[PATCH 0/2] Drop SCSI core dev->dma_mask check

[PATCH 0/2] Drop SCSI core dev->dma_mask check

[John Garry]

In commit be8fcd4a8217 ("scsi: sas: Skip opt_sectors when DMA reports no
real optimization hint"), the check for dev->dma_mask prior to calling
dma_opt_mapping_size() was dropped.

However, it is not safe to do so, as dma_opt_mapping_size() ->
dma_max_mapping_size() may try to dereference dev->dma_mask.

We just don't hit such a path as no SCSI HBA driver which has
dev->dma_mask unset would use the SAS transport.

Fix that issue by adding a dev->dma_mask check in
dma_max_mapping_size().

The dev->dma_mask check in scsi_add_host_with_dma() can then be also
dropped.

John Garry (2):
  dma-mapping: make dma_max_mapping_size() return 0 for no DMA
    capability
  scsi: core: Drop dev->dma_mask check in evaluating max_sectors

 drivers/scsi/hosts.c | 6 ++----
 kernel/dma/mapping.c | 3 +++
 2 files changed, 5 insertions(+), 4 deletions(-)

-- 
2.43.7

[PATCH 1/2] dma-mapping: make dma_max_mapping_size() return 0 for no DMA capability

[John Garry]

For when a device is not DMA capable, the max mapping size would be 0, so
make dma_max_mapping_size() reflect that.

Signed-off-by: John Garry <john.g.garry@oracle.com>
---
 kernel/dma/mapping.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
index 4fe04669e5e66..7e576e5c6b8be 100644
--- a/kernel/dma/mapping.c
+++ b/kernel/dma/mapping.c
@@ -979,6 +979,9 @@ size_t dma_max_mapping_size(struct device *dev)
 	const struct dma_map_ops *ops = get_dma_ops(dev);
 	size_t size = SIZE_MAX;
 
+	if (!dev->dma_mask)
+		return 0;
+
 	if (dma_map_direct(dev, ops))
 		size = dma_direct_max_mapping_size(dev);
 	else if (use_dma_iommu(dev))
-- 
2.43.7

Re: [PATCH 1/2] dma-mapping: make dma_max_mapping_size() return 0 for no DMA capability

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] dma_max_mapping_size() returning 0 causes division by zero in amdxdna_cbuf_map() and invalid zero-sized block layer limits in MMC drivers.
--

commit b19cb5dc0c3fd9b25263f65c9d042ee616692807
Author: John Garry <john.g.garry@oracle.com>

dma-mapping: make dma_max_mapping_size() return 0 for no DMA capability

This commit updates dma_max_mapping_size() to return 0 when a device
is not DMA capable, reflecting that the maximum mapping size for such
devices is 0.

> diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
> index 4fe04669e5e66..7e576e5c6b8be 100644
> --- a/kernel/dma/mapping.c
> +++ b/kernel/dma/mapping.c
> @@ -979,6 +979,9 @@ size_t dma_max_mapping_size(struct device *dev)
>  	const struct dma_map_ops *ops = get_dma_ops(dev);
>  	size_t size = SIZE_MAX;
>  
> +	if (!dev->dma_mask)
> +		return 0;

[Severity: High]
Could this new return value of 0 cause a division by zero in callers that
expect a strictly positive maximum mapping size?

For instance, in drivers/accel/amdxdna/amdxdna_cbuf.c:amdxdna_cbuf_map(),
the returned size is used directly as a divisor:

        max_seg = min_t(size_t, UINT_MAX, dma_max_mapping_size(dev));
        n_entries = (cbuf->node.size + max_seg - 1) / max_seg;

If a device lacking a dma_mask maps the attachment, max_seg becomes 0,
which would result in an immediate kernel panic.

Additionally, could this cause invalid zero-sized block layer queue limits
in MMC drivers? In drivers/mmc/host/bcm2835.c:bcm2835_add_host(), the
return value clamps max_req_size:

        mmc->max_req_size = min_t(size_t, 524288, dma_max_mapping_size(dev));

If the device lacks DMA support, max_req_size becomes 0, which propagates
to the block layer and can trigger panics when processing requests.

> +
>  	if (dma_map_direct(dev, ops))
>  		size = dma_direct_max_mapping_size(dev);
>  	else if (use_dma_iommu(dev))

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260629085310.2298552-1-john.g.garry@oracle.com?part=1

Re: [PATCH 1/2] dma-mapping: make dma_max_mapping_size() return 0 for no DMA capability

[Robin Murphy]

On 29/06/2026 9:53 am, John Garry wrote:
> For when a device is not DMA capable, the max mapping size would be 0, so
> make dma_max_mapping_size() reflect that.

Seems logical.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>

> Signed-off-by: John Garry <john.g.garry@oracle.com>
> ---
>   kernel/dma/mapping.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
> index 4fe04669e5e66..7e576e5c6b8be 100644
> --- a/kernel/dma/mapping.c
> +++ b/kernel/dma/mapping.c
> @@ -979,6 +979,9 @@ size_t dma_max_mapping_size(struct device *dev)
>   	const struct dma_map_ops *ops = get_dma_ops(dev);
>   	size_t size = SIZE_MAX;
>   
> +	if (!dev->dma_mask)
> +		return 0;
> +
>   	if (dma_map_direct(dev, ops))
>   		size = dma_direct_max_mapping_size(dev);
>   	else if (use_dma_iommu(dev))

Re: [PATCH 1/2] dma-mapping: make dma_max_mapping_size() return 0 for no DMA capability

[John Garry]

On 29/06/2026 11:11, Robin Murphy wrote:
> On 29/06/2026 9:53 am, John Garry wrote:
>> For when a device is not DMA capable, the max mapping size would be 0, so
>> make dma_max_mapping_size() reflect that.
> 
> Seems logical.
> 
> Reviewed-by: Robin Murphy <robin.murphy@arm.com>

Thanks, but sashiko has been reviewing this and has a few things to say.

I don't know how it chooses recipients.

On 29/06/2026 10:10, sashiko-bot@kernel.org wrote:
 >>
 >> +	if (!dev->dma_mask)
 >> +		return 0;
 > [Severity: High]
 > Could this new return value of 0 cause a division by zero in callers that
 > expect a strictly positive maximum mapping size?
 >
 > For instance, in drivers/accel/amdxdna/amdxdna_cbuf.c:amdxdna_cbuf_map(),
 > the returned size is used directly as a divisor:
 >
 >          max_seg = min_t(size_t, UINT_MAX, dma_max_mapping_size(dev));
 >          n_entries = (cbuf->node.size + max_seg - 1) / max_seg;

Later in amdxdna_cbuf_map() we call dma_map_resource() (-> 
dma_map_phys()) and this would fail and WARN for !dev->dma_mask. Indeed 
amdxdna_cbuf_map() is used as a callback for .map_dma_buf, so highly 
unrealistic to have !dev->dma_mask ever.

 >
 > If a device lacking a dma_mask maps the attachment, max_seg becomes 0,
 > which would result in an immediate kernel panic.
 >
 > Additionally, could this cause invalid zero-sized block layer queue 
limits
 > in MMC drivers? In drivers/mmc/host/bcm2835.c:bcm2835_add_host(), the
 > return value clamps max_req_size:
 >
 >          mmc->max_req_size = min_t(size_t, 524288, 
dma_max_mapping_size(dev));

bcm2835.c is a platform device driver, and platform devices have their 
dev->dma_mask set in setup_pdev_dma_masks()

Indeed, that driver does have a non-DMA mode of operation, but that 
looks to be selected independent of whether dev->dma_mask is set.

 >
 > If the device lacks DMA support, max_req_size becomes 0, which propagates
 > to the block layer and can trigger panics when processing requests.

All other users of dma_max_mapping_size() are drivers for real/virtio 
HW, so should be no issues.

> 
>> Signed-off-by: John Garry <john.g.garry@oracle.com>
>> ---
>>   kernel/dma/mapping.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
>> index 4fe04669e5e66..7e576e5c6b8be 100644
>> --- a/kernel/dma/mapping.c
>> +++ b/kernel/dma/mapping.c
>> @@ -979,6 +979,9 @@ size_t dma_max_mapping_size(struct device *dev)
>>       const struct dma_map_ops *ops = get_dma_ops(dev);
>>       size_t size = SIZE_MAX;
>> +    if (!dev->dma_mask)
>> +        return 0;
>> +
>>       if (dma_map_direct(dev, ops))
>>           size = dma_direct_max_mapping_size(dev);
>>       else if (use_dma_iommu(dev))
> 

Re: [PATCH 1/2] dma-mapping: make dma_max_mapping_size() return 0 for no DMA capability

[Robin Murphy]

