[PATCH 0/3] dmaengine: Remove obsolete 32-bit DMA mask fallbacks

[PATCH 0/3] dmaengine: Remove obsolete 32-bit DMA mask fallbacks

[Ruizhe Zhou]

A few DMAengine drivers set a >32 bit DMA mask and retry with a 32-bit
mask if the first call fails. This treats the return value of
dma_set_mask_and_coherent() as an indication that the platform requires
a narrower DMA width.

That is not a correct interpretation of dma_set_mask_and_coherent().
The mask passed to the DMA API describes the highest address a device can
reach. It asks the DMA layer to keep mappings within that limit; it is not
a probe asking the platform which addressing mode the device should use.
A wider mask includes every address permitted by a 32-bit mask, including
addresses from a platform that only produces 32-bit DMA addresses.

Since commit 91ef26f914171 ("dma-direct: relax addressability checks in
dma_direct_supported"), the DMA API guarantees support for masks
of 32 bits or wider. A failed wider request is therefore not evidence that
the driver should advertise a smaller capability. Retrying with a 32-bit
mask cannot repair the underlying DMA setup failure. The DMA API HOWTO
explicitly identifies this fallback pattern as incorrect [1]. See [2] and
[3] for details.

This series removes the fallbacks from the AMD PTDMA, PLX and
Spreadtrum DMA drivers without changing the mask selected for each device.
The return-value checks are retained. DMA setup errors are still reported
and still abort device initialization.

Testing was compile-only. No hardware testing was performed.

[1] DMA API HOWTO, "DMA addressing capabilities"
https://docs.kernel.org/core-api/dma-api-howto.html#dma-addressing-capabilities

[2] commit 91ef26f914171 ("dma-direct: relax addressability checks in
dma_direct_supported")
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=91ef26f914171cf753330f13724fd9142b5b1640

[3] commit f7ae20f2fc4e ("docs: dma: correct dma_set_mask() sample code")
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f7ae20f2fc4e

Best regards,
Ruizhe Zhou

Ruizhe Zhou (3):
  dmaengine: ptdma: Remove obsolete 32-bit DMA mask fallback
  dmaengine: plx_dma: Remove obsolete 32-bit DMA mask fallback
  dmaengine: sprd: Remove obsolete 32-bit DMA mask fallback

 drivers/dma/amd/ptdma/ptdma-pci.c | 8 ++------
 drivers/dma/plx_dma.c             | 2 --
 drivers/dma/sprd-dma.c            | 7 ++-----
 3 files changed, 4 insertions(+), 13 deletions(-)


base-commit: cee9395acd8043be0644b25c34bfa86623f2b935
--
2.27.0

[PATCH 1/3] dmaengine: ptdma: Remove obsolete 32-bit DMA mask fallback

[Ruizhe Zhou]

The DMA API guarantees support for masks of 32 bits or wider and
explicitly identifies retrying a 32-bit mask after a wider request as
incorrect:
https://docs.kernel.org/core-api/dma-api-howto.html#dma-addressing-capabilities

Remove the obsolete fallback while retaining the error check so that a
genuine DMA setup failure is still reported and aborts initialization.

Signed-off-by: Ruizhe Zhou <zhouruizhe@resnics.com>
---
 drivers/dma/amd/ptdma/ptdma-pci.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/drivers/dma/amd/ptdma/ptdma-pci.c b/drivers/dma/amd/ptdma/ptdma-pci.c
index 22739ff0c3c5..d36bb9c67325 100644
--- a/drivers/dma/amd/ptdma/ptdma-pci.c
+++ b/drivers/dma/amd/ptdma/ptdma-pci.c
@@ -178,12 +178,8 @@ static int pt_pci_probe(struct pci_dev *pdev, const struct pci_device_id *id)
 
 	ret = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(48));
 	if (ret) {
-		ret = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32));
-		if (ret) {
-			dev_err(dev, "dma_set_mask_and_coherent failed (%d)\n",
-				ret);
-			goto e_err;
-		}
+		dev_err(dev, "dma_set_mask_and_coherent failed (%d)\n", ret);
+		goto e_err;
 	}
 
 	dev_set_drvdata(dev, pt);
-- 
2.27.0

[PATCH 2/3] dmaengine: plx_dma: Remove obsolete 32-bit DMA mask fallback

[Ruizhe Zhou]

The DMA API guarantees support for masks of 32 bits or wider and
explicitly identifies retrying a 32-bit mask after a wider request as
incorrect:
https://docs.kernel.org/core-api/dma-api-howto.html#dma-addressing-capabilities

Remove the obsolete fallback while retaining the error check so that a
genuine DMA setup failure is still propagated and aborts initialization.

