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Subject: Re: [RFC PATCH 3/5] mtd: Add support for Hyperbus memory devices
To:     "Vignesh R (by way of Boris Brezillon <bbrezillon@kernel.org>)" 
        <vigneshr@ti.com>, David Woodhouse <dwmw2@infradead.org>,
        Brian Norris <computersforpeace@gmail.com>,
        Boris Brezillon <bbrezillon@kernel.org>,
        Marek Vasut <marek.vasut@gmail.com>,
        Richard Weinberger <richard@nod.at>,
        Rob Herring <robh+dt@kernel.org>
Cc:     devicetree@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
        tudor.ambarus@microchip.com,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        nsekhar@ti.com, linux-kernel@vger.kernel.org,
        linux-mtd@lists.infradead.org, linux-arm-kernel@lists.infradead.org
References: <20190219063607.29949-1-vigneshr@ti.com>
 <20190219063607.29949-4-vigneshr@ti.com>
From:   Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Organization: Cogent Embedded
Message-ID: <92bfdbf8-cc78-c691-298d-d2eb6193e1be@cogentembedded.com>
Date:   Mon, 25 Feb 2019 21:26:25 +0300
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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,

   HyperBus, please be consistent.

> its equivalent to x16 parallel NOR flash wrt bits per clk. But Hyperbus

   Again...

> 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 again.

> 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/drivers/mtd/hyperbus/core.c b/drivers/mtd/hyperbus/core.c
> new file mode 100644
> index 000000000000..d3d44aab7503
> --- /dev/null
> +++ b/drivers/mtd/hyperbus/core.c
> @@ -0,0 +1,167 @@
[...]
> +/* Calibrate HBMC by repeatedly reading "QRY" string from CFI space */
> +static int hb_calibrate(struct hb_device *hbdev)
> +{
> +	struct map_info *map = &hbdev->map;
> +	struct cfi_private cfi;
> +	int count = HB_CALIB_COUNT;
> +	int ret;
> +
> +	cfi.interleave = 1;
> +	cfi.device_type = CFI_DEVICETYPE_X16;
> +	cfi_send_gen_cmd(0xF0, 0, 0, map, &cfi, cfi.device_type, NULL);
> +	cfi_send_gen_cmd(0x98, 0x55, 0, map, &cfi, cfi.device_type, NULL);
> +
> +	while (count--)
> +		cfi_qry_present(map, 0, &cfi);

   Why do all 25 reads if you can encounter valid QRY in less reads than 25?

> +
> +	ret = cfi_qry_present(map, 0, &cfi);
> +	cfi_qry_mode_off(0, map, &cfi);
> +
> +	return ret;
> +}
[...]

MBR, Sergei