On 29/06/2026 11:46 am, John Garry wrote:
> On 29/06/2026 11:11, Robin Murphy wrote:
>> On 29/06/2026 9:53 am, John Garry wrote:
>>> For when a device is not DMA capable, the max mapping size would be 
>>> 0, so
>>> make dma_max_mapping_size() reflect that.
>>
>> Seems logical.
>>
>> Reviewed-by: Robin Murphy <robin.murphy@arm.com>
> 
> Thanks, but sashiko has been reviewing this and has a few things to say.
> 
> I don't know how it chooses recipients.
> 
> On 29/06/2026 10:10, sashiko-bot@kernel.org wrote:
>  >>
>  >> +    if (!dev->dma_mask)
>  >> +        return 0;
>  > [Severity: High]
>  > Could this new return value of 0 cause a division by zero in callers 
> that
>  > expect a strictly positive maximum mapping size?
>  >
>  > For instance, in drivers/accel/amdxdna/ 
> amdxdna_cbuf.c:amdxdna_cbuf_map(),
>  > the returned size is used directly as a divisor:
>  >
>  >          max_seg = min_t(size_t, UINT_MAX, dma_max_mapping_size(dev));
>  >          n_entries = (cbuf->node.size + max_seg - 1) / max_seg;
> 
> Later in amdxdna_cbuf_map() we call dma_map_resource() (-> 
> dma_map_phys()) and this would fail and WARN for !dev->dma_mask. Indeed 
> amdxdna_cbuf_map() is used as a callback for .map_dma_buf, so highly 
> unrealistic to have !dev->dma_mask ever.

Moreover, the driver would have failed to probe in the first place if 
its dma_set_mask_and_coherent() call had failed.

>  >
>  > If a device lacking a dma_mask maps the attachment, max_seg becomes 0,
>  > which would result in an immediate kernel panic.
>  >
>  > Additionally, could this cause invalid zero-sized block layer queue 
> limits
>  > in MMC drivers? In drivers/mmc/host/bcm2835.c:bcm2835_add_host(), the
>  > return value clamps max_req_size:
>  >
>  >          mmc->max_req_size = min_t(size_t, 524288, 
> dma_max_mapping_size(dev));
> 
> bcm2835.c is a platform device driver, and platform devices have their 
> dev->dma_mask set in setup_pdev_dma_masks()
> 
> Indeed, that driver does have a non-DMA mode of operation, but that 
> looks to be selected independent of whether dev->dma_mask is set.

That one seems pretty bogus already, given that DMA mode is apparently 
dependent on an external DMA channel, so "dev" is the wrong device to 
check (should be dmaengine_get_dma_device()), while conversely 
dma_max_mapping_size(anything) is a questionably meaningless number for 
PIO mode... :/

Cheers,
Robin.

> 
>  >
>  > If the device lacks DMA support, max_req_size becomes 0, which 
> propagates
>  > to the block layer and can trigger panics when processing requests.
> 
> All other users of dma_max_mapping_size() are drivers for real/virtio 
> HW, so should be no issues.
> 
>>
>>> Signed-off-by: John Garry <john.g.garry@oracle.com>
>>> ---
>>>   kernel/dma/mapping.c | 3 +++
>>>   1 file changed, 3 insertions(+)
>>>
>>> diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
>>> index 4fe04669e5e66..7e576e5c6b8be 100644
>>> --- a/kernel/dma/mapping.c
>>> +++ b/kernel/dma/mapping.c
>>> @@ -979,6 +979,9 @@ size_t dma_max_mapping_size(struct device *dev)
>>>       const struct dma_map_ops *ops = get_dma_ops(dev);
>>>       size_t size = SIZE_MAX;
>>> +    if (!dev->dma_mask)
>>> +        return 0;
>>> +
>>>       if (dma_map_direct(dev, ops))
>>>           size = dma_direct_max_mapping_size(dev);
>>>       else if (use_dma_iommu(dev))
>>
> 

[PATCH 2/2] scsi: core: Drop dev->dma_mask check in evaluating max_sectors

[John Garry]

When evaluating shost->max_sectors, we currently check dma_dev->dma_mask
is non-NULL, as dma_max_mapping_size(dma_dev) could previously not handle
unset dma_dev->dma_mask - this is no longer the case.

Signed-off-by: John Garry <john.g.garry@oracle.com>
---
 drivers/scsi/hosts.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
index e047747d4ecf8..d512080268afe 100644
--- a/drivers/scsi/hosts.c
+++ b/drivers/scsi/hosts.c
@@ -252,10 +252,8 @@ int scsi_add_host_with_dma(struct Scsi_Host *shost, struct device *dev,
 
 	shost->dma_dev = dma_dev;
 
-	if (dma_dev->dma_mask) {
-		shost->max_sectors = min_t(unsigned int, shost->max_sectors,
-				dma_max_mapping_size(dma_dev) >> SECTOR_SHIFT);
-	}
+	shost->max_sectors = min_not_zero(shost->max_sectors,
+			dma_max_mapping_size(dma_dev) >> SECTOR_SHIFT);
 
 	error = scsi_mq_setup_tags(shost);
 	if (error)
-- 
2.43.7