[PATCH] virtio: Simplify DMA setting

[PATCH] virtio: Simplify DMA setting

[Christophe JAILLET]

As stated in [1], dma_set_mask() with a 64-bit mask will never fail if
dev->dma_mask is non-NULL.
So, if it fails, the 32 bits case will also fail for the same reason.

Simplify code and remove some dead code accordingly.


While at it, include directly <linux/dma-mapping.h> instead on relying on
indirect inclusion.

[1]: https://lkml.org/lkml/2021/6/7/398

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
---
 drivers/virtio/virtio_mmio.c           | 4 +---
 drivers/virtio/virtio_pci_legacy_dev.c | 7 +++----
 drivers/virtio/virtio_pci_modern_dev.c | 6 ++----
 3 files changed, 6 insertions(+), 11 deletions(-)

diff --git a/drivers/virtio/virtio_mmio.c b/drivers/virtio/virtio_mmio.c
index 56128b9c46eb..aa1efa854de1 100644
--- a/drivers/virtio/virtio_mmio.c
+++ b/drivers/virtio/virtio_mmio.c
@@ -617,9 +617,7 @@ static int virtio_mmio_probe(struct platform_device *pdev)
 		rc = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(64));
 	}
 	if (rc)
-		rc = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(32));
-	if (rc)
-		dev_warn(&pdev->dev, "Failed to enable 64-bit or 32-bit DMA.  Trying to continue, but this might not work.\n");
+		dev_warn(&pdev->dev, "Failed to enable DMA.  Trying to continue, but this might not work.\n");
 
 	platform_set_drvdata(pdev, vm_dev);
 
diff --git a/drivers/virtio/virtio_pci_legacy_dev.c b/drivers/virtio/virtio_pci_legacy_dev.c
index 677d1f68bc9b..52b1c4dd43fe 100644
--- a/drivers/virtio/virtio_pci_legacy_dev.c
+++ b/drivers/virtio/virtio_pci_legacy_dev.c
@@ -2,6 +2,7 @@
 
 #include "linux/virtio_pci.h"
 #include <linux/virtio_pci_legacy.h>
+#include <linux/dma-mapping.h>
 #include <linux/module.h>
 #include <linux/pci.h>
 
@@ -26,9 +27,7 @@ int vp_legacy_probe(struct virtio_pci_legacy_device *ldev)
 		return -ENODEV;
 
 	rc = dma_set_mask(&pci_dev->dev, DMA_BIT_MASK(64));
-	if (rc) {
-		rc = dma_set_mask_and_coherent(&pci_dev->dev, DMA_BIT_MASK(32));
-	} else {
+	if (!rc) {
 		/*
 		 * The virtio ring base address is expressed as a 32-bit PFN,
 		 * with a page size of 1 << VIRTIO_PCI_QUEUE_ADDR_SHIFT.
@@ -38,7 +37,7 @@ int vp_legacy_probe(struct virtio_pci_legacy_device *ldev)
 	}
 
 	if (rc)
-		dev_warn(&pci_dev->dev, "Failed to enable 64-bit or 32-bit DMA.  Trying to continue, but this might not work.\n");
+		dev_warn(&pci_dev->dev, "Failed to enable DMA.  Trying to continue, but this might not work.\n");
 
 	rc = pci_request_region(pci_dev, 0, "virtio-pci-legacy");
 	if (rc)
diff --git a/drivers/virtio/virtio_pci_modern_dev.c b/drivers/virtio/virtio_pci_modern_dev.c
index e8b3ff2b9fbc..830dc269d68f 100644
--- a/drivers/virtio/virtio_pci_modern_dev.c
+++ b/drivers/virtio/virtio_pci_modern_dev.c
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0-or-later
 
 #include <linux/virtio_pci_modern.h>
+#include <linux/dma-mapping.h>
 #include <linux/module.h>
 #include <linux/pci.h>
 
@@ -256,10 +257,7 @@ int vp_modern_probe(struct virtio_pci_modern_device *mdev)
 
 	err = dma_set_mask_and_coherent(&pci_dev->dev, DMA_BIT_MASK(64));
 	if (err)
-		err = dma_set_mask_and_coherent(&pci_dev->dev,
-						DMA_BIT_MASK(32));
-	if (err)
-		dev_warn(&pci_dev->dev, "Failed to enable 64-bit or 32-bit DMA.  Trying to continue, but this might not work.\n");
+		dev_warn(&pci_dev->dev, "Failed to enable DMA.  Trying to continue, but this might not work.\n");
 
 	/* Device capability is only mandatory for devices that have
 	 * device-specific configuration.
-- 
2.32.0

Re: [PATCH] virtio: Simplify DMA setting

[Jason Wang]

在 2022/1/8 下午3:08, Christophe JAILLET 写道:
> As stated in [1], dma_set_mask() with a 64-bit mask will never fail if
> dev->dma_mask is non-NULL.
> So, if it fails, the 32 bits case will also fail for the same reason.


