Re: [Linaro-mm-sig] [PATCH v5 1/6] dma-buf: Add dma_buf_{begin,end}_access()

Paul Cercueil
2024-01-22T11:02:03Z

Hi Christian,

Le lundi 22 janvier 2024 à 11:35 +0100, Christian König a écrit :
> Am 19.01.24 um 15:13 schrieb Paul Cercueil:
> > These functions should be used by device drivers when they start
> > and
> > stop accessing the data of DMABUF. It allows DMABUF importers to
> > cache
> > the dma_buf_attachment while ensuring that the data they want to
> > access
> > is available for their device when the DMA transfers take place.
>
> As Daniel already noted as well this is a complete no-go from the
> DMA-buf design point of view.

What do you mean "as Daniel already noted"? It was him who suggested
this.

>
> Regards,
> Christian.

Cheers,
-Paul

>
> >
> > Signed-off-by: Paul Cercueil <paul@crapouillou.net>
> >
> > ---
> > v5: New patch
> > ---
> >   drivers/dma-buf/dma-buf.c | 66
> > +++++++++++++++++++++++++++++++++++++++
> >   include/linux/dma-buf.h   | 37 ++++++++++++++++++++++
> >   2 files changed, 103 insertions(+)
> >
> > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> > index 8fe5aa67b167..a8bab6c18fcd 100644
> > --- a/drivers/dma-buf/dma-buf.c
> > +++ b/drivers/dma-buf/dma-buf.c
> > @@ -830,6 +830,8 @@ static struct sg_table * __map_dma_buf(struct
> > dma_buf_attachment *attach,
> >    *     - dma_buf_mmap()
> >    *     - dma_buf_begin_cpu_access()
> >    *     - dma_buf_end_cpu_access()
> > + *     - dma_buf_begin_access()
> > + *     - dma_buf_end_access()
> >    *     - dma_buf_map_attachment_unlocked()
> >    *     - dma_buf_unmap_attachment_unlocked()
> >    *     - dma_buf_vmap_unlocked()
> > @@ -1602,6 +1604,70 @@ void dma_buf_vunmap_unlocked(struct dma_buf
> > *dmabuf, struct iosys_map *map)
> >   }
> >   EXPORT_SYMBOL_NS_GPL(dma_buf_vunmap_unlocked, DMA_BUF);
> >
> > +/**
> > + * @dma_buf_begin_access - Call before any hardware access from/to
> > the DMABUF
> > + * @attach: [in] attachment used for hardware access
> > + * @sg_table: [in] scatterlist used for the DMA transfer
> > + * @direction:  [in]    direction of DMA transfer
> > + */
> > +int dma_buf_begin_access(struct dma_buf_attachment *attach,
> > + struct sg_table *sgt, enum
> > dma_data_direction dir)
> > +{
> > + struct dma_buf *dmabuf;
> > + bool cookie;
> > + int ret;
> > +
> > + if (WARN_ON(!attach))
> > + return -EINVAL;
> > +
> > + dmabuf = attach->dmabuf;
> > +
> > + if (!dmabuf->ops->begin_access)
> > + return 0;
> > +
> > + cookie = dma_fence_begin_signalling();
> > + ret = dmabuf->ops->begin_access(attach, sgt, dir);
> > + dma_fence_end_signalling(cookie);
> > +
> > + if (WARN_ON_ONCE(ret))
> > + return ret;
> > +
> > + return 0;
> > +}
> > +EXPORT_SYMBOL_NS_GPL(dma_buf_begin_access, DMA_BUF);
> > +
> > +/**
> > + * @dma_buf_end_access - Call after any hardware access from/to
> > the DMABUF
> > + * @attach: [in] attachment used for hardware access
> > + * @sg_table: [in] scatterlist used for the DMA transfer
> > + * @direction:  [in]    direction of DMA transfer
> > + */
> > +int dma_buf_end_access(struct dma_buf_attachment *attach,
> > +        struct sg_table *sgt, enum
> > dma_data_direction dir)
> > +{
> > + struct dma_buf *dmabuf;
> > + bool cookie;
> > + int ret;
> > +
> > + if (WARN_ON(!attach))
> > + return -EINVAL;
> > +
> > + dmabuf = attach->dmabuf;
> > +
> > + if (!dmabuf->ops->end_access)
> > + return 0;
> > +
> > + cookie = dma_fence_begin_signalling();
> > + ret = dmabuf->ops->end_access(attach, sgt, dir);
> > + dma_fence_end_signalling(cookie);
> > +
> > + if (WARN_ON_ONCE(ret))
> > + return ret;
> > +
> > + return 0;
> > +}
> > +EXPORT_SYMBOL_NS_GPL(dma_buf_end_access, DMA_BUF);
> > +
> >   #ifdef CONFIG_DEBUG_FS
> >   static int dma_buf_debug_show(struct seq_file *s, void *unused)
> >   {
> > diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> > index 8ff4add71f88..8ba612c7cc16 100644
> > --- a/include/linux/dma-buf.h
> > +++ b/include/linux/dma-buf.h
> > @@ -246,6 +246,38 @@ struct dma_buf_ops {
> >    */
> >    int (*end_cpu_access)(struct dma_buf *, enum
> > dma_data_direction);
> >
> > + /**
> > + * @begin_access:
> > + *
> > + * This is called from dma_buf_begin_access() when a
> > device driver
> > + * wants to access the data of the DMABUF. The exporter
> > can use this
> > + * to flush/sync the caches if needed.
> > + *
> > + * This callback is optional.
> > + *
> > + * Returns:
> > + *
> > + * 0 on success or a negative error code on failure.
> > + */
> > + int (*begin_access)(struct dma_buf_attachment *, struct
> > sg_table *,
> > +     enum dma_data_direction);
> > +
> > + /**
> > + * @end_access:
> > + *
> > + * This is called from dma_buf_end_access() when a device
> > driver is
> > + * done accessing the data of the DMABUF. The exporter can
> > use this
> > + * to flush/sync the caches if needed.
> > + *
> > + * This callback is optional.
> > + *
> > + * Returns:
> > + *
> > + * 0 on success or a negative error code on failure.
> > + */
> > + int (*end_access)(struct dma_buf_attachment *, struct
> > sg_table *,
> > +   enum dma_data_direction);
> > +
> >    /**
> >    * @mmap:
> >    *
> > @@ -606,6 +638,11 @@ void dma_buf_detach(struct dma_buf *dmabuf,
> >   int dma_buf_pin(struct dma_buf_attachment *attach);
> >   void dma_buf_unpin(struct dma_buf_attachment *attach);
> >
> > +int dma_buf_begin_access(struct dma_buf_attachment *attach,
> > + struct sg_table *sgt, enum
> > dma_data_direction dir);
> > +int dma_buf_end_access(struct dma_buf_attachment *attach,
> > +        struct sg_table *sgt, enum
> > dma_data_direction dir);
> > +
> >   struct dma_buf *dma_buf_export(const struct dma_buf_export_info
> > *exp_info);
> >
> >   int dma_buf_fd(struct dma_buf *dmabuf, int flags);
>