Re: [RFC 1/3 v3] mm: iommu: An API to unify IOMMU, CPU and device memory management
* This is difficult to achieve without remapping kernel memory using L2 page tables, so we can unmap pages on 4K page granularity. That's going to increase TLB overhead and result in lower system performance as there'll be a greater number of MMU misses. * Given how the buffers in question can be on the orders of tens of MB (and I don't think they will ever be less than 1MB), would we be able to get the desired effect by unmapping and then remapping on a 1MB granularity (ie, L1 sections)? It seems to me like this should be sufficient, and would not require using L2 mappings. Thoughts? Thanks Steve Sent by an employee of the Qualcomm Innovation Center, Inc. The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum. -- To unsubscribe from this list: send the line unsubscribe linux-omap in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [RFC 1/3 v3] mm: iommu: An API to unify IOMMU, CPU and device memory management
Russell- If a driver wants to allow a device to access memory (and cache coherency is off/not present for device addesses), the driver needs to remap that memory as non-cacheable. Suppose there exists a chunk of physically-contiguous memory (say, memory reserved for device use) that happened to be already mapped into the kernel as normal memory (cacheable, etc). One way to remap this memory is to use ioremap (and then never touch the original virtual mapping, which would now have conflicting attributes). I feel as if there should be a better way to remap memory for device access, either by altering the attributes on the original mapping, or removing the original mapping and creating a new one with attributes set to non-cacheable. Is there a better way to do this than calling ioremap() on that memory? Please advise. Thanks Steve Sent by an employee of the Qualcomm Innovation Center, Inc. The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum. On Mon, Jul 19, 2010 at 10:55:15AM -0700, Michael Bohan wrote: On 7/16/2010 12:58 AM, Russell King - ARM Linux wrote: As the patch has been out for RFC since early April on the linux-arm-kernel mailing list (Subject: [RFC] Prohibit ioremap() on kernel managed RAM), and no comments have come back from Qualcomm folk. Would it be unreasonable to allow a map request to succeed if the requested attributes matched that of the preexisting mapping? What would be the point of creating such a mapping? -- To unsubscribe from this list: send the line unsubscribe linux-arm-msm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line unsubscribe linux-omap in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [RFC 1/3 v3] mm: iommu: An API to unify IOMMU, CPU and device memory management
What is the problem about mapping a 1MB buffer with the DMA API? Possibly, an IOMMU can't find space for 1MB but it's not the problem of the DMA API. As you have pointed out, one of the issues is that allocation can fail. While technically VCMM allocations can fail as well, these allocations can be made from one or more memory pools that have been set aside specifically to be used by devices. Thus, the kernel's normal allocator will not encroach on the large physically-contiguous chunks (of size 1MB or even 16MB) that are not easy to get back, and would be forced to deal with increasing memory pressure in other ways. Additionally, some of the memory pools may have special properties, such as being part of on-chip memory with higher performance than regular memory, and some devices may have special requirements regarding what type or memory they need. The VCMM allocator solves the problem in a generic way by being able to deal with multiple memory pools and supporting prioritization schemes for which subset of the memory pools is to be used for each physical allocation. Sent by an employee of the Qualcomm Innovation Center, Inc. The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum. -- To unsubscribe from this list: send the line unsubscribe linux-omap in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
