Re: [PATCH 3/4] media: mediatek: vcodec: Fix mtk_vcodec_mem_free() error log criteria

[AngeloGioacchino Del Regno]

Il 07/12/23 12:17, Fei Shao ha scritto:
> Hi Angelo,
>
> On Wed, Dec 6, 2023 at 6:19 PM AngeloGioacchino Del Regno
> <angelogioacchino.delregno@xxxxxxxxxxxxx> wrote:
> > Il 13/11/23 13:26, Fei Shao ha scritto:
> > > mtk_vcodec_mem_free() shouldn't print error if the target DMA buffer has
> > > never been allocated or was freed properly in the previous call. That
> > > makes log confusing.
> > >
> > > Update the error path to print log only when the caller attempts to free
> > > nonzero-size buffer with VA being NULL, which indicates something indeed
> > > went wrong.
> > >
> > > This brings another benefit that the callers no more need to check
> > > mem->va explicitly to avoid the error, which can make the code more
> > > compact and neat.
> > >
> > > Signed-off-by: Fei Shao <fshao@xxxxxxxxxxxx>
> >
> > I think that this error is supposed to catch two issues in one:
> >    - We're called to free no memory (something that does make no sense),
> >      this may happen for example when calling xxx_free() twice, and it
> >      is a mistake that *must* be fixed;
> When I made the change, I was thinking of kfree() that doesn't warn
> against a NULL pointer.
> I imagine mtk_vcodec_mem_free() calls with NULL VA and mem size 0
> probably have the similar nuance (if the buffer exists, free it; never
> mind otherwise), but I could have missed some important differences
> specific to the MTK vcodec driver.
>
> Looking at the mtk_vcodec_mem_free() usages, almost every one of those
> checks VA beforehand, but nothing else - they don't warn or do
> anything special when they encounter a NULL VA, and they should if
> that's a concern.
> Some even don't check at all (and I think that's why I ended up seeing
> the errors mentioned in the cover letter). As for that, I think
> there's nothing else we can fix except prepending "if (mem->va)".
> So from all this, I feel perhaps we don't need to worry much about
> those NULL VA, and we can further remove the checks (or at least move
> it into mtk_vcodec_mem_free()) to trim the lines in the driver. That's
> the reason for patch [4/4].
>
> Not sure if that makes sense to you.

What you say does make sense - and a lot - but still, I think that freeing
a NULL VA (= freeing nothing) is something that shouldn't happen...

> >    - We're failing to free memory for real (which you covered)
> >
> > ....that said, I think that if you want to clarify the error messages
> > in this function, it should look something like this:
> >
> > if (!mem->va) {
> >          mtk_v4l2_err(plat_dev, "%s: Tried to free a NULL VA", __func__);
> >          if (mem->size)
> >                  mtk_v4l2_err(plat_dev, "Failed to free %lu bytes", mem->size);
> >          return;
> > }
> Sure, I can revise the patch with this, but I also want to make sure
> if the NULL VA print needs to be an error.
> If you still think it should, I guess I'll drop the current patch
> [4/4] and instead add the check before every mtk_vcodec_mem_free()
> calls. This should also work for the issue I want to address in the
> first place.

... because if you notice, some of the calls to mtk_vcodec_mem_free() are not
checked with `if (something->va)` beforehand, so I think that those are cases
in which freeing with a NULL VA would actually be an indication of something
going wrong and/or not as expected anyway (checking beforehand = error won't
get printed from mtk_vcodec_mem_free(), not checking = print error if va==NULL)

It's an easy check:
cd drivers/media/platform/mediatek/vcodec
grep -rb1 mtk_vcodec_mem_free

P.S.: h264_if, av1_req_lat :-)

That's why I think that you should drop your [4/4] - unless MediaTek comes in
stating that the missed checks are something unintended, and that every instance
of VA==NULL should print an error.

I honestly wouldn't be surprised if they did so, because anyway this occurs only
in two decoders...

Regards,
Angelo