linux-kernel - Re: [PATCH 3/4] media: mediatek: vcodec: Fix mtk_vcodec_mem

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Message-ID: <6046a40b-800c-4bf8-ab45-d7f1015b2d9c@collabora.com>
Date:   Thu, 7 Dec 2023 13:54:55 +0100
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
To:     Fei Shao <fshao@...omium.org>,
        Yunfei Dong <yunfei.dong@...iatek.com>
Cc:     Hans Verkuil <hverkuil-cisco@...all.nl>,
        Andrew-CT Chen <andrew-ct.chen@...iatek.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Nicolas Dufresne <nicolas.dufresne@...labora.com>,
        Nícolas F. R. A. Prado 
        <nfraprado@...labora.com>, Tiffany Lin <tiffany.lin@...iatek.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org, linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH 3/4] media: mediatek: vcodec: Fix mtk_vcodec_mem_free()
 error log criteria

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@...labora.com> 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@...omium.org>
>>
>> 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