Re: [RFC PATCH] powerpc: fix get_arch_dma_ops() for NTB devices

[Michael Ellerman <mpe@ellerman.id.au>]

Christoph Hellwig <hch@lst.de> writes:
> On Wed, Jan 30, 2019 at 11:58:40PM +1100, Michael Ellerman wrote:
>> Alexander Fomichev <fomichev.ru@gmail.com> writes:
>> 
>> > get_dma_ops() falls into arch-dependant get_arch_dma_ops(), which
>> > historically returns NULL on PowerPC. Therefore dma_set_mask() fails.
>> > This affects Switchtec (and probably other) NTB devices, that they fail
>> > to initialize.
>> 
>> What's an NTB device?
>> 
>> drivers/ntb I assume?
>> 
>> So it's a PCI device of some sort, but presumably the device you're
>> calling dma_set_mask() on is an NTB device not a PCI device?
>> 
>> But then it works if you tell it to use the PCI DMA ops?
>> 
>> At the very least the code should be checking for the NTB bus type and
>> only returning the PCI ops in that specific case, not for all devices.
>
> Can you provide the context?  E.g. the patch and the rest of the commit
> log.  This all looks rather odd to me.

Sorry, here it is.

Or on lore: https://lore.kernel.org/linuxppc-dev/20190128133203.mon4a3nkrzijn43g@alfbook-pro.local/

Subject: [RFC PATCH] powerpc: fix get_arch_dma_ops() for NTB devices

get_dma_ops() falls into arch-dependant get_arch_dma_ops(), which
historically returns NULL on PowerPC. Therefore dma_set_mask() fails.
This affects Switchtec (and probably other) NTB devices, that they fail
to initialize.
The proposed patch should fix the issue.

---
 arch/powerpc/include/asm/dma-mapping.h | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/arch/powerpc/include/asm/dma-mapping.h b/arch/powerpc/include/asm/dma-mapping.h
index ebf6680..cb6ac96 100644
--- a/arch/powerpc/include/asm/dma-mapping.h
+++ b/arch/powerpc/include/asm/dma-mapping.h
@@ -70,14 +70,11 @@ extern struct dma_map_ops dma_iommu_ops;
 #endif
 extern const struct dma_map_ops dma_nommu_ops;

+extern const struct dma_map_ops *get_pci_dma_ops(void);
+
 static inline const struct dma_map_ops *get_arch_dma_ops(struct bus_type *bus)
 {
-	/* We don't handle the NULL dev case for ISA for now. We could
-	 * do it via an out of line call but it is not needed for now. The
-	 * only ISA DMA device we support is the floppy and we have a hack
-	 * in the floppy driver directly to get a device for us.
-	 */
-	return NULL;
+	return get_pci_dma_ops();
 }

 /*