linux-kernel - Re: [PATCH 2/2] spi: Add Baikal-T1 System Boot SPI Controller driver

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Date:   Fri, 8 May 2020 13:15:41 +0300
From:   Serge Semin <Sergey.Semin@...kalelectronics.ru>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
CC:     Mark Brown <broonie@...nel.org>,
        Ramil Zaripov <Ramil.Zaripov@...kalelectronics.ru>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Paul Burton <paulburton@...nel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Lee Jones <lee.jones@...aro.org>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Arnd Bergmann <arnd@...db.de>,
        Rob Herring <robh+dt@...nel.org>, <linux-mips@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        John Garry <john.garry@...wei.com>,
        Chuanhong Guo <gch981213@...il.com>,
        Eddie James <eajames@...ux.ibm.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Chris Packham <chris.packham@...iedtelesis.co.nz>,
        Tomer Maimon <tmaimon77@...il.com>,
        Masahisa Kojima <masahisa.kojima@...aro.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Jassi Brar <jaswinder.singh@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-spi <linux-spi@...r.kernel.org>
Subject: Re: [PATCH 2/2] spi: Add Baikal-T1 System Boot SPI Controller driver

On Fri, May 08, 2020 at 01:03:11PM +0300, Andy Shevchenko wrote:
> On Fri, May 8, 2020 at 12:37 PM Serge Semin
> <Sergey.Semin@...kalelectronics.ru> wrote:
> >
> > This SPI-controller is a part of the Baikal-T1 System Controller and
> > is based on the DW APB SSI IP-core, but with very limited resources:
> > no IRQ, no DMA, only a single native chip-select and just 8 bytes Tx/Rx
> > FIFO available. In order to provide a transparent initial boot code
> > execution this controller is also utilized by an vendor-specific block,
> > which provides an CS0 SPI flash direct mapping interface. Since both
> > direct mapping and SPI controller normal utilization are mutual exclusive
> > only a one of these interfaces can be used to access an external SPI
> > slave device. Taking into account the peculiarities of the controller
> > registers and physically mapped SPI flash access, very limited resources
> > and seeing the normal usecase of the controller is to access an external
> > SPI-nor flash, we decided to create a dedicated SPI driver for it.
> 
> It seems a lot of code.
> Why can't you use spi-dw-mmio.c et al.?

I said above why. Even though the registers set is similar It's too specific
to be integrated into the generic DW SSI driver.

-Sergey

> 
> > The driver provides callbacks for native messages-based SPI interface,
> > SPI-memory and direct mapping read operations. Due to not having any
> > asynchronous signaling interface provided by the core we have no choice
> > but to implement a polling-based data transmission/reception algorithm.
> > In addition to that in order to bypass the automatic native chip-select
> > toggle the driver disables the local interrupts during the memory-based
> > transfers if no complementary GPIO-based chip-select detected in the
> > platform.
> 
> 
> 
> -- 
> With Best Regards,
> Andy Shevchenko