Re: [PATCH 7/8] docs: dma-api: update streaming DMA API physical address constraints

[Marek Szyprowski <m.szyprowski@samsung.com>]

On 26.06.2025 07:06, Petr Tesarik wrote:
> On Thu, 26 Jun 2025 08:49:17 +0700
> Bagas Sanjaya <bagasdotme@gmail.com> wrote:
>
>> On Tue, Jun 24, 2025 at 03:39:22PM +0200, Petr Tesarik wrote:
>>> diff --git a/Documentation/core-api/dma-api.rst b/Documentation/core-api/dma-api.rst
>>> index cd432996949c..65132ec88104 100644
>>> --- a/Documentation/core-api/dma-api.rst
>>> +++ b/Documentation/core-api/dma-api.rst
>>> @@ -210,18 +210,12 @@ DMA_BIDIRECTIONAL	direction isn't known
>>>   	this API should be obtained from sources which guarantee it to be
>>>   	physically contiguous (like kmalloc).
>>>   
>>> -	Further, the DMA address of the memory must be within the dma_mask of
>>> -	the device.  To ensure that the memory allocated by kmalloc is within
>>> -	the dma_mask, the driver may specify various platform-dependent flags
>>> -	to restrict the DMA address range of the allocation (e.g., on x86,
>>> -	GFP_DMA guarantees to be within the first 16MB of available DMA
>>> -	addresses, as required by ISA devices).
>>> -
>>> -	Note also that the above constraints on physical contiguity and
>>> -	dma_mask may not apply if the platform has an IOMMU (a device which
>>> -	maps an I/O DMA address to a physical memory address).  However, to be
>>> -	portable, device driver writers may *not* assume that such an IOMMU
>>> -	exists.
>>> +	Mapping may also fail if the memory is not within the DMA mask of the
>>> +	device.  However, this constraint does not apply if the platform has
>>> +	an IOMMU (a device which maps an I/O DMA address to a physical memory
>>> +	address), or the kernel is configured with SWIOTLB (bounce buffers).
>>> +	It is reasonable to assume that at least one of these mechanisms
>>> +	allows streaming DMA to any physical address.
> Now I realize this last sentence may be contentious...
>
> @Marek, @Robin Do you agree that device drivers should not be concerned
> about the physical address of a buffer passed to the streaming DMA API?
>
> I mean, are there any real-world systems with:
>    * some RAM that is not DMA-addressable,
>    * no IOMMU,
>    * CONFIG_SWIOTLB is not set?
>
> FWIW if _I_ received a bug report that a device driver fails to submit
> I/O on such a system, I would politely explain the reporter that their
> kernel is misconfigured, and they should enable CONFIG_SWIOTLB.

What about the systems with legacy 16/24bit ZONE_DMA (i.e. ISA bus)? 
AFAIR they don't use SWIOTLB and probably they won't be able to use 
streaming DMA API for all system RAM.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland