From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marek Szyprowski Subject: Re: [PATCH v7 3/4] drivers: of: add initialization code for dma reserved memory Date: Mon, 16 Sep 2013 09:12:36 +0200 Message-ID: <5236AF64.80607@samsung.com> References: <1377527959-5080-1-git-send-email-m.szyprowski@samsung.com> <1377527959-5080-4-git-send-email-m.szyprowski@samsung.com> <20130829224637.D09183E1222@localhost> <52209278.8040106@samsung.com> <20130909160154.8D7183E0F23@localhost> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-reply-to: <20130909160154.8D7183E0F23@localhost> Sender: devicetree-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Grant Likely Cc: Kumar Gala , linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org, linaro-mm-sig-cunTk1MwBs8s++Sfvej+rw@public.gmane.org, devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Kyungmin Park , Arnd Bergmann , Michal Nazarewicz , Tomasz Figa , Sylwester Nawrocki , Sascha Hauer , Laura Abbott , Rob Herring , Olof Johansson , Pawel Moll , Mark Rutland , Stephen Warren , Ian Campbell , Tomasz Figa , Nishanth Peethambaran , Marc , Benjamin Herrenschmidt List-Id: devicetree@vger.kernel.org Hello, On 9/9/2013 6:01 PM, Grant Likely wrote: > On Fri, 30 Aug 2013 15:26:24 -0500, Kumar Gala wrote: > > On Aug 30, 2013, at 7:39 AM, Marek Szyprowski wrote: > > > On 8/30/2013 12:46 AM, Grant Likely wrote: > > >> On Mon, 26 Aug 2013 16:39:18 +0200, Marek Szyprowski wrote: > > >> > + - "reserved-memory-region" - compatibility is defined, given > > >> > + region is assigned for exclusive usage for by the respective > > >> > + devices. > > >> > > >> BTW, just so you're aware there is already a binding for marking regions > > >> as reserved. It was recently added to arch/powerpc/kernel/prom.c. > > >> Unfortunately it doesn't look like it got documented. Search for > > >> "reserved-ranges". However, I don't think it blocks your work here. That > > >> binding doesn't provide any way for matching devices to reserved ranges. > > >> It is only for telling the kernel "hands of that memory". > > > > > > ok, good to know. > > > > So I think the most generic compatible should effectively be the same > > as 'reserved-ranges', ie this region shouldn't be touched by the > > kernel. > > > > Than we can build on top of that with more specific compatibles. > > > > I have examples from FSL networking SoCs where we would carve out > > memory for backing storage managed completely by the HW and Linux > > wouldn't need to touch it, however we might have a > > "fsl,qman-backing-store" compat encase we might want some debug code > > in linux to look at the region of memory. > > > > I've got examples at qualcomm where we have shared data structures in > > specific memory regions between different processors that aren't cache > > coherent, so want the memory not to be marked as cacheable by Linux > > when it accesses it. Again we'd have a "qcom,smem" prop on top of the > > "reserved-memory-region". > > I think that is a reasonable approach, and works really well for regions > associated with specific hardware because the hardware driver can be > responsible for wiring it up to CMA or manage the region directly. > Whatever the driver desires to do. > > It still remains what to do for generic regions that any device can use. > As I've previously said, I don't like calling out "CMA" specifically in > the compatible property because that happens to be a Linux > implementation specific details. Two years from now someone may propose > a new allocator that should take over what used to be handled by CMA > (kind of like how CMA may end up taking over from ION).... > > Alright, I've thought about it some more and it probably does make sense > to use an additional compatible string. Marek originally suggested > "linux,contiguous-memory-region". I later suggested "shareable-memory-region", > but that's actually a worse name because it doesn't have any reference > to what the region is for (I was fixating on the kernel being able to > use unallocated pages; but that is also an implementation detail). I > exercise my right to change my mind and agree that contiguous is > appropriate here. But instead of calling out the contiguous allocator, > the binding should specifically lay out the usage model for these > regions. I would change the contiguous-memory binding to state the > following: > Contiguous-memory is a region of general purpose memory from > which large buffers of contiguous memory can be allocated. > Contiguous buffers are often used for DMA buffers. The operating > system may use the memory for any purpose, but must immediately > release the pages if a contiguous buffer is required. > > So the way I read things, the current proposal would be: > > The generic form for do-not-touch memory: > compatible = "reserved-memory"; > - I've dropped '-region' because I think it is redundant > - The kernel will not make use of this memory except for when a driver picks it up > > For contiguous memory: > compatible = "contiguous-memory", "reserved-memory" > > For contiguous memory that Linux will use by default if a specific > region isn't specified. > compatible = "contiguous-memory", "reserved-memory" > linux,default-contiguous-region; > - I stuck with a linux, prefix here because I haven't come up with a > non-linux way to describe this. > > Memory that specific hardware can pick up: > compatible = "qcom,smem", "reserved-memory" > > Nodes with a 'size' property and without a 'reg' property need to have a > region allocated and reserved. The allocated region needs to be cached > somewhere. We could create a data structure for tracking the reserved > regions, or could write it in directly. While I shudder at the thought of > modifying the device tree at runtime by the kernel, it might just be the > sanest implementation at this time. I'd like to see what the > implementation looks like. (Since this is potentially controversial, I > would recommend implementing the exact regions in on patch, and add > dynamically allocated regions as a follow-up patch) > > As far as proper implementation is concerned, I think it should be > explicit in the binding that the kernel should automatically exclude > anything under the reserved-memory node, regardless of the compatible > value. Merely being a child of the reserved-memory node immediately > means that it is a reserved memory region. It looks that my proposal for the reserved memory nodes causes much more controversy than I expected when I've sent a pull request to Linus: https://lkml.org/lkml/2013/9/15/151 http://www.spinics.net/lists/arm-kernel/msg273548.html Fixing those issues requires further discussion. Frankly, right now I really have no idea which way should I go. The /reserved-ranges node seems to be easy to match particular reserved memory region with a given device. I'm also not really convinced if it makes sense to add a code for finding and matching a reserved memory region to every device driver, which might need it. I would really like to get some more feedback on the Ben's comment. In any case, the code will also change significantly, so I assume that the best, what can be done now is to revert the current version and start from the scratch with a new, complete proposal. Best regards -- Marek Szyprowski Samsung R&D Institute Poland -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html