Signed-off-by: Ruizhe Zhou <zhouruizhe@resnics.com>
---
 drivers/dma/plx_dma.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/dma/plx_dma.c b/drivers/dma/plx_dma.c
index 84941a918b01..ad9187448ed2 100644
--- a/drivers/dma/plx_dma.c
+++ b/drivers/dma/plx_dma.c
@@ -562,8 +562,6 @@ static int plx_dma_probe(struct pci_dev *pdev,
 		return rc;
 
 	rc = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(48));
-	if (rc)
-		rc = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(32));
 	if (rc)
 		return rc;
 
-- 
2.27.0

[PATCH 3/3] dmaengine: sprd: Remove obsolete 32-bit DMA mask fallback

[Ruizhe Zhou]

The DMA API guarantees support for masks of 32 bits or wider and
explicitly identifies retrying a 32-bit mask after a wider request as
incorrect:
https://docs.kernel.org/core-api/dma-api-howto.html#dma-addressing-capabilities

Remove the obsolete fallback while retaining the error check so that a
genuine DMA setup failure is still reported and aborts initialization.

Signed-off-by: Ruizhe Zhou <zhouruizhe@resnics.com>
---
 drivers/dma/sprd-dma.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/drivers/dma/sprd-dma.c b/drivers/dma/sprd-dma.c
index 087fea3af2e4..b9f705dd80e5 100644
--- a/drivers/dma/sprd-dma.c
+++ b/drivers/dma/sprd-dma.c
@@ -1117,11 +1117,8 @@ static int sprd_dma_probe(struct platform_device *pdev)
 
 	ret = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(36));
 	if (ret) {
-		ret = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(32));
-		if (ret) {
-			dev_err(&pdev->dev, "unable to set coherent mask to 32\n");
-			return ret;
-		}
+		dev_err(&pdev->dev, "unable to set 36-bit DMA mask\n");
+		return ret;
 	}
 
 	/* Parse new and deprecated dma-channels properties */
-- 
2.27.0

Re: [PATCH 3/3] dmaengine: sprd: Remove obsolete 32-bit DMA mask fallback

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:

Pre-existing issues:
- [High] Premature IRQ request leading to potential NULL pointer dereference and uninitialized spinlock access.
- [Medium] PM runtime usage counter and enabled state leak on the `pm_runtime_get_sync()` error path.
--

commit 323a1a8d4f9b18f09fa8dd3e9872da5d0d05321b
Author: Ruizhe Zhou <zhouruizhe@resnics.com>

dmaengine: sprd: Remove obsolete 32-bit DMA mask fallback

This commit removes the obsolete 32-bit DMA mask fallback from
sprd_dma_probe(), relying solely on the 36-bit DMA mask request, while
retaining the error check to report genuine DMA setup failures.

> diff --git a/drivers/dma/sprd-dma.c b/drivers/dma/sprd-dma.c
> index 087fea3af2e41..b9f705dd80e52 100644
> --- a/drivers/dma/sprd-dma.c
> +++ b/drivers/dma/sprd-dma.c
> @@ -1117,11 +1117,8 @@ static int sprd_dma_probe(struct platform_device *pdev)
>  
>  	ret = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(36));
>  	if (ret) {
> -		ret = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(32));
> -		if (ret) {
> -			dev_err(&pdev->dev, "unable to set coherent mask to 32\n");
> -			return ret;
> -		}
> +		dev_err(&pdev->dev, "unable to set 36-bit DMA mask\n");
> +		return ret;
>  	}
>  
>  	/* Parse new and deprecated dma-channels properties */

[Severity: High]
This isn't a bug introduced by this patch, but looking further down in
sprd_dma_probe(), could the premature IRQ request lead to a NULL pointer
dereference or uninitialized spinlock access if an interrupt fires immediately?

