Re: [PATCH 19/28] dma-mapping: replace DMA_ATTR_NON_CONSISTENT with dma_{alloc, free}_pages
From: Tomasz Figa <tfiga@chromium.org>
Date: 2020-08-19 15:04:34
Also in:
alsa-devel, linux-doc, linux-iommu, linux-media, linux-mips, linux-mm, linux-nvme, linux-samsung-soc, linux-scsi, lkml, netdev, nouveau
Hi Christoph, On Wed, Aug 19, 2020 at 8:57 AM Christoph Hellwig [off-list ref] wrote:
Add a new API to allocate and free pages that are guaranteed to be
addressable by a device, but otherwise behave like pages allocated by
alloc_pages. The intended APIs to sync them for use with the device
and cpu are dma_sync_single_for_{device,cpu} that are also used for
streaming mappings.
Switch all drivers over to this new API, but keep the usage of the
crufty dma_cache_sync API for now, which will be cleaned up on a driver
by driver basis.
Signed-off-by: Christoph Hellwig <hch@lst.de>
---
Documentation/core-api/dma-api.rst | 68 +++++++++++------------
Documentation/core-api/dma-attributes.rst | 8 ---
arch/alpha/kernel/pci_iommu.c | 2 +
arch/arm/mm/dma-mapping-nommu.c | 2 +
arch/arm/mm/dma-mapping.c | 4 ++
arch/ia64/hp/common/sba_iommu.c | 2 +
arch/mips/jazz/jazzdma.c | 7 +--
arch/powerpc/kernel/dma-iommu.c | 2 +
arch/powerpc/platforms/ps3/system-bus.c | 4 ++
arch/powerpc/platforms/pseries/vio.c | 2 +
arch/s390/pci/pci_dma.c | 2 +
arch/x86/kernel/amd_gart_64.c | 2 +
drivers/iommu/dma-iommu.c | 2 +
drivers/iommu/intel/iommu.c | 4 ++
drivers/net/ethernet/i825xx/lasi_82596.c | 13 ++---
drivers/net/ethernet/seeq/sgiseeq.c | 12 ++--
drivers/parisc/ccio-dma.c | 2 +
drivers/parisc/sba_iommu.c | 2 +
drivers/scsi/53c700.c | 8 +--
drivers/scsi/sgiwd93.c | 12 ++--
drivers/xen/swiotlb-xen.c | 2 +
include/linux/dma-direct.h | 5 ++
include/linux/dma-mapping.h | 29 ++++++++--
include/linux/dma-noncoherent.h | 3 -
kernel/dma/direct.c | 51 ++++++++++++++++-
kernel/dma/mapping.c | 43 +++++++++++++-
kernel/dma/ops_helpers.c | 35 ++++++++++++
kernel/dma/virt.c | 2 +
sound/mips/hal2.c | 20 +++----
29 files changed, 254 insertions(+), 96 deletions(-)Thanks for the patch. The general design looks quite nice, but please see my comments inline.
quoted hunk ↗ jump to hunk
diff --git a/Documentation/core-api/dma-api.rst b/Documentation/core-api/dma-api.rst index 90239348b30f6f..047fcfffa0e5cf 100644 --- a/Documentation/core-api/dma-api.rst +++ b/Documentation/core-api/dma-api.rst@@ -516,48 +516,53 @@ routines, e.g.::: } -Part II - Advanced dma usage ----------------------------- +Part II - Non-coherent DMA allocations +-------------------------------------- -Warning: These pieces of the DMA API should not be used in the -majority of cases, since they cater for unlikely corner cases that -don't belong in usual drivers. +These APIs allow to allocate pages that can be used like normal pages +in the kernel direct mapping, but are guaranteed to be DMA addressable.
Could we elaborate a bit more on what "like normal pages in kernel direct mapping" mean from the driver perspective?
If you don't understand how cache line coherency works between a
processor and an I/O device, you should not be using this part of the
-API at all.
+API.
::
void *
- dma_alloc_attrs(struct device *dev, size_t size, dma_addr_t *dma_handle,
- gfp_t flag, unsigned long attrs)
+ dma_alloc_pages(struct device *dev, size_t size, dma_addr_t *dma_handle,
+ enum dma_data_direction dir, gfp_t gfp)
+
+This routine allocates a region of <size> bytes of consistent memory. It
+returns a pointer to the allocated region (in the processor's virtual address
+space) or NULL if the allocation failed. The returned memory is guanteed to
+behave like memory allocated using alloc_pages.There is one aspect that the existing dma_alloc_attrs() handles, but this new function doesn't: IOMMU support. The function will always allocate a physically-contiguous block memory, which is a costly operation and not even guaranteed to succeed, even if enough free memory is available. Modern SoCs employ IOMMUs to avoid the need to allocate physically-contiguous memory and those happen to be also the devices that could benefit from non-coherent allocations a lot. One of the tasks of the DMA API was making it possible to allocate suitable memory for a given device, without having the driver know about the SoC integration details, such as the presence of an IOMMU. Today, dma_alloc_attrs() uses the .alloc callback of the dma_ops struct and the IOMMU-aware implementations, like the dma-iommu helpers [1], would allocate discontiguous pages. Therefore, while I see the DMA-aware page allocation functionality as a useful functionality on its own for scatter-gather-capable hardware, I believe it is not a complete replacement for dma_alloc_attrs() with the DMA_ATTR_NON_CONSISTENT attribute. [1] https://elixir.bootlin.com/linux/v5.9-rc1/source/drivers/iommu/dma-iommu.c#L510 Best regards, Tomasz _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel