think it is more normative to compare with user original types as
this change do.
>> @data is storing a pointer type void __percpu * as shown by below
>> statement within __devm_alloc_percpu().
>> *(void __percpu **)p = pcpu;
> 
> Again, it's not very obvious so you better document the heck out of it
> in your changelog text.
> 
okay, will add comments after code review done.
> thanks,
> 
> greg k-h