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