On Thu, Oct 31, 2024 at 09:18:11PM +0000, Robin Murphy wrote:
> On 30/10/2024 3:12 pm, Leon Romanovsky wrote:
> > From: Christoph Hellwig <hch@xxxxxx>
> >
> > Add helper that allows a driver to skip calling dma_unmap_*
> > if the DMA layer can guarantee that they are no-nops.
> >
> > Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> > Signed-off-by: Leon Romanovsky <leonro@xxxxxxxxxx>
> > ---
> > include/linux/dma-mapping.h | 5 +++++
> > kernel/dma/mapping.c | 20 ++++++++++++++++++++
> > 2 files changed, 25 insertions(+)
> >
> > diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> > index 8074a3b5c807..6906edde505d 100644
> > --- a/include/linux/dma-mapping.h
> > +++ b/include/linux/dma-mapping.h
> > @@ -410,6 +410,7 @@ static inline bool dma_need_sync(struct device *dev, dma_addr_t dma_addr)
> > {
> > return dma_dev_need_sync(dev) ? __dma_need_sync(dev, dma_addr) : false;
> > }
> > +bool dma_need_unmap(struct device *dev);
> > #else /* !CONFIG_HAS_DMA || !CONFIG_DMA_NEED_SYNC */
> > static inline bool dma_dev_need_sync(const struct device *dev)
> > {
> > @@ -435,6 +436,10 @@ static inline bool dma_need_sync(struct device *dev, dma_addr_t dma_addr)
> > {
> > return false;
> > }
> > +static inline bool dma_need_unmap(struct device *dev)
> > +{
> > + return false;
> > +}
> > #endif /* !CONFIG_HAS_DMA || !CONFIG_DMA_NEED_SYNC */
> > struct page *dma_alloc_pages(struct device *dev, size_t size,
> > diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
> > index 864a1121bf08..daa97a650778 100644
> > --- a/kernel/dma/mapping.c
> > +++ b/kernel/dma/mapping.c
> > @@ -442,6 +442,26 @@ bool __dma_need_sync(struct device *dev, dma_addr_t dma_addr)
> > }
> > EXPORT_SYMBOL_GPL(__dma_need_sync);
> > +/**
> > + * dma_need_unmap - does this device need dma_unmap_* operations
> > + * @dev: device to check
> > + *
> > + * If this function returns %false, drivers can skip calling dma_unmap_* after
> > + * finishing an I/O. This function must be called after all mappings that might
> > + * need to be unmapped have been performed.
>
> In terms of the unmap call itself, why don't we just use dma_skip_sync to
> short-cut dma_direct_unmap_*() and make sure it's as cheap as possible?
