Re: IB: Add DMA mapping functions to allow device drivers to interpose

Re: IB: Add DMA mapping functions to allow device drivers to interpose

[Andrew Morton]

On Wed, 13 Dec 2006 03:59:45 GMT
Linux Kernel Mailing List <linux-kernel@vger.kernel.org> wrote:

>     IB: Add DMA mapping functions to allow device drivers to interpose
>     
>     The QLogic InfiniPath HCAs use programmed I/O instead of HW DMA.
>     This patch allows a verbs device driver to interpose on DMA mapping
>     function calls in order to avoid relying on bus_to_virt() and
>     phys_to_virt() to undo the mappings created by dma_map_single(),
>     dma_map_sg(), etc.
>     
>     Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
>     Signed-off-by: Roland Dreier <rolandd@cisco.com>

include/rdma/ib_verbs.h: In function 'ib_dma_alloc_coherent':
include/rdma/ib_verbs.h:1635: warning: passing argument 3 of 'dma_alloc_coherent' from incompatible pointer type
In file included from drivers/infiniband/hw/mthca/mthca_provider.h:41,
                 from drivers/infiniband/hw/mthca/mthca_dev.h:53,
                 from drivers/infiniband/hw/mthca/mthca_main.c:44:
include/rdma/ib_verbs.h: In function 'ib_dma_alloc_coherent':
include/rdma/ib_verbs.h:1635: warning: passing argument 3 of 'dma_alloc_coherent' from incompatible pointer type


That u64 needs to become a dma_addr_t.  That means that
ib_dma_mapping_ops.alloc_coherent() and ib_dma_mapping_ops.free_coherent() are
wrong as well.

> +struct ib_dma_mapping_ops {
> ...
> +	void		*(*alloc_coherent)(struct ib_device *dev,
> +					   size_t size,
> +					   u64 *dma_handle,
> +					   gfp_t flag);
> +	void		(*free_coherent)(struct ib_device *dev,
> +					 size_t size, void *cpu_addr,
> +					 u64 dma_handle);
> +};

I'd have picked this up if it had been in git-infiniband for even a couple
of days.  I'm assuming this all got slammed into mainline because of the
merge window thing.

I cannot find these patches on the kernel mailing list.  I cannot find the
pull request anywhere.

> +static inline u64 ib_dma_map_single(struct ib_device *dev,
> +				    void *cpu_addr, size_t size,
> +				    enum dma_data_direction direction)

no, dma_map_single() returns a dma_addr_t.

Re: IB: Add DMA mapping functions to allow device drivers to interpose

[Ralph Campbell]

On Tue, 2006-12-12 at 23:47 -0800, Andrew Morton wrote:
> On Wed, 13 Dec 2006 03:59:45 GMT
> Linux Kernel Mailing List <linux-kernel@vger.kernel.org> wrote:
> 
> >     IB: Add DMA mapping functions to allow device drivers to interpose
> >     
> >     The QLogic InfiniPath HCAs use programmed I/O instead of HW DMA.
> >     This patch allows a verbs device driver to interpose on DMA mapping
> >     function calls in order to avoid relying on bus_to_virt() and
> >     phys_to_virt() to undo the mappings created by dma_map_single(),
> >     dma_map_sg(), etc.
> >     
> >     Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
> >     Signed-off-by: Roland Dreier <rolandd@cisco.com>
> 
> include/rdma/ib_verbs.h: In function 'ib_dma_alloc_coherent':
> include/rdma/ib_verbs.h:1635: warning: passing argument 3 of 'dma_alloc_coherent' from incompatible pointer type
> In file included from drivers/infiniband/hw/mthca/mthca_provider.h:41,
>                  from drivers/infiniband/hw/mthca/mthca_dev.h:53,
>                  from drivers/infiniband/hw/mthca/mthca_main.c:44:
> include/rdma/ib_verbs.h: In function 'ib_dma_alloc_coherent':
> include/rdma/ib_verbs.h:1635: warning: passing argument 3 of 'dma_alloc_coherent' from incompatible pointer type
> 
> 
> That u64 needs to become a dma_addr_t.  That means that
> ib_dma_mapping_ops.alloc_coherent() and ib_dma_mapping_ops.free_coherent() are
> wrong as well.
> 
> > +struct ib_dma_mapping_ops {
> > ...
> > +	void		*(*alloc_coherent)(struct ib_device *dev,
> > +					   size_t size,
> > +					   u64 *dma_handle,
> > +					   gfp_t flag);
> > +	void		(*free_coherent)(struct ib_device *dev,
> > +					 size_t size, void *cpu_addr,
> > +					 u64 dma_handle);
> > +};
> 
> I'd have picked this up if it had been in git-infiniband for even a couple
> of days.  I'm assuming this all got slammed into mainline because of the
> merge window thing.
> 
> I cannot find these patches on the kernel mailing list.  I cannot find the
> pull request anywhere.
> 
> > +static inline u64 ib_dma_map_single(struct ib_device *dev,
> > +				    void *cpu_addr, size_t size,
> > +				    enum dma_data_direction direction)
> 
> no, dma_map_single() returns a dma_addr_t.

ib_dma_map_single() allows the ib_ipath device driver to interpose
on IOMMU allocations and not do them by returning the kernel
virtual address as the "DMA address".  I started with dma_addr_t
but it was pointed out to me that sparc64 defines dma_addr_t
as u32. This would cause addresses to be truncated.
Also, I chose u64 because the return value from ib_dma_*() is
stored in the ib_sge.addr field which is u64.