If an interrupt is pending at the controller level, the handler dma_irq_handle()
will execute as soon as it is requested:

	ret = devm_request_irq(&pdev->dev, sdev->irq, dma_irq_handle,
			       0, "sprd_dma", (void *)sdev);

The interrupt handler will then dereference sdev->glb_base, which is mapped
later:

	sdev->glb_base = devm_platform_ioremap_resource(pdev, 0);

And it will attempt to acquire the spinlock, which is initialized even later:

	vchan_init(&dma_chn->vc, &sdev->dma_dev);

[Severity: Medium]
This is a pre-existing issue, but does the pm_runtime_get_sync() error path
leak the PM runtime usage counter and leave PM runtime enabled?

	ret = pm_runtime_get_sync(&pdev->dev);
	if (ret < 0)
		goto err_rpm;

If pm_runtime_get_sync() fails and returns a negative error code, it jumps
to the err_rpm label:

err_rpm:
	sprd_dma_disable(sdev);
	return ret;

Since pm_runtime_get_sync() increments the usage counter even on failure,
shouldn't this path call pm_runtime_put_noidle() to drop the usage counter,
and balance the earlier pm_runtime_enable() call with pm_runtime_disable()?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260903115441.912500-1-zhouruizhe@resnics.com?part=3

Re: [PATCH 2/3] dmaengine: plx_dma: Remove obsolete 32-bit DMA mask fallback

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Potential NULL pointer dereference in plx_dma_isr() because the interrupt handler is registered before plxdev->bar is initialized.
--

commit ebe0c3fecaf3a5fe091d793b49cde50217868ab8
Author: Ruizhe Zhou <zhouruizhe@resnics.com>

dmaengine: plx_dma: Remove obsolete 32-bit DMA mask fallback

The DMA API guarantees support for masks of 32 bits or wider. This commit
removes the obsolete 32-bit DMA mask fallback while retaining the error
check so that a genuine DMA setup failure is still propagated.

> diff --git a/drivers/dma/plx_dma.c b/drivers/dma/plx_dma.c
> index 84941a918b01f..ad9187448ed20 100644
> --- a/drivers/dma/plx_dma.c
> +++ b/drivers/dma/plx_dma.c
> @@ -562,8 +562,6 @@ static int plx_dma_probe(struct pci_dev *pdev,
>  		return rc;
>  
>  	rc = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(48));
> -	if (rc)
> -		rc = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(32));
>  	if (rc)
>  		return rc;
>  

[Severity: High]
This is a pre-existing issue, but is there a potential NULL pointer
dereference in plx_dma_isr() due to the order of initialization in
plx_dma_create()?

When plx_dma_probe() calls plx_dma_create(), the interrupt handler is
registered before plxdev->bar is initialized:

drivers/dma/plx_dma.c:plx_dma_create() {
    ...
    rc = request_irq(pci_irq_vector(pdev, 0), plx_dma_isr, 0,
                     KBUILD_MODNAME, plxdev);
    if (rc)
        goto free_plx;

    ...
    plxdev->bar = pcim_iomap_table(pdev)[0];
    ...
}

If an interrupt triggers immediately after request_irq() (e.g., from a
spurious interrupt or a pending interrupt left by a previous kernel/kexec),
the handler will execute:

drivers/dma/plx_dma.c:plx_dma_isr() {
    ...
    status = readw(plxdev->bar + PLX_REG_INTR_STATUS);
    ...
}

Since plxdev is zero-allocated and plxdev->bar is initialized only after
the IRQ is registered, could this cause a kernel panic due to a NULL
pointer dereference? Should the driver state be fully initialized before
unmasking the interrupt?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260903115441.912500-1-zhouruizhe@resnics.com?part=2

Re: [PATCH 1/3] dmaengine: ptdma: Remove obsolete 32-bit DMA mask fallback

[Frank Li]

On Thu, Sep 03, 2026 at 07:54:39PM +0800, Ruizhe Zhou wrote:
> [You don't often get email from zhouruizhe@resnics.com. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ]
>
> The DMA API guarantees support for masks of 32 bits or wider and
> explicitly identifies retrying a 32-bit mask after a wider request as
> incorrect:
> https://docs.kernel.org/core-api/dma-api-howto.html#dma-addressing-capabilities
>
> Remove the obsolete fallback while retaining the error check so that a
> genuine DMA setup failure is still reported and aborts initialization.
>
> Signed-off-by: Ruizhe Zhou <zhouruizhe@resnics.com>
> ---
>  drivers/dma/amd/ptdma/ptdma-pci.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/dma/amd/ptdma/ptdma-pci.c b/drivers/dma/amd/ptdma/ptdma-pci.c
> index 22739ff0c3c5..d36bb9c67325 100644
> --- a/drivers/dma/amd/ptdma/ptdma-pci.c
> +++ b/drivers/dma/amd/ptdma/ptdma-pci.c
> @@ -178,12 +178,8 @@ static int pt_pci_probe(struct pci_dev *pdev, const struct pci_device_id *id)
>
>         ret = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(48));
>         if (ret) {
> -               ret = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32));
> -               if (ret) {
> -                       dev_err(dev, "dma_set_mask_and_coherent failed (%d)\n",
> -                               ret);
> -                       goto e_err;
> -               }
> +               dev_err(dev, "dma_set_mask_and_coherent failed (%d)\n", ret);
> +               goto e_err;

also needn't check return value, it always return success if mask >= 32.

