mirror of
https://github.com/torvalds/linux.git
synced 2026-10-08 03:26:02 +02:00
Linux kernel source tree
78d901897b
Move from 2*NUM_QUEUES dma_alloc_coherent() for DMA descriptor rings to
2 calls overall.
Issue is with how all queues share the same register for configuring the
upper 32-bits of Tx/Rx descriptor rings. Taking Tx, notice how TBQPH
does *not* depend on the queue index:
#define GEM_TBQP(hw_q) (0x0440 + ((hw_q) << 2))
#define GEM_TBQPH(hw_q) (0x04C8)
queue_writel(queue, TBQP, lower_32_bits(queue->tx_ring_dma));
#ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT
if (bp->hw_dma_cap & HW_DMA_CAP_64B)
queue_writel(queue, TBQPH, upper_32_bits(queue->tx_ring_dma));
#endif
To maximise our chances of getting valid DMA addresses, we do a single
dma_alloc_coherent() across queues. This improves the odds because
alloc_pages() guarantees natural alignment. Other codepaths (IOMMU or
dev/arch dma_map_ops) don't give high enough guarantees
(even page-aligned isn't enough).
Two consideration:
- dma_alloc_coherent() gives us page alignment. Here we remove this
constraint meaning each queue's ring won't be page-aligned anymore.
- This can save some tiny amounts of memory. Fewer allocations means
(1) less overhead (constant cost per alloc) and (2) less wasted bytes
due to alignment constraints.
Example for (2): 4 queues, default ring size (512), 64-bit DMA
descriptors, 16K pages:
- Before: 8 allocs of 8K, each rounded to 16K => 64K wasted.
- After: 2 allocs of 32K => 0K wasted.
Fixes:
|
||
|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| rust | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .clippy.toml | ||
| .cocciconfig | ||
| .editorconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| .pylintrc | ||
| .rustfmt.toml | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.