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

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Message-ID: <20200326091313.GA603801@dell>
Date:   Thu, 26 Mar 2020 09:13:13 +0000
From:   Lee Jones <lee.jones@...aro.org>
To:     Sergey Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Paul Burton <paulburton@...nel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mfd: Add Baikal-T1 Boot Controller driver

On Wed, 25 Mar 2020, Sergey Semin wrote:

> Hello Lee,
> 
> On Wed, Mar 25, 2020 at 10:09:40AM +0000, Lee Jones wrote:
> > On Fri, 06 Mar 2020, Sergey.Semin@...kalelectronics.ru wrote:
> > 
> > > From: Serge Semin <Sergey.Semin@...kalelectronics.ru>
> > > 
> > > Baikal-T1 Boot Controller is an IP block embedded into the SoC and
> > > responsible for the chip proper pre-initialization and further
> > > booting up from some memory device. From the Linux kernel point of view
> > > it's just a multi-functional device, which exports three physically mapped
> > > ROMs and a single SPI controller.
> > > 
> > > Primarily the ROMs are intended to be used for transparent access of
> > > the memory devices with system bootup code. First ROM device is an
> > > embedded into the SoC firmware, which is supposed to be used just for
> > > the chip debug and tests. Second ROM device provides a MMIO-based
> > > access to an external SPI flash. Third ROM mirrors either the Internal
> > > or SPI ROM region, depending on the state of the external BOOTCFG_{0,1}
> > > chip pins selecting the system boot device.
> > > 
> > > External SPI flash can be also accessed by the DW APB SSI SPI controller
> > > embedded into the Baikal-T1 Boot Controller. In this case the memory mapped
> > > SPI flash region shouldn't be accessed.
> > > 
> > > Taking into account all the peculiarities described above, we created
> > > an MFD-based driver for the Baikal-T1 controller. Aside from ordinary
> > > OF-based sub-device registration it also provides a simple API to
> > > serialize an access to the external SPI flash from either the MMIO-based
> > > SPI interface or embedded SPI controller.
> > 
> > Not sure why this is being classified as an MFD.
> > 
> > This is clearly 'just' a memory device.
> > 
> 
> Hm, I see this as a normal MFD device. The Boot controller provides a
> set of physically mapped ROMs and a DW APB SSI-based embedded SPI
> controller. Yes, the SPI controller is normally utilized to access an external
> flash device, and at boot stage it is used for it. But still it's a SPI
> controller which driver belongs to the kernel SPI subsystem. Moreover
> nothing prevents a platform designer from using it together with custom
> GPIO-based chip-selects to have additional devices on the SPI bus.
> 
> As I said the problem is that an SPI flash might be accessed either with
> use of a physically mapped ROM or via the normal DW APB SPI controller.
> These two interfaces can't be used simultaneously, because the ROM is
> just an rtl state-machine, which is built to translate MMIO operations
> through the SPI controller registers to an external SPI-nor at CS0 of
> the interface. That's why first I need to make sure the interface is locked,
> then being enabled, then the corresponding driver can use it, then get
> to unlock. That's the point of having the __bt1_bc_spi_lock() and
> bt1_bc_spi_unlock() methods exported from the driver.
> 
> I've got two drivers for MTD ROM and SPI controller based on that
> methods. But I haven't submitted them yet, because they belong to two
> different subsystems and I need to have this one being accepted first.

This is not a totally unique device/situation.  I've seen (and NACKed)
this type of device before.  You need to explain this to the MTD
(SPI-NOR?) maintainers.  They should be in a good position to provide
guidance.

> Recently I've sent an RFC regarding a different question, but it
> concerns the Baikal-T1 system controller and the boot controller as being part
> of it:
> 
> https://lkml.org/lkml/2020/3/22/393

-- 
Lee Jones [李琼斯]
Linaro Services Technical Lead
Linaro.org │ Open source software for ARM SoCs
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