Frank
>         }
>
>         dev_set_drvdata(dev, pt);
> --
> 2.27.0
>

Re: [PATCH 2/3] dmaengine: plx_dma: Remove obsolete 32-bit DMA mask fallback

[Logan Gunthorpe]

On 2026-09-03 5:54 a.m., Ruizhe Zhou wrote:
> The DMA API guarantees support for masks of 32 bits or wider and
> explicitly identifies retrying a 32-bit mask after a wider request as
> incorrect:
> https://docs.kernel.org/core-api/dma-api-howto.html#dma-addressing-capabilities
> 
> Remove the obsolete fallback while retaining the error check so that a
> genuine DMA setup failure is still propagated and aborts initialization.
> 
> Signed-off-by: Ruizhe Zhou <zhouruizhe@resnics.com>

Makes sense to me, thanks.

Reviewed-by: Logan Gunthorpe <logang@deltatee.com>

Re:Re: [PATCH 1/3] dmaengine: ptdma: Remove obsolete 32-bit DMA mask fallback

[周睿哲]

Hi Frank,


From: Frank Li <Frank.li@oss.nxp.com>
Date: 2026-09-04 03:26:36
To:  Ruizhe Zhou <zhouruizhe@resnics.com>
Cc:  Vinod Koul <vkoul@kernel.org>,Basavaraj Natikar <Basavaraj.Natikar@amd.com>,Logan Gunthorpe <logang@deltatee.com>,Orson Zhai <orsonzhai@gmail.com>,Baolin Wang <baolin.wang@linux.alibaba.com>,Frank Li <Frank.Li@kernel.org>,Chunyan Zhang <zhang.lyra@gmail.com>,dmaengine@vger.kernel.org,linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3] dmaengine: ptdma: Remove obsolete 32-bit DMA mask fallback>On Thu, Sep 03, 2026 at 07:54:39PM +0800, Ruizhe Zhou wrote:
>> [You don't often get email from zhouruizhe@resnics.com. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification ]
>>
>> The DMA API guarantees support for masks of 32 bits or wider and
>> explicitly identifies retrying a 32-bit mask after a wider request as
>> incorrect:
>> https://docs.kernel.org/core-api/dma-api-howto.html#dma-addressing-capabilities
>>
>> Remove the obsolete fallback while retaining the error check so that a
>> genuine DMA setup failure is still reported and aborts initialization.
>>
>> Signed-off-by: Ruizhe Zhou <zhouruizhe@resnics.com>
>> ---
>>  drivers/dma/amd/ptdma/ptdma-pci.c | 8 ++------
>>  1 file changed, 2 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/dma/amd/ptdma/ptdma-pci.c b/drivers/dma/amd/ptdma/ptdma-pci.c
>> index 22739ff0c3c5..d36bb9c67325 100644
>> --- a/drivers/dma/amd/ptdma/ptdma-pci.c
>> +++ b/drivers/dma/amd/ptdma/ptdma-pci.c
>> @@ -178,12 +178,8 @@ static int pt_pci_probe(struct pci_dev *pdev, const struct pci_device_id *id)
>>
>>         ret = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(48));
>>         if (ret) {
>> -               ret = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32));
>> -               if (ret) {
>> -                       dev_err(dev, "dma_set_mask_and_coherent failed (%d)\n",
>> -                               ret);
>> -                       goto e_err;
>> -               }
>> +               dev_err(dev, "dma_set_mask_and_coherent failed (%d)\n", ret);
>> +               goto e_err;
>
>also needn't check return value, it always return success if mask >= 32.

I did some more checking after your reply to make sure I understand
whether dma_set_mask_and_coherent() can actually fail for a mask wider
than 32 bits, and whether keeping the return-value check is necessary.

