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PMSAv7 defines curious alignment requirements to the regions: - size must be power of 2, and - region start must be aligned to the region size Because of that we currently adjust lowmem bounds plus we assign only one MPU region to cover memory all these lead to significant amount of memory could be wasted. As an example, consider 64Mb of memory at 0x70000000 - it fits alignment requirements nicely; now, imagine that 2Mb of memory is reserved for coherent DMA allocation, so now Linux is expected to see 62Mb of memory... and here annoying thing happens - memory gets truncated to 32Mb (we've lost 30Mb!), i.e. MPU layout looks like: 0: base 0x70000000, size 0x2000000 This patch tries to allocate as much as possible MPU slots to minimise amount of truncated memory. Moreover, with this patch MPU subregions starting to get used. MPU subregions allow us reduce the number of MPU slots used. For example given above, MPU layout looks like: 0: base 0x70000000, size 0x2000000 1: base 0x72000000, size 0x1000000 2: base 0x73000000, size 0x1000000, disable subreg 7 (0x73e00000 - 0x73ffffff) Where without subregions we'd get: 0: base 0x70000000, size 0x2000000 1: base 0x72000000, size 0x1000000 2: base 0x73000000, size 0x800000 3: base 0x73800000, size 0x400000 4: base 0x73c00000, size 0x200000 To achieve better layout we fist try to cover specified memory as is (maybe with help of subregions) and if we failed, we truncate memory to fit alignment requirements (so it occupies one MPU slot) and perform one more attempt with the reminder, and so on till we either cover all memory or run out of MPU slots. Tested-by: Szemző András <sza@esh.hu> Tested-by: Alexandre TORGUE <alexandre.torgue@st.com> Tested-by: Benjamin Gaignard <benjamin.gaignard@linaro.org> Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com> Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.