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

From: punnaiah choudary kalluri
Date: Thu Aug 27 2015 - 07:49:38 EST

On Thu, Aug 27, 2015 at 3:45 PM, Jagan Teki <jteki@xxxxxxxxxxxx> wrote:
> On 27 August 2015 at 14:18, punnaiah choudary kalluri
> <punnaia@xxxxxxxxxx> wrote:
>> On Thu, Aug 27, 2015 at 11:53 AM, Jagan Teki <jteki@xxxxxxxxxxxx> wrote:
>>> 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.
>> <snip>
>>>>> 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.
>> True and this is the reason we have controller drivers and protocol drivers.
>> GQSPI is the controller driver and spi-nor and spi-nand are the
>> protocol drivers.
>>> 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.
>> Just to clear, dual parallel( 2 CS and 8 IO lines) is not only specific
>> to flash parts, one can use for any other custom streaming protocols
>> I would say exporting dual parallel connection to protocol drivers is
>> something like depicting the spi bus topology to the protocol layer.
> So dual parallel may not used for spi-nor flash it can also used other
> spi slaves that's what your saying is it?

Yes. As i said above, the main intention of this feature is to improve
the data rate with an overhead of few IO lines.

>> AFAIK, spi-nor and spi-nand are protocol drivers for accessing the
>> nor and nand flash devices sitting on the spi bus using the spi
>> controller driver.
> Yes, I do agree with your point, but though driver stacks are
> different with same kind of bus here, I'm trying to spit the GQSPI
> into 3 different controller drivers as Linux understand it and fit on
> to Linux stack with out disturbing the generic-ness.

I feel this is not a nice idea. if there are 'n' functionalities and having
'n' controller drivers doesn't seem good in any direction.

Protocol driver can query the spi core about the bus topology and it is the
responsibility of the spi core and controller driver providing this information
to the upper layers.


> Assumption is GQSPI shall split to various platform_drivers (if each
> platform driver treated as a controller) thought it made up of spi
> bus.
>>> 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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