Re: [PATCH] usb: gadget: f_fs: Remove deprecated dma_fence->ops

["Christian König" <christian.koenig@amd.com>]

On 10/29/26 15:18, Paul Cercueil wrote:
> Le jeudi 29 octobre 2026 à 14:49 +0100, Philipp Stanner a écrit :
>> Hi,
>>
>> On Thu, 2026-10-29 at 14:19 +0100, Paul Cercueil wrote:
>>> Hi Philipp,
>>>
>>> For context, this .release() callback was added to avoid the implicit free
>>> of
>>> the base class, which IIRC was a request I got during the review process of
>>> this
>>> change set.
>>
>> Can't say anything about the request you got, but fences are reference-
>> counted and the ability to free itself exists since at least
>> 3c3b177a9369 (2014).
> 
> Yes, it already existed when I wrote the code and I was requested to make it
> explicit. Not trying to make a point here, just stating the reasons why it was
> written that way.
> 
>> From dma_fence's perspective it's certainly the desired canonical
>> pattern since the release() ops prevents fences from the important
>> feature of decoupling fence producer and consumer.
> 
> ACK.
> 
>>> I'm personally fine with the change, but do we need a "keep this field at
>>> the
>>> top" comment for the base class in the ffs_dma_fence struct? Or what's the
>>> policy here?
>>
>> When I got into the kernel I was told that it's common practice to
>> always have the base class as the first element. That also allows for
>> obtaining the parent class by a mere pointer-cast instead of
>> container_of.
>>
>> But the reality is also that some users are not aware of this rule. I
>> can document it in a v2 if you like.
> 
> Let's wait on feedback from more people. I don't have a strong opinion either
> way.

As far as I know It is indeed common practice , if you want a comment or not on your structure that the base class needs to be the first element or the dma_fence release will break is totally up to you I think.

On the other hand it is maybe possible to provide a dma_fence_alloc_obj() macro which wraps kmalloc_obj and compile time asserts that the member you give for the base class is at offset 0.

Regards,
Christian.

> 
> Cheers,
> -Paul