My preference would be to change the offending uses of dma_addr_t
to u64.  Do you have a better solution?

Re: IB: Add DMA mapping functions to allow device drivers to interpose

[Andrew Morton]

On Wed, 13 Dec 2006 12:11:27 -0800
Ralph Campbell <ralph.campbell@qlogic.com> wrote:

> > I'd have picked this up if it had been in git-infiniband for even a couple
> > of days.  I'm assuming this all got slammed into mainline because of the
> > merge window thing.
> > 
> > I cannot find these patches on the kernel mailing list.  I cannot find the
> > pull request anywhere.
> > 
> > > +static inline u64 ib_dma_map_single(struct ib_device *dev,
> > > +				    void *cpu_addr, size_t size,
> > > +				    enum dma_data_direction direction)
> > 
> > no, dma_map_single() returns a dma_addr_t.
> 
> ib_dma_map_single() allows the ib_ipath device driver to interpose
> on IOMMU allocations and not do them by returning the kernel
> virtual address as the "DMA address".  I started with dma_addr_t
> but it was pointed out to me that sparc64 defines dma_addr_t
> as u32. This would cause addresses to be truncated.
> Also, I chose u64 because the return value from ib_dma_*() is
> stored in the ib_sge.addr field which is u64.
> 
> My preference would be to change the offending uses of dma_addr_t
> to u64.  Do you have a better solution?

We should be able to use dma_addr_t for this?  Is it not the case that the
values we're dealing with here _are_ DMA addresses?  I think a more complete
description of the problem we're trying to solve here would help.

I'm not sure what the problem is with sparc64 - perhaps its dma_addr_t
really is a "cookie" and isn't a physical bus address?  But you want a value
which is really a physical bus address?  Dunno.

Perhaps dma64_addr_t can be used here.

Re: IB: Add DMA mapping functions to allow device drivers to interpose

[Ralph Campbell]

On Wed, 2006-12-13 at 12:30 -0800, Andrew Morton wrote:
> On Wed, 13 Dec 2006 12:11:27 -0800
> Ralph Campbell <ralph.campbell@qlogic.com> wrote:
snip.
> > My preference would be to change the offending uses of dma_addr_t
> > to u64.  Do you have a better solution?
> 
> We should be able to use dma_addr_t for this?  Is it not the case that the
> values we're dealing with here _are_ DMA addresses?  I think a more complete
> description of the problem we're trying to solve here would help.
> 
> I'm not sure what the problem is with sparc64 - perhaps its dma_addr_t
> really is a "cookie" and isn't a physical bus address?  But you want a value
> which is really a physical bus address?  Dunno.
> 
> Perhaps dma64_addr_t can be used here.

The fundamental problem is that ib_get_dma_mr() returns a
pointer representing a memory region for all of physical memory
and the IB interface consumer is expected to call dma_map_single()
to get a dma_addr_t to pass to the HCA driver with the memory
region pointer.

For an HCA like Mellanox, the dma_addr_t really is a DMA address
which the hardware uses to DMA data from the chip to memory.

The ib_ipath HCA driver does not generally use DMA addresses.
The hardware does DMA the receive packet to a ring of buffers
but the receive interrupt handler then has to copy the data
from the receive buffer to the address specified by the
ib_post_recv() function.  This means the driver needs a
kernel virtual address instead of the memory region + DMA address
that is passed by ib_post_recv().

The ib_dma_*() functions were added to allow the ib_ipath
driver to interpose on the dma_*() functions so that a
kernel virtual address can be returned instead of allocating
an IOMMU DMA address.  Without the interposing functions,
there is no way for the ib_ipath driver to convert the IOMMU
address back into a kernel virtual address since it never
sees the original inputs used to allocate the IOMMU address.

I don't want to make ib_dma_map_single() return a dma_addr_t
cookie and use it to lookup the kernel virtual address since
this is a performance critical code path in the receive
interrupt handler and the "cookie" needs to be a byte address
which can be offset within a page.

The secondary problem is that on sparc64, dma_addr_t is a
u32 so if ib_dma_map_single() returns a void * cast to dma_addr_t,
it is truncated.  We might be able to convince the sparc64
maintainers to make dma_addr_t u64 but dma64_addr_t won't
work because that is specific to sparc64.  The type has
to be architecture neutral.