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Date:   Wed, 13 Sep 2023 10:30:48 +0200
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Subject: Re: [PATCH 3/9] dma-heap: Provide accessors so that in-kernel drivers
 can allocate dmabufs from specific heaps
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To:     Nicolas Dufresne <nicolas@ndufresne.ca>,
        =?UTF-8?B?WW9uZyBXdSAo5ZC05YuHKQ==?= <Yong.Wu@mediatek.com>,
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References: <20230911023038.30649-1-yong.wu@mediatek.com>
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 <8140975aa1f5c3cbdfa2e6f42aecebe3701f29da.camel@ndufresne.ca>
From:   =?UTF-8?Q?Christian_K=c3=b6nig?= <ckoenig.leichtzumerken@gmail.com>
In-Reply-To: <8140975aa1f5c3cbdfa2e6f42aecebe3701f29da.camel@ndufresne.ca>
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Am 12.09.23 um 16:58 schrieb Nicolas Dufresne:
> Le mardi 12 septembre 2023 à 16:46 +0200, Christian König a écrit :
>> Am 12.09.23 um 10:52 schrieb Yong Wu (吴勇):
>>> [SNIP]
>>>> But what we should try to avoid is that newly merged drivers provide
>>>> both a driver specific UAPI and DMA-heaps. The justification that
>>>> this
>>>> makes it easier to transit userspace to the new UAPI doesn't really
>>>> count.
>>>>
>>>> That would be adding UAPI already with a plan to deprecate it and
>>>> that
>>>> is most likely not helpful considering that UAPI must be supported
>>>> forever as soon as it is upstream.
>>> Sorry, I didn't understand this. I think we have not change the UAPI.
>>> Which code are you referring to?
>> Well, what do you need this for if not a new UAPI?
>>
>> My assumption here is that you need to export the DMA-heap allocation
>> function so that you can server an UAPI in your new driver. Or what else
>> is that good for?
>>
>> As far as I understand you try to upstream your new vcodec driver. So
>> while this change here seems to be a good idea to clean up existing
>> drivers it doesn't look like a good idea for a newly created driver.
> MTK VCODEC has been upstream for quite some time now. The other patchset is
> trying to add secure decoding/encoding support to that existing upstream driver.
>
> Regarding the uAPI, it seems that this addition to dmabuf heap internal API is
> exactly the opposite. By making heaps available to drivers, modification to the
> V4L2 uAPI is being reduced to adding "SECURE_MODE" + "SECURE_HEAP_ID" controls
> (this is not debated yet has an approach). The heaps is being used internally in
> replacement to every allocation, user visible or not.

Thanks a lot for that explanation, I was really wondering what the use 
case for this is if it's not to serve new UAPI.

In this case I don't see any reason why we shouldn't do it. It's indeed 
much cleaner.

Christian.

>
> Nicolas
>
>> Regards,
>> Christian.
>>
>>>>> So I think this patch is a little confusing in this series, as I
>>>> don't
>>>>> see much of it actually being used here (though forgive me if I'm
>>>>> missing it).
>>>>>
>>>>> Instead, It seems it get used in a separate patch series here:
>>>>>      
>>>> https://lore.kernel.org/all/20230911125936.10648-1-yunfei.dong@mediatek.com/
>>>>
>>>> Please try to avoid stuff like that it is really confusing and eats
>>>> reviewers time.
>>> My fault, I thought dma-buf and media belonged to the different tree,
>>> so I send them separately. The cover letter just said "The consumers of
>>> the new heap and new interface are our codecs and DRM, which will be
>>> sent upstream soon", and there was no vcodec link at that time.
>>>
>>> In the next version, we will put the first three patches into the
>>> vcodec patchset.
>>>
>>> Thanks.
>>>