linux-kernel - Re: [PATCH] dma-buf: add no-op stubs when CONFIG_DMA_SHARED

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Message-ID: <79688e4f-203c-4e06-b73a-641dcc3f4c73@amd.com>
Date: Thu, 27 Nov 2025 11:26:23 +0100
From: Christian König <christian.koenig@....com>
To: Viresh Kumar <viresh.kumar@...aro.org>
Cc: Sumit Semwal <sumit.semwal@...aro.org>, linux-media@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma-buf: add no-op stubs when CONFIG_DMA_SHARED_BUFFER is
 disabled

On 11/27/25 09:23, Viresh Kumar wrote:
> On 27-11-25, 09:07, Christian König wrote:
>> On 11/27/25 08:40, Viresh Kumar wrote:
>>> Move several dma-buf function declarations under
>>> CONFIG_DMA_SHARED_BUFFER and provide static inline no-op implementations
>>> for the disabled case to allow the callers to build when the feature is
>>> not compiled in.
>>
>> Good point, but which driver actually needs that?
> 
> This broke some WIP stuff [1] which isn't posted upstream yet. That's why I
> didn't mention anything in the commit log, though I could have added a comment
> about that in the non-commit-log part.

Well then better send that out with the full patch set.

>> In other words there should be a concrete example of what breaks in the commit message.
> 
> There is time for those changes to be posted and not sure if they will be
> accepted or not. But either way, this change made sense in general and so I
> thought there is nothing wrong to get this upstream right away.

Yeah when it is unused intermediately then that is usually a no-go even if I agree that it makes sense.

>>> +static inline struct dma_buf *dma_buf_get(int fd)
>>> +{
>>> +	return NULL;
>>
>> And here ERR_PTR(-EINVAL).
> 
> I am not really sure if this should be EINVAL in this case. EOPNOTSUPP still
> makes sense as the API isn't supported.

When the API isn't compiled in the fd can't be valid (because you can't create a dma_buf object in the first place).

So returning -EINVAL still makes a lot of sense.

Regards,
Christian.

> 
>>> +static inline struct dma_buf *dma_buf_iter_begin(void)
>>> +{
>>> +	return NULL;
>>> +}
>>> +
>>> +static inline struct dma_buf *dma_buf_iter_next(struct dma_buf *dmbuf)
>>> +{
>>> +	return NULL;
>>> +}
>>
>> Those two are only for BPF and not driver use.
> 
> Will drop them.
>