From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kiszka Subject: Re: [PATCH v7 2/5] of: change overlay apply input data from unflattened to FDT Date: Thu, 5 Apr 2018 21:38:26 +0200 Message-ID: <255dffbd-4ff1-42a4-033b-311d49339102@web.de> References: <1520122673-11003-1-git-send-email-frowand.list@gmail.com> <1520122673-11003-3-git-send-email-frowand.list@gmail.com> <09e3db63-cbf9-52a2-ee77-520979f17fea@web.de> <7bbf615b-3cdd-6bb4-6918-33e48de4225d@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Return-path: Sender: jailhouse-dev@googlegroups.com In-Reply-To: Content-Language: en-US List-Post: , List-Help: , List-Archive: , List-Unsubscribe: , To: Frank Rowand , Rob Herring , pantelis.antoniou@konsulko.com, Pantelis Antoniou Cc: devicetree@vger.kernel.org, linux-kernel@vger.kernel.org, geert@linux-m68k.org, laurent.pinchart+renesas@ideasonboard.com, Jailhouse List-Id: devicetree@vger.kernel.org On 2018-04-05 21:28, Frank Rowand wrote: > On 04/05/18 12:13, Jan Kiszka wrote: >> On 2018-04-05 20:59, Frank Rowand wrote: >>> Hi Jan, >>> >>> On 04/04/18 15:35, Jan Kiszka wrote: >>>> Hi Frank, >>>> >>>> On 2018-03-04 01:17, frowand.list@gmail.com wrote: >>>>> From: Frank Rowand >>>>> >>>>> Move duplicating and unflattening of an overlay flattened devicetree >>>>> (FDT) into the overlay application code. To accomplish this, >>>>> of_overlay_apply() is replaced by of_overlay_fdt_apply(). >>>>> >>>>> The copy of the FDT (aka "duplicate FDT") now belongs to devicetree >>>>> code, which is thus responsible for freeing the duplicate FDT. The >>>>> caller of of_overlay_fdt_apply() remains responsible for freeing the >>>>> original FDT. >>>>> >>>>> The unflattened devicetree now belongs to devicetree code, which is >>>>> thus responsible for freeing the unflattened devicetree. >>>>> >>>>> These ownership changes prevent early freeing of the duplicated FDT >>>>> or the unflattened devicetree, which could result in use after free >>>>> errors. >>>>> >>>>> of_overlay_fdt_apply() is a private function for the anticipated >>>>> overlay loader. >>>> >>>> We are using of_fdt_unflatten_tree + of_overlay_apply in the >>>> (out-of-tree) Jailhouse loader driver in order to register a virtual >>>> device during hypervisor activation with Linux. The DT overlay is >>>> created from a a template but modified prior to application to account >>>> for runtime-specific parameters. See [1] for the current implementation. >>>> >>>> I'm now wondering how to model that scenario best with the new API. >>>> Given that the loader lost ownership of the unflattened tree but the >>>> modification API exist only for the that DT state, I'm not yet seeing a >>>> clear solution. Should we apply the template in disabled form (status = >>>> "disabled"), modify it, and then activate it while it is already applied? >>> >>> Thank you for the pointer to the driver - that makes it much easier to >>> understand the use case and consider solutions. >>> >>> If you can make the changes directly on the FDT instead of on the >>> expanded devicetree, then you could move to the new API. >> >> Are there some examples/references on how to edit FDTs in-place in the >> kernel? I'd like to avoid writing the n-th FDT parser/generator. > > I don't know of any existing in-kernel edits of the FDT (but they might > exist). The functions to access an FDT are in libfdt, which is in > scripts/dtc/libfdt/. > Ah, libfdt is available for kernel drivers as well. That looks like a viable path on first sight. I'll try that and come back in case it does not solve all issues. > >>> >>> Looking at the driver, I see one potential issue with that approach. >>> The property "interrupt-map" is added directly to the changeset >>> instead of being an existing property in the overlay. Is it possible >>> to have this property in the overlay when needed? >> >> Well, the size of that property has a runtime dependency on the gic's >> #address-cells. If that is easy to account for depends a bit on the >> available FDT manipulation services. Or it would take multiple templates >> to handle the different cases (0, 1, or 2 IIRC). > > If I understand create_vpci_of_overlay() correctly, it is assuming a > fixed size of 4 cells. Line 314 is: for (n = 0, cell = 0; n < 4; n++) { > > Off the top of my head, it is theoretically possible to create a property > that can contain the largest possible size for the property, then shrink > the size by inserting NOPs at the end of the property to shrink it. Well, I even find fdt_appendprop which sounds like we could keep adding that property on the fly. How does memory management work with libfdt? Do I have to ensure that the fdt is already backed by an area large enough also for it modified form? Thanks, Jan