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

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Message-ID: <CANDhNCqS+WKhTWjeC7yBL+x4erK4S4bievTxdneaCu1haA8=hA@mail.gmail.com>
Date: Thu, 2 Oct 2025 12:45:41 -0700
From: John Stultz <jstultz@...gle.com>
To: Maxime Ripard <mripard@...hat.com>
Cc: Jens Wiklander <jens.wiklander@...aro.org>, 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>, 
	"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

On Thu, Oct 2, 2025 at 12:47 AM Maxime Ripard <mripard@...hat.com> wrote:
> 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?

For things like distro kernels (or in Android's case, the GKI),
there's a benefit for modules that can be loaded permanently (not
having a module_exit hook).
One doesn't have to bloat the base kernel image/memory usage for
everyone, while still not having to necessarily deal with
complications from module unloading issues.

thanks
-john