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

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Message-ID: <CAF6CsJzAzieEumnunohi9VrBZ8D+1rqG0wLO9EW2vLKUZS4keg@mail.gmail.com>
Date: Thu, 15 Jan 2026 16:51:35 +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)

Hello Andy,

I appreciate you taking time to explain to me the refactor using kcalloc.

After reviewing the requirements of the task, I realized that this is
beyond my current understanding of the kernel. As this is my first
contribution, I would prefer to build up more foundational knowledge
before tackling a refactor of this scope.

I am stepping back from this patch for now and working on simpler
contributions while I learn more about kernel development. If this issue
is still open after I have gained more experience, I would be interested
in revisiting it.

Thank you for your time and the learning opportunity.

Best regards,
Anubhav

On Tue, Jan 13, 2026 at 9:38 PM Anubhav Kokane <dev.anubhavk@...il.com> wrote:
>
> 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