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

From: Serge Semin <Sergey.Semin@baikalelectronics.ru>
To: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Mark Brown <broonie@kernel.org>,
	Ramil Zaripov <Ramil.Zaripov@baikalelectronics.ru>,
	Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	Paul Burton <paulburton@kernel.org>,
	Ralf Baechle <ralf@linux-mips.org>,
	Lee Jones <lee.jones@linaro.org>,
	Miquel Raynal <miquel.raynal@bootlin.com>,
	Arnd Bergmann <arnd@arndb.de>, Rob Herring <robh+dt@kernel.org>,
	<linux-mips@vger.kernel.org>,
	devicetree <devicetree@vger.kernel.org>,
	John Garry <john.garry@huawei.com>,
	Chuanhong Guo <gch981213@gmail.com>,
	Eddie James <eajames@linux.ibm.com>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Chris Packham <chris.packham@alliedtelesis.co.nz>,
	Tomer Maimon <tmaimon77@gmail.com>,
	Masahisa Kojima <masahisa.kojima@linaro.org>,
	Krzysztof Kozlowski <krzk@kernel.org>,
	Florian Fainelli <f.fainelli@gmail.com>,
	Jassi Brar <jaswinder.singh@linaro.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-spi <linux-spi@vger.kernel.org>
Subject: Re: [PATCH 2/2] spi: Add Baikal-T1 System Boot SPI Controller driver
Date: Fri, 8 May 2020 13:15:41 +0300	[thread overview]
Message-ID: <20200508101541.e3yxaocuilaiyutg@mobilestation> (raw)
In-Reply-To: <CAHp75VdtzdX-sOvq2cZdXqGUmU=0rdzQW_USGD_q0D59pUMTWg@mail.gmail.com>

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@baikalelectronics.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