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Thu, 08 Aug 2019 08:02:42 -0700 (PDT) X-Gm-Message-State: APjAAAUmys3pAavT+xY9md5Y4hnqn48GIHylCk7HXNC9dMs5wpu5S3V0 wRD3OY/TolemKAasmclqZv6rDB3M1/Ga3F2Rpw== X-Google-Smtp-Source: APXvYqwoUwqMMoJlRpUQeZhstAAUALmk5OLfWeCSiKB8+lRLcDvufCgBEJ1TDb+hHOMFdyjeedwU8GdoRwm/uZYI8ik= X-Received: by 2002:ac8:7593:: with SMTP id s19mr6019854qtq.136.1565276561735; Thu, 08 Aug 2019 08:02:41 -0700 (PDT) MIME-Version: 1.0 References: <20190731154752.16557-1-nsaenzjulienne@suse.de> <20190731154752.16557-4-nsaenzjulienne@suse.de> <2050374ac07e0330e505c4a1637256428adb10c4.camel@suse.de> <12eb3aba207c552e5eb727535e7c4f08673c4c80.camel@suse.de> In-Reply-To: <12eb3aba207c552e5eb727535e7c4f08673c4c80.camel@suse.de> From: Rob Herring Date: Thu, 8 Aug 2019 09:02:30 -0600 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH 3/8] of/fdt: add function to get the SoC wide DMA addressable memory size To: Nicolas Saenz Julienne X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190808_080243_265046_A2A01F83 X-CRM114-Status: GOOD ( 34.41 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: phill@raspberryi.org, devicetree@vger.kernel.org, "moderated list:BROADCOM BCM2835 ARM ARCHITECTURE" , Florian Fainelli , Will Deacon , Eric Anholt , Marc Zyngier , Catalin Marinas , Frank Rowand , "linux-kernel@vger.kernel.org" , linux-mm@kvack.org, Linux IOMMU , Matthias Brugger , wahrenst@gmx.net, Andrew Morton , Robin Murphy , Christoph Hellwig , "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" , Marek Szyprowski Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Aug 6, 2019 at 12:12 PM Nicolas Saenz Julienne wrote: > > Hi Rob, > > On Mon, 2019-08-05 at 13:23 -0600, Rob Herring wrote: > > On Mon, Aug 5, 2019 at 10:03 AM Nicolas Saenz Julienne > > wrote: > > > Hi Rob, > > > Thanks for the review! > > > > > > On Fri, 2019-08-02 at 11:17 -0600, Rob Herring wrote: > > > > On Wed, Jul 31, 2019 at 9:48 AM Nicolas Saenz Julienne > > > > wrote: > > > > > Some SoCs might have multiple interconnects each with their own DMA > > > > > addressing limitations. This function parses the 'dma-ranges' on each of > > > > > them and tries to guess the maximum SoC wide DMA addressable memory > > > > > size. > > > > > > > > > > This is specially useful for arch code in order to properly setup CMA > > > > > and memory zones. > > > > > > > > We already have a way to setup CMA in reserved-memory, so why is this > > > > needed for that? > > > > > > Correct me if I'm wrong but I got the feeling you got the point of the patch > > > later on. > > > > No, for CMA I don't. Can't we already pass a size and location for CMA > > region under /reserved-memory. The only advantage here is perhaps the > > CMA range could be anywhere in the DMA zone vs. a fixed location. > > Now I get it, sorry I wasn't aware of that interface. > > Still, I'm not convinced it matches RPi's use case as this would hard-code > CMA's size. Most people won't care, but for the ones that do, it's nicer to > change the value from the kernel command line than editing the dtb. Sure, I fully agree and am not a fan of the CMA DT overlays I've seen. > I get that > if you need to, for example, reserve some memory for the video to work, it's > silly not to hard-code it. Yet due to the board's nature and users base I say > it's important to favor flexibility. It would also break compatibility with > earlier versions of the board and diverge from the downstream kernel behaviour. > Which is a bigger issue than it seems as most users don't always understand > which kernel they are running and unknowingly copy configuration options from > forums. > > As I also need to know the DMA addressing limitations to properly configure > memory zones and dma-direct. Setting up the proper CMA constraints during the > arch's init will be trivial anyway. It was really just commentary on commit text as for CMA alone we have a solution already. I agree on the need for zones. > > > > > IMO, I'd just do: > > > > > > > > if (of_fdt_machine_is_compatible(blob, "brcm,bcm2711")) > > > > dma_zone_size = XX; > > > > > > > > 2 lines of code is much easier to maintain than 10s of incomplete code > > > > and is clearer who needs this. Maybe if we have dozens of SoCs with > > > > this problem we should start parsing dma-ranges. > > > > > > FYI that's what arm32 is doing at the moment and was my first instinct. But > > > it > > > seems that arm64 has been able to survive so far without any machine > > > specific > > > code and I have the feeling Catalin and Will will not be happy about this > > > solution. Am I wrong? > > > > No doubt. I'm fine if the 2 lines live in drivers/of/. > > > > Note that I'm trying to reduce the number of early_init_dt_scan_* > > calls from arch code into the DT code so there's more commonality > > across architectures in the early DT scans. So ideally, this can all > > be handled under early_init_dt_scan() call. > > How does this look? (I'll split it in two patches and add a comment explaining > why dt_dma_zone_size is needed) > > diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c > index f2444c61a136..1395be40b722 100644 > --- a/drivers/of/fdt.c > +++ b/drivers/of/fdt.c > @@ -30,6 +30,8 @@ > > #include "of_private.h" > > +u64 dt_dma_zone_size __ro_after_init; Avoiding a call from arch code by just having a variable isn't really better. I'd rather see a common, non DT specific variable that can be adjusted. Something similar to initrd_start/end. Then the arch code doesn't have to care what hardware description code adjusted the value. Rob _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel