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Recent CPUs from Imagination Technologies such as the I6400 or P6600 are able to speculatively fetch data from memory into caches. This means that if used in a system with non-coherent DMA they require that caches be invalidated after a device performs DMA, and before the CPU reads the DMA'd data, in order to ensure that stale values weren't speculatively prefetched. Such CPUs also introduced Memory Accessibility Attribute Registers (MAARs) in order to control the regions in which they are allowed to speculate. Thus we can use the presence of MAARs as a good indication that the CPU requires the above cache maintenance. Use the presence of MAARs to determine the result of cpu_needs_post_dma_flush() in the default case, in order to handle these recent CPUs correctly. Note that the return type of cpu_needs_post_dma_flush() is changed to bool, such that it's clearer what's happening when cpu_has_maar is cast to bool for the return value. If this patch were backported to a pre-v4.7 kernel then MIPS_CPU_MAAR was 1ull<<34, so when cast to an int we would incorrectly return 0. It so happens that MIPS_CPU_MAAR is currently 1ull<<30, so when truncated to an int gives a non-zero value anyway, but even so the implicit conversion from long long int to bool makes it clearer to understand what will happen than the implicit conversion from long long int to int would. The bool return type also fits this usage better semantically, so seems like an all-round win. Thanks to Ed for spotting the issue for pre-v4.7 kernels & suggesting the return type change. Signed-off-by: Paul Burton <paul.burton@imgtec.com> Reviewed-by: Bryan O'Donoghue <pure.logic@nexus-software.ie> Tested-by: Bryan O'Donoghue <pure.logic@nexus-software.ie> Cc: Ed Blake <ed.blake@imgtec.com> Cc: linux-mips@linux-mips.org Patchwork: https://patchwork.linux-mips.org/patch/16363/ Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
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.