linux-kernel - Re: [PATCH v12 2/9] dma-buf: dma-heap: export declared functions

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Message-ID: <CAHUa44GtAY7=r_bA7ETOsBq+K4w6r1CtY7T9rFn224mshA5GYg@mail.gmail.com>
Date: Thu, 2 Oct 2025 14:17:27 +0200
From: Jens Wiklander <jens.wiklander@...aro.org>
To: Maxime Ripard <mripard@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-media@...r.kernel.org, 
	dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org, 
	op-tee@...ts.trustedfirmware.org, linux-arm-kernel@...ts.infradead.org, 
	Olivier Masse <olivier.masse@....com>, Thierry Reding <thierry.reding@...il.com>, 
	Yong Wu <yong.wu@...iatek.com>, Sumit Semwal <sumit.semwal@...aro.org>, 
	Benjamin Gaignard <benjamin.gaignard@...labora.com>, Brian Starkey <Brian.Starkey@....com>, 
	John Stultz <jstultz@...gle.com>, "T . J . Mercier" <tjmercier@...gle.com>, 
	Christian König <christian.koenig@....com>, 
	Sumit Garg <sumit.garg@...nel.org>, Matthias Brugger <matthias.bgg@...il.com>, 
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>, azarrabi@....qualcomm.com, 
	Simona Vetter <simona.vetter@...ll.ch>, Daniel Stone <daniel@...ishbar.org>, 
	Rouven Czerwinski <rouven.czerwinski@...aro.org>, robin.murphy@....com, 
	Sumit Garg <sumit.garg@....qualcomm.com>
Subject: Re: [PATCH v12 2/9] dma-buf: dma-heap: export declared functions

Hi,

On Thu, Oct 2, 2025 at 9:47 AM Maxime Ripard <mripard@...hat.com> wrote:
>
> Hi,
>
> On Thu, Sep 11, 2025 at 03:49:43PM +0200, Jens Wiklander wrote:
> > Export the dma-buf heap functions to allow them to be used by the OP-TEE
> > driver. The OP-TEE driver wants to register and manage specific secure
> > DMA heaps with it.
> >
> > Reviewed-by: Sumit Garg <sumit.garg@....qualcomm.com>
> > Reviewed-by: T.J. Mercier <tjmercier@...gle.com>
> > Acked-by: Sumit Semwal <sumit.semwal@...aro.org>
> > Signed-off-by: Jens Wiklander <jens.wiklander@...aro.org>
> > ---
> >  drivers/dma-buf/dma-heap.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/drivers/dma-buf/dma-heap.c b/drivers/dma-buf/dma-heap.c
> > index 3cbe87d4a464..8ab49924f8b7 100644
> > --- a/drivers/dma-buf/dma-heap.c
> > +++ b/drivers/dma-buf/dma-heap.c
> > @@ -11,6 +11,7 @@
> >  #include <linux/dma-buf.h>
> >  #include <linux/dma-heap.h>
> >  #include <linux/err.h>
> > +#include <linux/export.h>
> >  #include <linux/list.h>
> >  #include <linux/nospec.h>
> >  #include <linux/syscalls.h>
> > @@ -202,6 +203,7 @@ void *dma_heap_get_drvdata(struct dma_heap *heap)
> >  {
> >       return heap->priv;
> >  }
> > +EXPORT_SYMBOL_NS_GPL(dma_heap_get_drvdata, "DMA_BUF_HEAP");
> >
> >  /**
> >   * dma_heap_get_name - get heap name
> > @@ -214,6 +216,7 @@ const char *dma_heap_get_name(struct dma_heap *heap)
> >  {
> >       return heap->name;
> >  }
> > +EXPORT_SYMBOL_NS_GPL(dma_heap_get_name, "DMA_BUF_HEAP");
> >
> >  /**
> >   * dma_heap_add - adds a heap to dmabuf heaps
> > @@ -303,6 +306,7 @@ struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info)
> >       kfree(heap);
> >       return err_ret;
> >  }
> > +EXPORT_SYMBOL_NS_GPL(dma_heap_add, "DMA_BUF_HEAP");
>
> It's not clear to me why we would need to export those symbols.
>
> As far as I know, heaps cannot be removed, and compiling them as module
> means that we would be able to remove them.
>
> Now, if we don't expect the users to be compiled as modules, then we
> don't need to export these symbols at all.
>
> Am I missing something?

In this case, it's the TEE module that _might_ need to instantiate a
DMA heap. Whether it will be instantiated depends on the TEE backend
driver and the TEE firmware. If a heap is instantiated, then it will
not be possible to unload the TEE module. That might not be perfect,
but in my opinion, it's better than other options, such as always
making the TEE subsystem built-in or disabling DMA-heap support when
compiled as a module.

Thanks,
Jens