linux-kernel - Re: [PATCH kernel v10 13/34] powerpc/powernv: Do not set "read" flag if direction==DMA

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Date:	Thu, 14 May 2015 10:00:48 +1000
From:	Gavin Shan <gwshan@...ux.vnet.ibm.com>
To:	Alexey Kardashevskiy <aik@...abs.ru>
Cc:	linuxppc-dev@...ts.ozlabs.org,
	David Gibson <david@...son.dropbear.id.au>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Alex Williamson <alex.williamson@...hat.com>,
	Gavin Shan <gwshan@...ux.vnet.ibm.com>,
	Wei Yang <weiyang@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH kernel v10 13/34] powerpc/powernv: Do not set "read" flag
 if direction==DMA_NONE

On Tue, May 12, 2015 at 01:39:02AM +1000, Alexey Kardashevskiy wrote:
>Normally a bitmap from the iommu_table is used to track what TCE entry
>is in use. Since we are going to use iommu_table without its locks and
>do xchg() instead, it becomes essential not to put bits which are not
>implied in the direction flag as the old TCE value (more precisely -
>the permission bits) will be used to decide whether to put the page or not.
>
>This adds iommu_direction_to_tce_perm() (its counterpart is there already)
>and uses it for powernv's pnv_tce_build().
>
>Signed-off-by: Alexey Kardashevskiy <aik@...abs.ru>
>Reviewed-by: David Gibson <david@...son.dropbear.id.au>

Reviewed-by: Gavin Shan <gwshan@...ux.vnet.ibm.com>

>---
>Changes:
>v9:
>* added comment why we must put only valid permission bits
>---
> arch/powerpc/include/asm/iommu.h     |  1 +
> arch/powerpc/kernel/iommu.c          | 15 +++++++++++++++
> arch/powerpc/platforms/powernv/pci.c |  7 +------
> 3 files changed, 17 insertions(+), 6 deletions(-)
>
>diff --git a/arch/powerpc/include/asm/iommu.h b/arch/powerpc/include/asm/iommu.h
>index e94a5e3..d91bd69 100644
>--- a/arch/powerpc/include/asm/iommu.h
>+++ b/arch/powerpc/include/asm/iommu.h
>@@ -200,6 +200,7 @@ extern int iommu_take_ownership(struct iommu_table *tbl);
> extern void iommu_release_ownership(struct iommu_table *tbl);
>
> extern enum dma_data_direction iommu_tce_direction(unsigned long tce);
>+extern unsigned long iommu_direction_to_tce_perm(enum dma_data_direction dir);
>
> #endif /* __KERNEL__ */
> #endif /* _ASM_IOMMU_H */
>diff --git a/arch/powerpc/kernel/iommu.c b/arch/powerpc/kernel/iommu.c
>index 8673c94..31319f8 100644
>--- a/arch/powerpc/kernel/iommu.c
>+++ b/arch/powerpc/kernel/iommu.c
>@@ -863,6 +863,21 @@ void iommu_free_coherent(struct iommu_table *tbl, size_t size,
> 	}
> }
>
>+unsigned long iommu_direction_to_tce_perm(enum dma_data_direction dir)
>+{
>+	switch (dir) {
>+	case DMA_BIDIRECTIONAL:
>+		return TCE_PCI_READ | TCE_PCI_WRITE;
>+	case DMA_FROM_DEVICE:
>+		return TCE_PCI_WRITE;
>+	case DMA_TO_DEVICE:
>+		return TCE_PCI_READ;
>+	default:
>+		return 0;

It might be nice to have a WARN_ON() or log for the default case. If the TCE
entry is going to be updated without permission bits by ppc_md.tce_build().
The DMA operation covered by this TCE entry will cause EEH error. More
logs would be helpful to locate the root cause of the EEH error :-)

>+	}
>+}
>+EXPORT_SYMBOL_GPL(iommu_direction_to_tce_perm);
>+

The function converts generic permission flags to PCI specific flags as
the names (TCE_PCI_{READ,WRITE}) indicates. I'm not sure if it's reasonable
to have function name iommu_direction_to_pci_tce_perm(). Platform devices
who have DMA capability might have different flags other than TCE_PCI_{READ,WRITE}
and possibly use iommu.c to manage platform specific TCE table. We might
not have the use case for now, so I'm not sure it makes sense to have a
more specific function name.

Thanks,
Gavin

> #ifdef CONFIG_IOMMU_API
> /*
>  * SPAPR TCE API
>diff --git a/arch/powerpc/platforms/powernv/pci.c b/arch/powerpc/platforms/powernv/pci.c
>index bca2aeb..b7ea245 100644
>--- a/arch/powerpc/platforms/powernv/pci.c
>+++ b/arch/powerpc/platforms/powernv/pci.c
>@@ -576,15 +576,10 @@ static int pnv_tce_build(struct iommu_table *tbl, long index, long npages,
> 			 unsigned long uaddr, enum dma_data_direction direction,
> 			 struct dma_attrs *attrs, bool rm)
> {
>-	u64 proto_tce;
>+	u64 proto_tce = iommu_direction_to_tce_perm(direction);
> 	__be64 *tcep, *tces;
> 	u64 rpn;
>
>-	proto_tce = TCE_PCI_READ; // Read allowed
>-
>-	if (direction != DMA_TO_DEVICE)
>-		proto_tce |= TCE_PCI_WRITE;
>-
> 	tces = tcep = ((__be64 *)tbl->it_base) + index - tbl->it_offset;
> 	rpn = __pa(uaddr) >> tbl->it_page_shift;
>
>-- 
>2.4.0.rc3.8.gfb3e7d5
>

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