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

[Petr Tesarik]

On Thu, 26 Jun 2025 09:09:34 +0200
Marek Szyprowski <m.szyprowski@xxxxxxxxxxx> wrote:

> On 26.06.2025 07:06, Petr Tesarik wrote:
> > On Thu, 26 Jun 2025 08:49:17 +0700
> > Bagas Sanjaya <bagasdotme@xxxxxxxxx> 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.

ISA is probably dead, but yeah, there may still be some systems with
LPC, which inherits the same addressing limitations.

I haven't really tested, but I believe these systems should be able to
enable SWIOTLB. Is there a specific reason they can't use SWIOTLB?

But if there is doubt, I can probably test such configuration.

Petr T