linux-kernel - Re: [patch v6 0/3] JTAG driver introduction

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Date:   Thu, 24 Aug 2017 23:07:29 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Oleksandr Shamray <oleksandrs@...lanox.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        OpenBMC Maillist <openbmc@...ts.ozlabs.org>,
        Joel Stanley <joel@....id.au>, jiri@...nulli.us,
        Tobias Klauser <tklauser@...tanz.ch>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        mec@...ut.net, vadimp@...llanox.com,
        system-sw-low-level@...lanox.com, Rob Herring <robh+dt@...nel.org>,
        openocd-devel-owner@...ts.sourceforge.net,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [patch v6 0/3] JTAG driver introduction

On Tue, Aug 22, 2017 at 6:10 PM, Oleksandr Shamray
<oleksandrs@...lanox.com> wrote:

> SoC which are not equipped with JTAG master interface, can be built
> on top of JTAG core driver infrastructure, by applying bit-banging of
> TDI, TDO, TCK and TMS pins within the hardware specific driver.

I guess you mean it should then use GPIO lines for bit-banging?

I was wondering about how some JTAG clients like openOCD does
this in some cases.

In my worst nightmare they export GPIO lines using
the horrid ABI in /sys/gpio/*

In best case they use the GPIO character device or even
libgpiod.

But having a JTAG abstraction inside the kernel that can
grab a few lines for JTAG defined in a device tree, ACPI DSDT
or similar makes sense too, as it abstracts the hardware so the
JTAG client can then just open whatever /dev/jtag0 is on the machine
and go ahead without having to bother about what GPIO lines
are connected exactly where.

Yours,
Linus Walleij