On Mon, May 26, 2014 at 7:42 PM, Alexandre Courbot <gnurou@xxxxxxxxx> wrote: > On Tue, May 27, 2014 at 10:07 AM, Stéphane Marchesin > <stephane.marchesin@xxxxxxxxx> wrote: >> On Mon, May 26, 2014 at 5:02 PM, Alexandre Courbot <gnurou@xxxxxxxxx> wrote: >>> On Mon, May 26, 2014 at 6:21 PM, Lucas Stach <l.stach@xxxxxxxxxxxxxx> wrote: >>>> Am Montag, den 26.05.2014, 09:45 +0300 schrieb Terje Bergström: >>>>> On 23.05.2014 17:40, Alex Courbot wrote: >>>>> > On 05/23/2014 06:59 PM, Lucas Stach wrote: >>>>> > So after checking with more knowledgeable people, it turns out this is >>>>> > the expected behavior on ARM and BAR regions should be mapped uncached >>>>> > on GK20A. All the more reasons to avoid using the BAR at all. >>>>> >>>>> This is actually specific to Tegra. >>>>> >>>>> >> You may want to make yourself aware of all the quirks required for >>>>> >> sharing memory between the GPU and CPU on an ARM host. I think there are >>>>> >> far more involved than what you see now and writing an replacement for >>>>> >> TTM will not be an easy task. >>>>> >> >>>>> >> Doing away with the concept of two memory areas will not get you to a >>>>> >> single unified address space. You would have to deal with things like >>>>> >> not being able to change the caching state of pages in the systems >>>>> >> lowmem yourself. You will still have to deal with remapping pages that >>>>> >> aren't currently visible to the CPU (ok this is not an issue on Jetson >>>>> >> right now as it only has 2GB of RAM), because it's in systems highmem, >>>>> >> or even in a different LPAE area. >>>>> >> >>>>> >> You really want to be sure you are aware of all the consequences of >>>>> >> this, before considering this task. >>>>> > >>>>> > Yep, that's why I am seeking advice here. My first hope is that with a >>>>> > few tweaks we will be able to keep using TTM and the current nouveau_bo >>>>> > implementation. But unless I missed something this is not going to be easy. >>>>> > >>>>> > We can also use something like the patch I originally sent to make it >>>>> > work, although not with good performance, on GK20A. Not very graceful, >>>>> > but it will allow applications to run. >>>>> > >>>>> > In the long run though, we will want to achieve better performance, and >>>>> > it seems like a BO implementation targeted at UMA devices would also be >>>>> > beneficial to quite a few desktop GPUs. So as tricky as it may be I'm >>>>> > interested in gathering thoughts and why not giving it a first try with >>>>> > GK20A, even if it imposes some limitations like having buffers in lowmem >>>>> > in a first time (we can probably live with this one for a short while, >>>>> > and 64 bits will also be coming to the rescue :)) >>>>> >>>>> I don't think lowmem or LPAE is any problem, if the memory manager is >>>>> designed with that in mind. Vast majority of the buffers kernel >>>>> allocates do not need to be touched in kernel space. >>>>> >>>>> Actually I can't think of any buffers that we allocate on behalf of user >>>>> space that would need to be permanently mapped also to kernel. In case >>>>> or relocs only push buffer needs to be temporarily mapped to kernel. >>>>> >>>>> Ultimately even relocs are not necessary if we expose GPU virtual >>>>> addresses directly to user space. But that's another topic. >>>>> >>>> Nouveau already exposes constant virtual addresses to userspace and >>>> skips the pushbuf patching when the presumed offset from userspace is >>>> the same as what the kernel thinks it should be. >>>> >>>> The problem with lowmem on ARM is that you can't unmap those pages from >>>> the kernel cached mapping. So if you alloc a page, give it to userspace >>>> and userspace decides to map the page WC you just produced a conflicting >>>> mapping, which may yield undefined results on ARMv7. You may think this >>>> is not a problem as you are not touching the kernel cached mapping, but >>>> in fact it is. The CPUs prefetcher can still access this mapping. >>> >>> Why would this memory be mapped into the kernel? >> >> On ARM the kernel keeps a linear mapping of lowmem using sections >> (ARM's version of huge pages). This is always cached, and because the >> sections are not 4k, it's a pain to remove parts of it. See >> arch/arm/mm/mmu.c > > Ah, are we talking about the directly-mapped low memory region > starting at PAGE_OFFSET? Ok, it makes sense now, thanks. > > But it seems to me that such different mappings can also happen in > many other scenarios as well, don't they? How is the issue handled in > these cases? It depends. A lot of cache controllers actually implement a solution for that in hardware, in the cache controller. For example I think Tegra2 is one of those platforms. And then a lot of platforms just ignore the issue completely because it has very low probability. Stéphane -- To unsubscribe from this list: send the line "unsubscribe linux-tegra" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
