Hi Russell, On Fri, Aug 12, 2011 at 9:35 PM, Gupta, Ramesh <grgupta@xxxxxx> wrote: > On Thu, Aug 11, 2011 at 5:29 PM, Russell King - ARM Linux > <linux@xxxxxxxxxxxxxxxx> wrote: >> On Thu, Aug 11, 2011 at 02:28:39PM -0500, Gupta, Ramesh wrote: >>> Hi Russel, >> >> grr. > > Sorry, typo. > >> >>> On Thu, Apr 28, 2011 at 11:48 AM, Gupta, Ramesh <grgupta@xxxxxx> wrote: >>> > Hi Russel, >>> > >>> > On Thu, Apr 28, 2011 at 8:40 AM, Russell King - ARM Linux >>> > <linux@xxxxxxxxxxxxxxxx> wrote: >>> >> We _could_ invent a new API to deal with this, which is probably going >>> >> to be far better in the longer term for page table based iommus. That's >>> >> going to need some thought - eg, do we need to pass a struct device >>> >> argument for the iommu cache flushing so we know whether we need to flush >>> >> or not (eg, if we have cache coherent iommus)... >>> >>> my apologies for a late mail on this topic. >>> >>> do you think of any other requirements for this new API? >>> >>> Could we use the existing dmac_flush_range(), outer_flush_range() >>> for this purpose instead of a new API? >>> >>> I see a comment in the arch/arm/include/asm/cacheflush.h >>> for _not_ to use these APIs directly, but I am not really understand >>> the reason for that. >> >> When I create APIs, I create them to solve a _purpose_ to code which wants >> to do something. They're not created to provide some facility which can >> be re-used for unrelated stuff. >> >> This has been proven many times to be the correct approach. Over time, >> things change. >> > > I agree. > >> Let's say for arguments sake that you decide to use the DMA API stuff >> to achieve your goals. Then, lets say that ARM DMA becomes fully >> cache coherent, but your IOMMU tables still need to be flushed from >> the L1 caches. >> >> Suddenly, dmac_flush_range() starts doing absolutely nothing. Your >> driver breaks. I get whinged at because a previously working driver >> stops working. In effect, that usage _prevents_ me making the changes >> necessary to keep the core architecture support moving forward as >> things develop. Or, alternatively I just ignore your driver, make the >> changes anyway and leave it to rot. >> >> So, APIs get created to provide a purpose. Like - handling the DMA >> issues when mapping a buffer to DMA. Like handling the DMA issues >> when unmapping a buffer from DMA. If you start using those _because_ >> they happen to clean or invalidate the cache for you, you're really >> asking for your driver to be broken at some point in the future. >> >> What is far better to do is to ensure that we have the right APIs in >> place for the purposes for which they are to be used. So, if we need >> an API to flush out the IOMMU page table entries, then that's what >> we need, not some bodged lets-reuse-the-dma-flushing-functions thing. >> Inside the processors implementation, yes, it may well be the same >> thing, but that's a decision for the _processor_ support code to make, >> not the IOMMU writer. > > I completely agree, thank you for explaining the need to use the correct API. > > >> As to what shape a new API should be - as I said above, maybe it should >> take a struct device argument, virtual base address and size. Or maybe >> if we don't have any coherent IOMMUs then just ignore the device >> argument for the time being, and just pass the virtual base address and >> size. >> >> The next issue is whether it should require the virtual base address to >> be in the kernel direct mapped region. If you're touching L2, then >> that's a yes, because we need to use virt_to_phys on it to get at the >> phys address for the L2 operations. >> >> So, I think: extend the cpu cache operations structure to have a method >> for dealing with IOMMUs. Add an inline function to deal with calling >> that, and the L2 ops if that's what's required. my apologies for a late response. I have posted a patch for a new L1 cache maintenance api for handling iommu as you suggested. I will send iommu patches to use this api. Thank you and regards Ramesh Gupta G -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