I'd expect to be more verbose here. E.g I see dma_supported() who has a 
brunch of checks need to be called if dma_mask is non-NULL.

Thanks


>
> Simplify code and remove some dead code accordingly.
>
>
> While at it, include directly <linux/dma-mapping.h> instead on relying on
> indirect inclusion.
>
> [1]: https://lkml.org/lkml/2021/6/7/398
>
> Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
> ---
>   drivers/virtio/virtio_mmio.c           | 4 +---
>   drivers/virtio/virtio_pci_legacy_dev.c | 7 +++----
>   drivers/virtio/virtio_pci_modern_dev.c | 6 ++----
>   3 files changed, 6 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/virtio/virtio_mmio.c b/drivers/virtio/virtio_mmio.c
> index 56128b9c46eb..aa1efa854de1 100644
> --- a/drivers/virtio/virtio_mmio.c
> +++ b/drivers/virtio/virtio_mmio.c
> @@ -617,9 +617,7 @@ static int virtio_mmio_probe(struct platform_device *pdev)
>   		rc = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(64));
>   	}
>   	if (rc)
> -		rc = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(32));
> -	if (rc)
> -		dev_warn(&pdev->dev, "Failed to enable 64-bit or 32-bit DMA.  Trying to continue, but this might not work.\n");
> +		dev_warn(&pdev->dev, "Failed to enable DMA.  Trying to continue, but this might not work.\n");
>   
>   	platform_set_drvdata(pdev, vm_dev);
>   
> diff --git a/drivers/virtio/virtio_pci_legacy_dev.c b/drivers/virtio/virtio_pci_legacy_dev.c
> index 677d1f68bc9b..52b1c4dd43fe 100644
> --- a/drivers/virtio/virtio_pci_legacy_dev.c
> +++ b/drivers/virtio/virtio_pci_legacy_dev.c
> @@ -2,6 +2,7 @@
>   
>   #include "linux/virtio_pci.h"
>   #include <linux/virtio_pci_legacy.h>
> +#include <linux/dma-mapping.h>
>   #include <linux/module.h>
>   #include <linux/pci.h>
>   
> @@ -26,9 +27,7 @@ int vp_legacy_probe(struct virtio_pci_legacy_device *ldev)
>   		return -ENODEV;
>   
>   	rc = dma_set_mask(&pci_dev->dev, DMA_BIT_MASK(64));
> -	if (rc) {
> -		rc = dma_set_mask_and_coherent(&pci_dev->dev, DMA_BIT_MASK(32));
> -	} else {
> +	if (!rc) {
>   		/*
>   		 * The virtio ring base address is expressed as a 32-bit PFN,
>   		 * with a page size of 1 << VIRTIO_PCI_QUEUE_ADDR_SHIFT.
> @@ -38,7 +37,7 @@ int vp_legacy_probe(struct virtio_pci_legacy_device *ldev)
>   	}
>   
>   	if (rc)
> -		dev_warn(&pci_dev->dev, "Failed to enable 64-bit or 32-bit DMA.  Trying to continue, but this might not work.\n");
> +		dev_warn(&pci_dev->dev, "Failed to enable DMA.  Trying to continue, but this might not work.\n");
>   
>   	rc = pci_request_region(pci_dev, 0, "virtio-pci-legacy");
>   	if (rc)
> diff --git a/drivers/virtio/virtio_pci_modern_dev.c b/drivers/virtio/virtio_pci_modern_dev.c
> index e8b3ff2b9fbc..830dc269d68f 100644
> --- a/drivers/virtio/virtio_pci_modern_dev.c
> +++ b/drivers/virtio/virtio_pci_modern_dev.c
> @@ -1,6 +1,7 @@
>   // SPDX-License-Identifier: GPL-2.0-or-later
>   
>   #include <linux/virtio_pci_modern.h>
> +#include <linux/dma-mapping.h>
>   #include <linux/module.h>
>   #include <linux/pci.h>
>   
> @@ -256,10 +257,7 @@ int vp_modern_probe(struct virtio_pci_modern_device *mdev)
>   
>   	err = dma_set_mask_and_coherent(&pci_dev->dev, DMA_BIT_MASK(64));
>   	if (err)
> -		err = dma_set_mask_and_coherent(&pci_dev->dev,
> -						DMA_BIT_MASK(32));
> -	if (err)
> -		dev_warn(&pci_dev->dev, "Failed to enable 64-bit or 32-bit DMA.  Trying to continue, but this might not work.\n");
> +		dev_warn(&pci_dev->dev, "Failed to enable DMA.  Trying to continue, but this might not work.\n");
>   
>   	/* Device capability is only mandatory for devices that have
>   	 * device-specific configuration.

