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. Just my two cents, Petr T > > .. warning:: > > > > LGTM, thanks! > > Reviewed-by: Bagas Sanjaya <bagasdotme@xxxxxxxxx> Thank you for the review, Bagas.