I also read your August discussion with Michal Pecio on the same
question:

https://lore.kernel.org/all/aoMty9Dvt2bnWm74@SMW015318/

In that discussion you pointed out that dev->dma_mask should already be
initialized by the bus before driver probe, and asked whether there are
any dma_supported() implementations which can actually reject such a
mask.

I agree that !dev->dma_mask does not look like an interesting failure
case for a normally probed device. However, after going through the
current dma_supported() paths and the in-tree .dma_supported callbacks,
I found several other cases which appear to make the stronger "cannot
fail for >32 bits" statement incorrect.

The current path in kernel/dma/mapping.c is:

static int dma_supported(struct device *dev, u64 mask)
{
        const struct dma_map_ops *ops = get_dma_ops(dev);

        if (use_dma_iommu(dev)) {
                if (WARN_ON(ops))
                        return false;
                return true;
        }

        if (ops) {
                if (!ops->dma_supported)
                        return true;
                return ops->dma_supported(dev, mask);
        }

        return dma_direct_supported(dev, mask);
}

and dma_set_mask() does:

if (!dev->dma_mask || !dma_supported(dev, mask))
        return -EIO;

For the generic direct-DMA path, the situation is clear:

int dma_direct_supported(struct device *dev, u64 mask)
{
        ...

        if (mask >= DMA_BIT_MASK(32))
                return 1;

        ...
}

So dma_direct_supported() itself cannot reject a >=32-bit mask.

However, this does not appear to be true for every path through
dma_supported().

My audit of the relevant current callbacks looks roughly like this:

+----------------------------+---------------------+-----------------------+
| Backend / callback            | >32 can fail?           | 64 can fail?                |
+----------------------------+---------------------+-----------------------+
| dma_direct_supported()        | No                          | No                          |
| default dma-iommu              | Yes, conflicting       | Yes, conflicting       |
|                                                | dma_ops state       | dma_ops state        |
| ibmebus_dma_supported()    | Yes, !=64 fails         | No                          |
| xen_grant_dma_supported()  | Yes, !=64 fails         | No                          |
| xen_swiotlb_dma_supported | Yes, threshold         | Not due to width   |
| ppc dma_iommu_supported()  | Yes                       | Yes, if no table        |
| parisc sba_dma_supported()  | Yes                         | Yes, if no IOC           |
| parisc ccio_supported()         | No for >=32          | No for valid dev       |
| alpha_pci_supported()           | Yes, threshold /     | Can fail if no             |
|                                                | mapping dependent   | usable DMA path|
| dma_dummy_supported()      | Yes, always           | Yes, always                |
+----------------------------+---------------------+-----------------------+

There seem to be two separate issues here.

First, "any mask wider than 32 bits cannot fail" has direct
counterexamples.

For example, arch/powerpc/platforms/pseries/ibmebus.c contains:

static int ibmebus_dma_supported(struct device *dev, u64 mask)
{
        return mask == DMA_BIT_MASK(64);
}

and installs it in ibmebus_dma_ops.

Therefore a call such as:

dma_set_mask(dev, DMA_BIT_MASK(40))

will fail even though the mask is wider than 32 bits.

drivers/xen/grant-dma-ops.c has the same rule:

static int xen_grant_dma_supported(struct device *dev, u64 mask)
{
        return mask == DMA_BIT_MASK(64);
}

and xen_grant_dma_ops installs this as .dma_supported.

So, for that backend as well, DMA_BIT_MASK(40), for example, is
rejected.

These two cases seem to directly contradict the more general statement
that dma_set_mask_and_coherent() cannot fail for a mask wider than
32 bits.

There are also cases where even DMA_BIT_MASK(64) can fail.

One example is the PowerPC legacy IOMMU backend in
arch/powerpc/kernel/dma-iommu.c:

int dma_iommu_dma_supported(struct device *dev, u64 mask)
{
        struct iommu_table *tbl;

        ...

        tbl = get_iommu_table_base(dev);

        if (!tbl) {
                dev_err(dev,
                        "Warning: IOMMU dma not supported: "
                        "mask 0x%08llx, table unavailable\n",
                        mask);
                return 0;
        }

        if (tbl->it_offset >
            (mask >> tbl->it_page_shift)) {
                ...
                return 0;
        }

        return 1;
}

If get_iommu_table_base() returns NULL, the callback rejects the mask
regardless of whether it is 32, 40, or 64 bits.

I understand that an unavailable IOMMU table may represent an
unexpected or unusable DMA configuration rather than an ordinary
address-width limitation, but that seems exactly like a reason for the
driver to retain the error check and abort probe instead of proceeding
as if DMA setup succeeded.

There is a similar state-dependent failure in
drivers/parisc/sba_iommu.c:

ioc = GET_IOC(dev);
if (!ioc)
        return 0;

/*
 * The max IO Virt address will *always* < 30 bits.
 */
return mask >= ...;

For a valid IOC the required address range is below 32 bits, so a
64-bit mask works. But if GET_IOC(dev) fails, DMA_BIT_MASK(64) is still
rejected.

The default dma-iommu path also appears to contain an intentional
failure condition:

if (use_dma_iommu(dev)) {
        if (WARN_ON(ops))
                return false;
        return true;
}

The intended state is:

use_dma_iommu(dev) == true
get_dma_ops(dev)   == NULL

because the default IOMMU DMA implementation does not rely on a
dma_map_ops instance.

However, if use_dma_iommu(dev) is true and get_dma_ops(dev) is
non-NULL, dma_supported() deliberately returns false, independent of
mask width.

This looks particularly significant because this was explicitly
discussed when the default dma-iommu implementation was changed from
dma_ops indirect calls to direct calls.

Christoph suggested moving the consistency check out of the fast path
and doing it in dma_set_mask(), "And fail the call while we're at it."
Leon replied that he would add it to dma_supported():

https://lore.kernel.org/all/20240718070406.GK5630@unreal/

The same discussion also says that the default-IOMMU state implies
!ops.

So the resulting WARN_ON(ops) path appears to be an intentional reason
for dma_set_mask() to return an error when the DMA backend state is
inconsistent, including when mask == DMA_BIT_MASK(64).

Another concrete case is dma_dummy_ops.

kernel/dma/dummy.c has:

static int dma_dummy_supported(struct device *hwdev, u64 mask)
{
        return 0;
}

so dma_set_mask() fails for every mask, including DMA_BIT_MASK(64).

I initially wondered whether this was only a theoretical sentinel that
could never be seen by a driver probe, but there is a real setup path
for it. acpi_dma_configure_id() does:

if (attr == DEV_DMA_NOT_SUPPORTED) {
        set_dma_ops(dev, &dma_dummy_ops);
        return 0;
}

Since this returns 0 from DMA configuration after installing
dma_dummy_ops, the device can continue through the driver-core setup
with DMA deliberately marked unsupported. If such a DMA-using driver
then calls dma_set_mask_and_coherent(), the return-value check is what
prevents it from continuing with an unusable DMA backend.

I agree this is not representative of the normal case for a healthy
PCI device, but it does seem to demonstrate that a 64-bit mask is not an
unconditional success guarantee of the DMA API itself.

The formal DMA API documentation also seems to reflect this. In
Documentation/core-api/dma-api.rst, under "DMA addressing
limitations", it says:

All the below functions which set a DMA mask may fail if the
requested mask cannot be used with the device, or if the device is
not capable of doing DMA.

and dma_set_mask_and_coherent() is documented as returning zero on
success and a negative error on failure.

So I think there are two different statements here:

1. For a normally configured DMA-capable device, a 64-bit DMA mask
   should be supportable.

2. dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64)) cannot return
   an error, so checking its return value is unnecessary.

The first looks like the intended normal-case invariant, but I do not
see how the second follows given the current dma_supported() paths
above.

In particular, the PowerPC IOMMU table failure, the
use_dma_iommu(dev) && ops consistency check, and dma_dummy_ops are all
paths where the mask width itself is not the problem but
dma_set_mask_and_coherent(..., DMA_BIT_MASK(64)) can nevertheless
report failure.

That is why I currently think code like:

ret = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64));
if (ret)
        return ret;

still has value: the check is not probing whether 64-bit DMA
addressing is supported versus some narrower address width. It is
checking whether the DMA setup as a whole succeeded before the driver
starts using DMA.

Again, I do not work within the DMA subsystem, and therefore I won't exactly
claim myself to be an expert on this. The above are some findings after some
digging. So correct me if I am wrong. 

Am I missing an invariant which makes these failure paths unreachable
for a driver calling dma_set_mask_and_coherent()? If not, then I believe the return
value check is needed, and the DMA HOWTO document might need to mention that.

Thanks,
Ruizhe




>
>Frank
>>         }
>>
>>         dev_set_drvdata(dev, pt);
>> --
>> 2.27.0
>>
>