From: Petr Tesarik <petr.tesarik.ext@xxxxxxxxxx> The goal of my work is to provide more flexibility in the sizing of SWIOTLB. The software IO TLB was designed with these assumptions: 1. It would not be used much, especially on 64-bit systems. 2. A small fixed memory area (64 MiB by default) is sufficient to handle the few cases which require a bounce buffer. 3. 64 MiB is little enough that it has no impact on the rest of the system. First, if SEV is active, all DMA must be done through shared unencrypted pages, and SWIOTLB is used to make this happen without changing device drivers. The software IO TLB size is increased to 6% of total memory in sev_setup_arch(), but that is more of an approximation. The actual requirements may vary depending on the amount of I/O and which drivers are used. These factors may not be know at boot time, i.e. when SWIOTLB is allocated. Second, other colleagues have noticed that they can reliably get rid of occasional OOM kills on an Arm embedded device by reducing the SWIOTLB size. This can be achieved with a kernel parameter, but determining the right value puts additional burden on pre-release testing, which could be avoided if SWIOTLB is allocated small and grows only when necessary. Changes from RFC: - Track dynamic buffers per device instead of per swiotlb - Use a linked list instead of a maple tree - Move initialization of swiotlb fields of struct device to a helper function - Rename __lookup_dyn_slot() to lookup_dyn_slot_locked() - Introduce per-device flag if dynamic buffers are in use - Add one more user of DMA_ATTR_MAY_SLEEP - Add kernel-doc comments for new (and some old) code - Properly escape '*' in dma-attributes.rst Petr Tesarik (7): swiotlb: Use a helper to initialize swiotlb fields in struct device swiotlb: Move code around in preparation for dynamic bounce buffers dma-mapping: introduce the DMA_ATTR_MAY_SLEEP attribute swiotlb: Dynamically allocated bounce buffers swiotlb: Add a boot option to enable dynamic bounce buffers drm: Use DMA_ATTR_MAY_SLEEP from process context swiotlb: per-device flag if there are dynamically allocated buffers .../admin-guide/kernel-parameters.txt | 6 +- Documentation/core-api/dma-attributes.rst | 10 + drivers/base/core.c | 4 +- drivers/gpu/drm/drm_gem_shmem_helper.c | 2 +- drivers/gpu/drm/drm_prime.c | 2 +- include/linux/device.h | 12 + include/linux/dma-mapping.h | 6 + include/linux/swiotlb.h | 54 ++- kernel/dma/swiotlb.c | 382 ++++++++++++++++-- 9 files changed, 443 insertions(+), 35 deletions(-) -- 2.25.1
