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Date:   Wed, 27 Feb 2019 10:59:37 +0100
From:   Boris Brezillon <bbrezillon@kernel.org>
To:     Vignesh Raghavendra <vigneshr@ti.com>
Cc:     Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Brian Norris <computersforpeace@gmail.com>,
        Marek Vasut <marek.vasut@gmail.com>,
        Richard Weinberger <richard@nod.at>,
        Rob Herring <robh+dt@kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        "tudor.ambarus@microchip.com" <tudor.ambarus@microchip.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Nori, Sekhar" <nsekhar@ti.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mtd@lists.infradead.org" <linux-mtd@lists.infradead.org>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        Miquel Raynal <miquel.raynal@bootlin.com>
Subject: Re: [RFC PATCH 3/5] mtd: Add support for Hyperbus memory devices
Message-ID: <20190227105937.527c6573@kernel.org>
In-Reply-To: <3952b327-3aff-a32e-4280-6ab24c32cfd9@ti.com>
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        <20190219063607.29949-4-vigneshr@ti.com>
        <7e79fff7-2565-28f7-7b2b-bb3098f1a928@cogentembedded.com>
        <3952b327-3aff-a32e-4280-6ab24c32cfd9@ti.com>
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On Wed, 27 Feb 2019 15:22:19 +0530
Vignesh Raghavendra <vigneshr@ti.com> wrote:

> On 26/02/19 11:46 PM, Sergei Shtylyov wrote:
> > On 02/19/2019 09:36 AM, Vignesh R (by way of Boris Brezillon <bbrezillon@kernel.org>) wrote:
> >   
> >> Cypress HyperBus is Low Signal Count, High Performance Double Data Rate Bus
> >> interface between a host system master and one or more slave interfaces.
> >> HyperBus is used to connect microprocessor, microcontroller, or ASIC
> >> devices with random access NOR flash memory(called HyperFlash) or
> >> self refresh DRAM(called HyperRAM).
> >>
> >> Its a 8-bit data bus (DQ[7:0]) with  Read-Write Data Strobe (RWDS)
> >> signal and either Single-ended clock(3.0V parts) or Differential clock
> >> (1.8V parts). It uses ChipSelect lines to select b/w multiple slaves.
> >> At bus level, it follows a separate protocol described in HyperBus
> >> specification[1].
> >>
> >> HyperFlash follows CFI AMD/Fujitsu Extended Command Set (0x0002) similar
> >> to that of existing parallel NORs. Since Hyperbus is x8 DDR bus,
> >> its equivalent to x16 parallel NOR flash wrt bits per clk. But Hyperbus
> >> operates at >166MHz frequencies.
> >> HyperRAM provides direct random read/write access to flash memory
> >> array.
> >>
> >> But, Hyperbus memory controllers seem to abstract implementation details
> >> and expose a simple MMIO interface to access connected flash.
> >>
> >> Add support for registering HyperFlash devices with MTD framework. MTD
> >> maps framework along with CFI chip support framework are used to support
> >> communicate with flash.
> >>
> >> Framework is modelled along the lines of spi-nor framework. HyperBus
> >> memory controller(HBMC) drivers call hb_register_device() to register a
> >> single HyperFlash device. HyperFlash core parses MMIO access
> >> information from DT, sets up the map_info struct, probes CFI flash and
> >> registers it with MTD framework.
> >>
> >> Some HBMC masters need calibration/training sequence[3] to be carried
> >> out, in order for DLL inside the controller to lock, by reading a known
> >> string/pattern. This is done by repeatedly reading CFI Query
> >> Identification String. Calibration needs to be done before try to detect
> >> flash as part of CFI flash probe.
> >>
> >> HyperRAM is not supported atm.
> >>
> >> HyperBus specification can be found at[1]
> >> HyperFlash datasheet can be found at[2]
> >>
> >> [1] https://www.cypress.com/file/213356/download
> >> [2] https://www.cypress.com/file/213346/download
> >> [3] http://www.ti.com/lit/ug/spruid7b/spruid7b.pdf
> >>     Table 12-5741. HyperFlash Access Sequence
> >>
> >> Signed-off-by: Vignesh R <vigneshr@ti.com>  
> > [...]  
> >> diff --git a/include/linux/mtd/hyperbus.h b/include/linux/mtd/hyperbus.h
> >> new file mode 100644
> >> index 000000000000..0aa11458c424
> >> --- /dev/null
> >> +++ b/include/linux/mtd/hyperbus.h
> >> @@ -0,0 +1,73 @@
> >> +/* SPDX-License-Identifier: GPL-2.0
> >> + *
> >> + * Copyright (C) 2019 Texas Instruments Incorporated - http://www.ti.com/
> >> + */
> >> +
> >> +#ifndef __LINUX_MTD_HYPERBUS_H__
> >> +#define __LINUX_MTD_HYPERBUS_H__
> >> +
> >> +#include <linux/mtd/map.h>
> >> +
> >> +enum hb_memtype {
> >> +	HYPERFLASH,
> >> +	HYPERRAM,
> >> +};
> >> +
> >> +/**
> >> + * struct hb_device - struct representing Hyperbus slave device
> >> + * @map: map_info struct for accessing MMIO Hyperbus flash memory
> >> + * @dev: device pointer of Hyperbus Controller  
> > 
> >    I think we need a separate structure for the HyperBus controller, not just
> > for the slave devices...
> >   
> >> + * @np:	pointer to Hyperbus slave device node
> >> + * @mtd: pointer to MTD struct
> >> + * @ops: pointer to custom Hyperbus ops
> >> + * @memtype: type of memory device: Hyperflash or HyperRAM
> >> + * @needs_calib: flag to indicate whether calibration sequence is needed
> >> + * @registered: flag to indicate whether device is registered with MTD core
> >> + */
> >> +
> >> +struct hb_device {
> >> +	struct map_info map;
> >> +	struct device *dev;
> >> +	struct device_node *np;
> >> +	struct mtd_info *mtd;
> >> +	struct hb_ops *ops;
> >> +	enum hb_memtype memtype;
> >> +	bool needs_calib;
> >> +	bool registered;
> >> +};
> >> +
> >> +/**
> >> + * struct hb_ops - struct representing custom Hyperbus operations
> >> + * @read16: read 16 bit of data, usually from register/ID-CFI space
> >> + * @write16: write 16 bit of data, usually to register/ID-CFI space
> >> + * copy_from: copy data from flash memory
> >> + * copy_to: copy data to flash_memory
> >> + */
> >> +
> >> +struct hb_ops {
> >> +	u16 (*read16)(struct hb_device *hbdev, unsigned long addr);
> >> +	void (*write16)(struct hb_device *hbdev, unsigned long addr, u16 val);
> >> +
> >> +	void (*copy_from)(struct hb_device *hbdev, void *to,
> >> +			  unsigned long from, ssize_t len);
> >> +	void (*copy_to)(struct hb_device *dev, unsigned long to,
> >> +			const void *from, ssize_t len);  
> > 
> >    ... else these methods won't fly if you need to "massage" the controller
> > registers inside them...
> >   
> 
> If accessing controller register is the only need, wouldn't a private
> data pointer within struct hb_device be sufficient to hold pointer to
> controller specific struct?
> 
> struct hb_device {
> 	...
> 	void *priv; /* points to controller's private data */
> };
> 
> 
> Or do you see a need for separate structure for the HyperBus controller?

Sorry to chime in. Just want to share my experience here: properly
splitting the controller/device representation is always a good thing.
When it's not done from the beginning and people start to add their own
controller drivers as if it was a flash device driver it becomes messy
pretty quickly and people add hacks to support that (look at the raw
NAND framework if you need a proof). So, I'd recommend having this
separation now, even if the onle controllers we support have a 1:1
relationship between HB controller and HB device.

> 
> 
> >> +};  
> > [...]
> > 
> > MBR, Sergei
> >   
>