Re: [LINUX RFC v2 0/4] spi: add dual parallel mode support in Zynq MPSoC GQSPI controller

[Jagan Teki <jteki@xxxxxxxxxxxx>]

On 26 August 2015 at 21:02, punnaiah choudary kalluri
<punnaia@xxxxxxxxxx> wrote:
> On Wed, Aug 26, 2015 at 5:49 PM, Jagan Teki <jteki@xxxxxxxxxxxx> wrote:
>> On 26 August 2015 at 11:56, Ranjit Waghmode <ranjit.waghmode@xxxxxxxxxx> wrote:
>>> This series adds dual parallel mode support for Zynq Ultrascale+
>>> MPSoC GQSPI controller driver.
>>>
>>> What is dual parallel mode?
>>> ---------------------------
>>> ZynqMP GQSPI controller supports Dual Parallel mode with following functionalities:
>>> 1) Supporting two SPI flash memories operating in parallel. 8 I/O lines.
>>> 2) Chip selects and clock are shared to both the flash devices
>>> 3) This mode is targeted for faster read/write speed and also doubles the size
>>> 4) Commands/data can be transmitted/received from both the devices(mirror),
>>>    or only upper or only lower flash memory devices.
>>> 5) Data arrangement:
>>>    With stripe enabled,
>>>    Even bytes i.e. 0, 2, 4,... are transmitted on Lower Data Bus
>>>    Odd bytes i.e. 1, 3, 5,.. are transmitted on Upper Data Bus.
>>>
>>> This series also updated MTD layer files for adding parallel mode support.
>>>
>>> 1) Added Support for two flashes
>>> 2) Support to enable/disable data stripe as and when required.
>>> 3) Added required parameters to spi_nor structure. Initialized all
>>>    added parameters in spi_nor_scan()
>>> 4) Added support for dual parallel in spi_nor_read/write/erase functions by:
>>>    a) Increasing page_size, sector_size, erase_size and toatal flash size
>>>       as and when required.
>>>    b) Dividing address by 2
>>>    c) Updating spi->master->flags for qspi driver to change CS
>>> 5) Updated read_sr() to get status of both flashes
>>> 6) Also updated read_fsr() to get status of both flashes
>>>
>>> These all are very high level changes and expected to make an idea clear.
>>> Comments and suggestions are always welcomed
>>>
>>> ---
>>> V2 Changes:
>>> a) Splitted patches based on logical changes
>>> b) Added error handling for newly added APIs in SPI core
>>> ---
>>>
>>> Ranjit Waghmode (4):
>>>   spi: add support of two chip selects & data stripe
>>>   mtd: add spi_device instance to spi_nor struct
>>>   spi-nor: add dual parallel mode support
>>>   spi: zynqmp: gqspi: add support for dual parallel mode configuration
>>
>> I don't find any previous discussion about way to inform flash
>> dual-ness into spi-nor
>> from spi drivers.
>>
>> Here is my idea, probably others may think same.
>> Informing dual_flash from drivers/spi through flags or any other mode
>> bits is not a better approach as dual flash feature is specific to
>> spi-nor flash controller (controller specially designed for spi-nor
>> flash not the generic spi controller).  So if the driver sits on
>> drivers/mtd/spi-nor/ (ex: fsl-quadspi.c), may be we can inform flash
>> specific things to spi-nor as it's not touching generic spi stack in
>> Linux. But there is a defined-drawback if the driver is moved to
>> drivers/mtd/spi-nor ie it can't use spi core API's at-all.
>
> Xilinx GQSPI is a generic quad spi controller. The primary goal is to support
> Generic/Future command sequences and Future NOR/NAND flash devices.
> This core can also be used for legacy SPI devices. Due to the generic nature
> of the core, software can generate any command sequence. It also has additional
> features like parallel and stacked configurations to double the data
> rate and size.
> Accessing spi-nor flash device is one particular use case and like
> that there will be
> many. So, we decided to keep this driver in generic spi framework and
> that is the ideal
> thing to do for the GQSPI controller.

Yes, I understand the generic nature of the GQSPI and it's good to
have all-in-one like generic spi, spi-nor and spi-nand and more
together, but Linux stacks were implemented in such a way to support
the each type of controller with connected slaves separably for better
handling.

Currently GQSPI driver is added in drivers/spi as it supports generic
spi nature and now it enhanced more through flags for supporting
spi-nor, what if we add spi-nand support for the same controller? do
we add one more driver in spi-nand framework (drivers/mtd/spi-nand -
an on going implementation)? My only observation here is even if the
controller is more generic to support more number of device classes,
and adding same driver and populate the slave stuff through flags or
different kind of mechanism to different driver stack, this is not a
better approach I thought.

Based on the above comments, there is an approach to handle this
support and I'm not 100% sure whether this fits or not but we
implemented the same -  this is "probing child devices from parent"
(there was a discussion with Arnd earlier wrt this, but I'm unable to
get the mailing thread)

Added Arnd (probably will give more inputs or corrections)

Let me explain how we implemented on our design.
We have PCIe controller that support basic root complex handling, dma
and controller hotplug (not in-build pcie hp) and ideally we need to
write driver for handling root complex on drivers/pci/host and one
hotplug driver in drivers/pci and one more driver in drivers/dma for
handling pcie dma stuff. And some pcie calls need to navigate from
root complex driver to dma and hotplug driver that means there is call
transition from driver/pci to driver/dma which is absolutely not a
good approach (spi to spi-nor and spi-nand transition - in GQSPI case)

So the implementation we follow is like there is a pcie root complex
driver(probably generic spi driver in drivers/spi/*) and inside probe
we have register platform_device for hotplug (spi-nor) and dma
(spi-nand) and the dma driver in drivers/dma and hotplug driver in
driver/pci/ are platform drivers which is of legacy binding (not with
dts) so there should be a common dts for root complex driver
(drivers/spi/*) and individual child driver need to take those while
registering platform_device.

example pseudo:

drivers/dma/dma-child2.c

Legacy platform_driver binding and handling dma future as normal dma
driver, spi-nand in your case

drivers/pci/hotplug/hp-child1.c

Legacy platform_driver binding and handling hotplug future as normal
hotplug driver, spi-nor in your case.

drivers/pci/host/rc-parent-pci.c

static int rc_parent_pcie_probe_bridge(struct platform_device *pdev)
{
   // Generic rc handling (genric spi stuff)

   // Hotplug handling (spi-nor)
   - platform_device_alloc
   - assign need resources
   - register pdev using platform_device_add

    // DMA handling (spi-nand)
   - same as above
}

static const struct of_device_id rc_parent_pcie_match_table[] = {
        {.compatible = "abc,rc-parent",},
        {},
};

static struct platform_driver rc_parent_pcie_driver = {
        .driver = {
                   .name = "rc-parent",
                   .of_match_table = of_match_ptr(rc_parent_pcie_match_table),
        },
        .probe = rc_parent_pcie_probe_bridge,
};
module_platform_driver(rc_parent_pcie_driver);

I couldn't find any driver mainlined wrt this design, think more on
GQSPI front, whether this design fits well or not.

thanks!
-- 
Jagan | openedev.
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