linux-kernel - Re: [PATCH] staging: media: atomisp: refactor sizeof(struct type) to sizeof(*ptr)

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Message-ID: <CAF6CsJxKZ2FvRWVR0SjyfYR=Nj+4uCMgaJgO4BVBtbMvakXhVQ@mail.gmail.com>
Date: Tue, 13 Jan 2026 21:38:56 +0530
From: Anubhav Kokane <dev.anubhavk@...il.com>
To: Andy Shevchenko <andy.shevchenko@...il.com>
Cc: Andy Shevchenko <andriy.shevchenko@...el.com>, hansg@...nel.org, mchehab@...nel.org, 
	andy@...nel.org, sakari.ailus@...ux.intel.com, gregkh@...uxfoundation.org, 
	linux-media@...r.kernel.org, linux-staging@...ts.linux.dev, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] staging: media: atomisp: refactor sizeof(struct type) to sizeof(*ptr)

On Tue, Jan 13, 2026 at 4:16 PM Andy Shevchenko
<andy.shevchenko@...il.com> wrote:
> > I looked into implementing kcalloc() as suggested. But the issue is struct
> > atomisp_s3a_buf (and the other buffers) are defined as list nodes with
> > struct list_head list embedded in them.
>
> Yes, and how does it affect the allocation?
>
> > The driver relies on adding these
> > individually to asd->s3a_stats and freeing them individually using kfree()
> > in multiple cleanup paths (including error path here).
>
> Is it the issue? Instead of incrementing by a pointer size, you will
> increment an address by a structure size, this is how + operator works
> in C from the beginning (or close enough to that time).
>
> > Switching to kcalloc() would mean the s3a_buf is no longer a standalone
> > object but a slice of an array. This would lead to invalid or double frees
> > if the existing code tries kfree() on this array element.
>
> How? As I showed above you need to carefully move and replace
> individual handling by a common one. So, instead of allocation per
> item it will be an allocation per bucket.
>
> > Addressing this requires a larger refactor of the buffer management logic
> > across the driver,
>
> Exactly! And that's what I think is the best way moving forward. You
> will kill two birds with one stone: fixing the issue at hand and
> improving the memory allocations in the driver in this area a lot.
>
> > would you prefer I stick to the sizeof(*ptr) hardening for
> > now to fix the checkpatch warning?
>
> See above. As now I think this is unneeded churn as the idea would
> still be the same — moving towards kcalloc().

Hi Andy,

Thanks for the explanation regarding the pointer arithmetic and bucket
allocation.

I understand the approach now, will work on refactoring the allocation
to use kcalloc() and updating the cleanup paths to handle the array
correctly.

I'll send a v2 once I have verified the changes, though it might take me
a little time to ensure the cleanup logic is robust.

Regards,
Anubhav