[RFC PATCH v3 03/12] iommu/io-pgtable: Introduce map_pages() as a page table op

[Will Deacon]

On Tue, Apr 06, 2021 at 02:07:41PM -0700, isaacm at codeaurora.org wrote:
> On 2021-04-06 04:57, Will Deacon wrote:
> > On Mon, Apr 05, 2021 at 12:11:03PM -0700, Isaac J. Manjarres wrote:
> > > Mapping memory into io-pgtables follows the same semantics
> > > that unmapping memory used to follow (i.e. a buffer will be
> > > mapped one page block per call to the io-pgtable code). This
> > > means that it can be optimized in the same way that unmapping
> > > memory was, so add a map_pages() callback to the io-pgtable
> > > ops structure, so that a range of pages of the same size
> > > can be mapped within the same call.
> > > 
> > > Signed-off-by: Isaac J. Manjarres <isaacm at codeaurora.org>
> > > Suggested-by: Will Deacon <will at kernel.org>
> > > ---
> > >  include/linux/io-pgtable.h | 4 ++++
> > >  1 file changed, 4 insertions(+)
> > > 
> > > diff --git a/include/linux/io-pgtable.h b/include/linux/io-pgtable.h
> > > index 2ed0c057d9e7..019149b204b8 100644
> > > --- a/include/linux/io-pgtable.h
> > > +++ b/include/linux/io-pgtable.h
> > > @@ -143,6 +143,7 @@ struct io_pgtable_cfg {
> > >   * struct io_pgtable_ops - Page table manipulation API for IOMMU
> > > drivers.
> > >   *
> > >   * @map:          Map a physically contiguous memory region.
> > > + * @map_pages:    Map a physically contiguous range of pages of the
> > > same size.
> > >   * @unmap:        Unmap a physically contiguous memory region.
> > >   * @unmap_pages:  Unmap a range of virtually contiguous pages of
> > > the same size.
> > >   * @iova_to_phys: Translate iova to physical address.
> > > @@ -153,6 +154,9 @@ struct io_pgtable_cfg {
> > >  struct io_pgtable_ops {
> > >  	int (*map)(struct io_pgtable_ops *ops, unsigned long iova,
> > >  		   phys_addr_t paddr, size_t size, int prot, gfp_t gfp);
> > > +	int (*map_pages)(struct io_pgtable_ops *ops, unsigned long iova,
> > > +			 phys_addr_t paddr, size_t pgsize, size_t pgcount,
> > > +			 int prot, gfp_t gfp, size_t *mapped);
> > 
> > How about returning 'size_t' and using IS_ERR_VALUE() instead of adding
> > the extra 'mapped' argument (i.e. return the size of the region mapped
> > or an error code)? I don't think we realistically need to care about map
> > sizes that overlap with the error region.
> > 
> I'd given that a shot before, but the problem that I kept running into was
> that
> in case of an error, if I return an error code, I don't know how much memory
> was mapped, so that I can invoke iommu_unmap from __iommu_map with that size
> to
> undo the partial mappings from a map_pages() call.

Ah yes, sorry, I see it now. So keep this as you've got it. Pushing the
cleanup path deeper doesn't feel like the right thing to do.

Will