Re: [PATCH] virtio: Simplify DMA setting

[Christophe JAILLET]

Le 10/01/2022 à 07:14, Jason Wang a écrit :
> 
> 在 2022/1/8 下午3:08, Christophe JAILLET 写道:
>> As stated in [1], dma_set_mask() with a 64-bit mask will never fail if
>> dev->dma_mask is non-NULL.
>> So, if it fails, the 32 bits case will also fail for the same reason.
> 
> 
> I'd expect to be more verbose here. E.g I see dma_supported() who has a 
> brunch of checks need to be called if dma_mask is non-NULL.
> 
> Thanks
> 
> 

Hi,

If Christoph Hellwig's references ([1], [2]) are not enough, here is my 
own analysis.
I put him in copy in case he has be better wording or explanation than me.



I've searched all dma_supported() function with grep
     grep -r --include=*.[ch]  \\.dma_supported * > dma_supported.txt

I've removed duplicates, then, I've audited audit all functions.
Short summary below.

Adding all this in the commit looks an overkill to me.
Maybe we can add a link to this mail if it looks good to you.

CJ



arch/arm/common/dmabounce.c:			.dma_supported =	dmabounce_dma_supported
==> Same as arm_dma_supported()

arch/arm/mm/dma-mapping.c:			.dma_supported =	arm_dma_supported
==> If a mask fails, smaller masks will fail as well.

arch/alpha/kernel/pci_iommu.c:			.dma_supported =	alpha_pci_supported
==> If a mask fails, smaller masks will fail as well.

arch/x86/kernel/amd_gart_64.c:			.dma_supported =	dma_direct_supported
==> Succeeds if >= 32 bits. If a mask fails, smaller masks will fail as 
well.

arch/powerpc/kernel/dma-iommu.c:		.dma_supported =	dma_iommu_dma_supported
==> Tricky because of dma_iommu_bypass_supported(), but if a mask fails, 
smaller masks will fail as well.

arch/powerpc/platforms/ps3/system-bus.c:	.dma_supported =	ps3_dma_supported
==> Succeeds if >= 32 bits

arch/powerpc/platforms/pseries/ibmebus.c:	.dma_supported = 
ibmebus_dma_supported
==> Succeeds if == 64 bits

arch/sparc/kernel/iommu.c:			.dma_supported =	dma_4u_supported
==> One corner case which accept only 31 bits
==> If a mask fails, smaller masks will fail as well

arch/sparc/kernel/pci_sun4v.c:			.dma_supported =	dma_4v_supported
==> Same as dma_4u_supported() above

arch/ia64/hp/common/sba_iommu.c:		.dma_supported =	sba_dma_supported
==> Succeeds if >= 32 bits

drivers/xen/swiotlb-xen.c:			.dma_supported =	xen_swiotlb_dma_supported
==> If a mask fails, smaller masks will fail as well.

drivers/parisc/ccio-dma.c:			.dma_supported =	ccio_dma_supported
==> Succeeds if >= 32 bits

drivers/parisc/sba_iommu.c:			.dma_supported =	sba_dma_supported
==> If a mask fails, smaller masks will fail as well.

kernel/dma/dummy.c:				.dma_supported =	dma_dummy_supported
==> Always fails, whatever the value of the mask


[1]: https://lore.kernel.org/linux-kernel/YL3vSPK5DXTNvgdx@infradead.org/

[2]: https://lore.kernel.org/kernel-janitors/YdK4IIFvi5O5eXHC@infradead.org/