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

Paul Cercueil
2024-01-23T10:10:40Z

Hi Christian,

Le lundi 22 janvier 2024 à 14:41 +0100, Christian König a écrit :
> Am 22.01.24 um 12:01 schrieb Paul Cercueil:
> > 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.
>
> Sorry, I haven't fully catched up to the discussion then.
>
> In general DMA-buf is build around the idea that the data can be
> accessed coherently by the involved devices.
>
> Having a begin/end of access for devices was brought up multiple
> times
> but so far rejected for good reasons.

I would argue that if it was brought up multiple times, then there are
also good reasons to support such a mechanism.

> That an exporter has to call extra functions to access his own
> buffers
> is a complete no-go for the design since this forces exporters into
> doing extra steps for allowing importers to access their data.

Then what about we add these dma_buf_{begin,end}_access(), with only
implementations for "dumb" exporters e.g. udmabuf or the dmabuf heaps?
And only importers (who cache the mapping and actually care about non-
coherency) would have to call these.

At the very least, is there a way to check that "the data can be
accessed coherently by the involved devices"? So that my importer can
EPERM if there is no coherency vs. a device that's already attached.

Cheers,
-Paul

> That in turn is pretty much un-testable unless you have every
> possible
> importer around while testing the exporter.
>
> Regards,
> Christian.
>
> >
> > > 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);
>