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[209.132.180.67]) by mx.google.com with ESMTP id w18si38499375otl.54.2020.01.06.00.14.30; Mon, 06 Jan 2020 00:14:44 -0800 (PST) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726296AbgAFIMw (ORCPT + 99 others); Mon, 6 Jan 2020 03:12:52 -0500 Received: from mail-ot1-f67.google.com ([209.85.210.67]:40219 "EHLO mail-ot1-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725446AbgAFIMv (ORCPT ); Mon, 6 Jan 2020 03:12:51 -0500 Received: by mail-ot1-f67.google.com with SMTP id w21so62870207otj.7; Mon, 06 Jan 2020 00:12:50 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=dM6755OiAVxNNT/EGnm6GG38dRM8wbwsdCcOva50lrs=; b=IEIOyi3XR0Jg2+UNFDUwTo2b0HKfSd3KRh4XmECpCeyLQFyiwlFS8LQSc3GyyEX+sN o1hnzHuulSxw2dNRZFueVNJelWo5277dB2FutRhIlGpYVGFIfZfhFIDteepgsVwuOSLY wHZmY89zMaVu7rDZmLqHjNe1ipt7/+RuLj/H5TrBV9khrBRM80u7gHpo2dCytnC482bW 1paPBoXnF1R/Mso0yjo/5dDiM/4QbNwlvkNpCOZRqe85eGkiVVTeFO3klyAS26Vx5oMi +wfOA+PQUhrZlOVJpjOr1PvnlkdUazJp/Pk3a9q03QoFKWFJ2ctHtqtybKLgOWwnjfCV iBdA== X-Gm-Message-State: APjAAAURtreuF5c5xJZ+/9EWIwIIWrSeswtZzaa0TX0PT5TuuNGrQ7WP ui7LDm1wgvM6j5I1AQrgdv1WdTWzokg9Z67Wpdc= X-Received: by 2002:a9d:7984:: with SMTP id h4mr116794369otm.297.1578298370564; Mon, 06 Jan 2020 00:12:50 -0800 (PST) MIME-Version: 1.0 References: <20191127084253.16356-1-geert+renesas@glider.be> <20191127084253.16356-5-geert+renesas@glider.be> <20191205210653.GA29969@bogus> In-Reply-To: From: Geert Uytterhoeven Date: Mon, 6 Jan 2020 09:12:39 +0100 Message-ID: Subject: Re: [PATCH v3 4/7] dt-bindings: gpio: Add gpio-repeater bindings To: Rob Herring Cc: Geert Uytterhoeven , Linus Walleij , Bartosz Golaszewski , Jonathan Corbet , Mark Rutland , Harish Jenny K N , Eugeniu Rosca , Alexander Graf , Peter Maydell , Paolo Bonzini , Phil Reid , Marc Zyngier , Christoffer Dall , Magnus Damm , "open list:GPIO SUBSYSTEM" , "open list:DOCUMENTATION" , "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" , Linux-Renesas , Linux Kernel Mailing List , QEMU Developers Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Rob, On Fri, Dec 6, 2019 at 4:04 PM Rob Herring wrote: > On Fri, Dec 6, 2019 at 3:17 AM Geert Uytterhoeven wrote: > > On Thu, Dec 5, 2019 at 10:06 PM Rob Herring wrote: > > > On Wed, Nov 27, 2019 at 09:42:50AM +0100, Geert Uytterhoeven wrote: > > > > Add Device Tree bindings for a GPIO repeater, with optional translation > > > > of physical signal properties. This is useful for describing explicitly > > > > the presence of e.g. an inverter on a GPIO line, and was inspired by the > > > > non-YAML gpio-inverter bindings by Harish Jenny K N > > > > [1]. > > > > > > > > Note that this is different from a GPIO Nexus Node[2], which cannot do > > > > physical signal property translation. > > > > > > It can't? Why not? The point of the passthru mask is to not do > > > translation of flags, but without it you are always doing translation of > > > cells. > > > > Thanks for pushing me deeper into nexuses! > > You're right, you can map from one type to another. > > However, you cannot handle the "double inversion" of an ACTIVE_LOW > > signal with a physical inverter added: > > > > nexus: led-nexus { > > #gpio-cells = <2>; > > gpio-map = <0 0 &gpio2 19 GPIO_ACTIVE_LOW>, // inverted > > <1 0 &gpio2 20 GPIO_ACTIVE_HIGH>, // noninverted > > <2 0 &gpio2 21 GPIO_ACTIVE_LOW>; // inverted > > gpio-map-mask = <3 0>; > > // default gpio-map-pass-thru = <0 0>; > > }; > > > > leds { > > compatible = "gpio-leds"; > > led6-inverted { > > gpios = <&nexus 0 GPIO_ACTIVE_HIGH>; > > }; > > led7-noninverted { > > gpios = <&nexus 1 GPIO_ACTIVE_HIGH>; > > }; > > led8-double-inverted { // FAILS: still inverted > > gpios = <&nexus 2 GPIO_ACTIVE_LOW>; > > }; > > }; > > > > It "works" if the last entry in gpio-map is changed to GPIO_ACTIVE_HIGH. > > Still, the consumer would see the final translated polarity, and not the > > actual one it needs to program the consumer for. > > I'm not really following. Why isn't a double inversion just the same > as no inversion? Because the nexus can only mask and/or substitute bits. It cannot do a XOR operation on the GPIO flags. > > > > While an inverter can be described implicitly by exchanging the > > > > GPIO_ACTIVE_HIGH and GPIO_ACTIVE_LOW flags, this has its limitations. > > > > Each GPIO line has only a single GPIO_ACTIVE_* flag, but applies to both > > > > th provider and consumer sides: > > > > 1. The GPIO provider (controller) looks at the flags to know the > > > > polarity, so it can translate between logical (active/not active) > > > > and physical (high/low) signal levels. > > > > 2. While the signal polarity is usually fixed on the GPIO consumer > > > > side (e.g. an LED is tied to either the supply voltage or GND), > > > > it may be configurable on some devices, and both sides need to > > > > agree. Hence the GPIO_ACTIVE_* flag as seen by the consumer must > > > > match the actual polarity. > > > > There exists a similar issue with interrupt flags, where both the > > > > interrupt controller and the device generating the interrupt need > > > > to agree, which breaks in the presence of a physical inverter not > > > > described in DT (see e.g. [3]). > > > > > > Adding an inverted flag as I've suggested would also solve this issue. > > > > As per your suggestion in "Re: [PATCH V4 2/2] gpio: inverter: document > > the inverter bindings"? > > https://lore.kernel.org/linux-devicetree/CAL_JsqLp___2O-naU+2PPQy0QmJX6+aN3hByz-OB9+qFvWgN9Q@mail.gmail.com/ > > > > Oh, now I understand. I was misguided by Harish' interpretation > > https://lore.kernel.org/linux-devicetree/dde73334-a26d-b53f-6b97-4101c1cdc185@mentor.com/ > > which assumed an "inverted" property, e.g. > > > > inverted = /bits/ 8 <0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0>; > > > > But you actually meant a new GPIO_INVERTED flag, to be ORed into the 2nd > > cell of a GPIO specifier? I.e. add to include/dt-bindings/gpio/gpio.h" > > > > /* Bit 6 expresses the presence of a physical inverter */ > > #define GPIO_INVERTED 64 > > Exactly. OK, makes sense. > > We need to be very careful in defining to which side the GPIO_ACTIVE_* > > applies to (consumer?), and which side the GPIO_INVERTED flag (provider?). > > Still, this doesn't help if e.g. a FET is used instead of a push-pull > > inverter, as the former needs translation of other flags (which the > > nexus can do, the caveats above still applies, though). > > Yes. Historically the cells values are meaningful to the provider and > opaque to the consumer. Standardized cell values changes that > somewhat. I think we want the active flag to be from the provider's > prospective because the provider always needs to know. The consumer > often doesn't need to know. That also means things work without the > GPIO_INVERTED flag if the consumer doesn't care which is what we have > today already and we can't go back in time. > > > > Same for adding IRQ_TYPE_INVERTED. > > I suppose so, yes. > > > Related issue: how to handle physical inverters on SPI chip select lines, > > if the SPI slave can be configured for both polarities? > > Good question. Perhaps in a different way because we have to handle > both h/w controlled and gpio chip selects. > > However, how would one configure the polarity in the device in the > first place? You have to assert the CS first to give a command to > reprogram it. That's indeed true for a simple SPI slave. But if it is a smarter device (e.g. a generic micro controller), it may use the system's DTB to configure itself. Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds