Hi Wen, On Thursday 10 February 2011 10:44:58 Wang, Wen W wrote: > Hi Laurent, > > You make this very clear, thank you! > > The ISP on Medfield do have its own IOMMU. And I also think an IOMMU layer > for ISP is needed. > > I'm not quite understand "unless hardware design doesn't give you a choice > about that,". Can you explain more on that? Basically, Linux aims for driver reusability. That means that separate pieces of hardware, even when they are designed to work together, should be handled by separate, indepedent drivers. This might make the initial development a bit longer, as proper abstraction APIs need to be designed when they don't exist, but it cuts down time to market later when a hardware block is reused in a different silicon or board. A good example of that approach is I2C sensors. They're supported by drivers that are completely independent of the ISP they are connected to, and use the V4L2 subdev in-kernel API to communicate with the ISP in a hardware-indepedent way. IOMMU is such an API. It lets you implement support for your particular IOMMU in a self-contained drivers, which can then be used by device drivers (such as the ISP driver). For this to work, the hardware needs to have at least some level of separation between the different components. With I2C that's easy, the sensor can be controlled completely indepedently from the ISP. With the ISP IOMMU, it would more or less depend on how the registers are layed out in memory. If the ISP IOMMU registers are grouped together and separated from the other ISP registers, you should be fine. If they are mixed with ISP registers (an hypotetical bad case would for instance be if the ISP IOMMU required you to set bits in the TLB entries that describe the format of the video data stored in a page), such an abstraction would be much more difficult to achieve, and sometimes even impossible. > Also regarding to the VCMM (Virtual Contiguous Memory Manager) or CMA, is > it also an option? I'm not sure about VCMM, it seems to be an attempt to unify memory management across IOMMUs and system MMU. I don't think you need to worry about it now, IOMMU should be enough for your needs. CMA, as its name implies, is a contiguous memory allocator. As your ISP has an IOMMU, you don't need to allocate contiguous memory, so CMA isn't useful for your hardware. -- Regards, Laurent Pinchart -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
