Re: [PATCH 07/18] dma-mapping: Implement link/unlink ranges API

[Leon Romanovsky <leon@kernel.org>]

On Mon, Oct 28, 2024 at 08:22:52AM +0200, Leon Romanovsky wrote:
> On Mon, Oct 28, 2024 at 10:00:25AM +0800, Baolu Lu wrote:
> > On 2024/10/27 22:21, Leon Romanovsky wrote:
> > > +/**
> > > + * dma_iova_sync - Sync IOTLB
> > > + * @dev: DMA device
> > > + * @state: IOVA state
> > > + * @offset: offset into the IOVA state to sync
> > > + * @size: size of the buffer
> > > + * @ret: return value from the last IOVA operation
> > > + *
> > > + * Sync IOTLB for the given IOVA state. This function should be called on
> > > + * the IOVA-contigous range created by one ore more dma_iova_link() calls
> > > + * to sync the IOTLB.
> > > + */
> > > +int dma_iova_sync(struct device *dev, struct dma_iova_state *state,
> > > +		size_t offset, size_t size, int ret)
> > > +{
> > > +	struct iommu_domain *domain = iommu_get_dma_domain(dev);
> > > +	struct iommu_dma_cookie *cookie = domain->iova_cookie;
> > > +	struct iova_domain *iovad = &cookie->iovad;
> > > +	dma_addr_t addr = state->addr + offset;
> > > +	size_t iova_start_pad = iova_offset(iovad, addr);
> > > +
> > > +	addr -= iova_start_pad;
> > > +	size = iova_align(iovad, size + iova_start_pad);
> > > +
> > > +	if (!ret)
> > > +		ret = iommu_sync_map(domain, addr, size);
> > > +	if (ret)
> > > +		iommu_unmap(domain, addr, size);
> > 
> > It appears strange that mapping is not done in this helper, but
> > unmapping is added in the failure path. Perhaps I overlooked anything?
> 
> Like iommu_sync_map() is performed on whole continuous range, the iommu_unmap()
> should be done on the same range. So, technically you can unmap only part of
> the range which called to dma_iova_link() and failed, but you will need
> to make sure that iommu_sync_map() is still called for "successful" part of
> iommu_map().
> 
> In that case, you will need to undo everything anyway and it means that
> you will call to iommu_unmap() on the successful part of the range
> anyway.
> 
> dma_iova_sync() is single operation for the whole range and
> iommu_unmap() too, so they are bound together.
> 
> > To my understanding, it should like below:
> > 
> > 	return iommu_sync_map(domain, addr, size);
> > 
> > In the drivers that make use of this interface should do something like
> > below:
> > 
> > 	ret = dma_iova_sync(...);
> > 	if (ret)
> > 		dma_iova_destroy(...)
> 
> It is actually what is happening in the code, but in less direct way due
> to unwinding of the code.

After more thoughts on the topic, I think that it will be better to make
this dma_iova_sync() less cryptic and more direct. I will change it to be
as below in my next version:

  1972 int dma_iova_sync(struct device *dev, struct dma_iova_state *state,
  1973                 size_t offset, size_t size)
  1974 {
  1975         struct iommu_domain *domain = iommu_get_dma_domain(dev);
  1976         struct iommu_dma_cookie *cookie = domain->iova_cookie;
  1977         struct iova_domain *iovad = &cookie->iovad;
  1978         dma_addr_t addr = state->addr + offset;
  1979         size_t iova_start_pad = iova_offset(iovad, addr);
  1980
  1981         return iommu_sync_map(domain, addr - iova_start_pad,
  1982                       iova_align(iovad, size + iova_start_pad));
  1983 }
  1984 EXPORT_SYMBOL_GPL(dma_iova_sync);

Thanks