Re: [PATCH v1 08/17] dma-mapping: add a dma_need_unmap helper

[Leon Romanovsky <leon@kernel.org>]

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@lst.de>
> > 
> > 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@lst.de>
> > Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
> > ---
> >   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?

From what I see dma_skip_sync is not available when kernel is built
without CONFIG_DMA_NEED_SYNC.

> 
> In terms of not having to unmap implying not having to store addresses at
> all, it doesn't seem super-useful when you still have to store them for long
> enough to find out that you don't :/

Why? The decision if DMA addresses are needed is taken when allocating
relevant arrays, before we have any DMA address to store. If we know
that we don't need to unmap, we can skip allocation of the array for
free. So what and when "you still have to store them"?

Thanks

> 
> Thanks,
> Robin.
> 
> > + */
> > +bool dma_need_unmap(struct device *dev)
> > +{
> > +	if (!dma_map_direct(dev, get_dma_ops(dev)))
> > +		return true;
> > +#ifdef CONFIG_DMA_NEED_SYNC
> > +	if (!dev->dma_skip_sync)
> > +		return true;
> > +#endif
> > +	return IS_ENABLED(CONFIG_DMA_API_DEBUG);
> > +}
> > +EXPORT_SYMBOL_GPL(dma_need_unmap);
> > +
> >   static void dma_setup_need_sync(struct device *dev)
> >   {
> >   	const struct dma_map_ops *ops = get_dma_ops(dev);
>