Re: Support for configurable PCIe endpoint

[Kishon Vijay Abraham I <kishon@ti.com>]

Hi Arnd,

On Wednesday 03 August 2016 01:12 PM, Arnd Bergmann wrote:
> On Wednesday, August 3, 2016 11:33:19 AM CEST Kishon Vijay Abraham I wrote:
>> Hi,
>>
>> The PCIe controller present in TI's DRA7 SoC is capable of operating either in
>> Root Complex mode or Endpoint mode. (It uses Synopsys Designware Core).I'd
>> assume most of the PCIe controllers on other platforms that use Designware core
>> should also be capable to operate in endpoint mode. But linux kernel right now
>> supports only RC mode.
>>
>> PCIe endpoint support discussion came up briefly before [1] but it was felt the
>> practical use case will find firmware more suitable and endpoint support in
>> kernel can be used only for validation or demo.
>>
>> Validation or demo is itself a valid use case in my opinion (consider something
>> similar to gadget zero for USB). There can be other use cases as well. The RC
>> can use the SoC with EP mode support as an accelerator to accomplish specific
>> task. Here RC gives data to the EP. The EP processes the data. The processing
>> can be done either in ARM itself or it can use other hardware accelerators
>> (like DSP, IVA-HD etc..) present in the EP system. If HW accelerator is used,
>> the linux kernel running in ARM can be used to accomplish other tasks. Once EP
>> mode support is added, I think more use cases will be added.
>>
>> From the high level this should look _similar_ to the gadget framework of USB.
>> One difference from USB would be this should allow HW components (like DSP, PRU
>> etc.. and maybe even some peripheral) in the EP system to be used by RC system.
> 
> (Adding Jon Mason)
> 
> We have the drivers/ntb framework, which in theory should be able to handle
> this, but in practice is only used for a very small number of bridge
> implementations, and is also limited in the way it can be configured
> (compared to USB gadget)
> 
>> So these are the high-level steps that I thought would be needed to add EP
>> support in linux.
>> *) move pcie-designware.c out of drivers/pci/host (maybe create a
>> drivers/pci/designware/ folder?). All users of pcie-designware.c should be
>> moved here.
>> This is in preparation for adding EP mode support to designware.
> 
> A lot of the other drivers in drivers/pci/host support endpoint mode too,
> I don't think moving them all elsewhere is helpful or necessary here.

having drivers that support endpoint in *host* directory might be misleading IMO.
> 
>> *) Add library functions in pcie-designware.c specific to EP mode like
>> initializing general ecam registers, BAR registers etc.. These functions should
>> be invoked from a *function* driver (function driver should determine the use
>> of a particular EP).
>>
>> *) Add a sample *function* driver that can be used just for validation. This
>> function driver will invoke the previously added functions in PCIe controller
>> to initialize ecam, BAR etc.. This will invoke the PCIe controller functions
>> through *ep-core* layer. That way the same function driver can be made to work
>> with different PCIe controllers. (A PCIe driver corresponding to this function
>> driver should also be implemented in RC side)
>>
>> *) Add *ep-core* layer to bind the function driver to the controller driver and
>> using which the function driver will invoke controller driver callbacks (to
>> initialize ecam, BAR registers etc..)
> 
> I think we should first have a rough idea what the framework should look like,
> and how it handles having multiple hardware implementation and multiple
> protocol implementations, before we modify a particular driver.
> 
> So this step here should come a bit earlier than the others.

Okay. That makes sense to me as well.
> 
>> *) Modify platform driver to support EP mode (in my case pci-dra7xx.c).
>>
>> *) dt binding specific to EP mode should be created.
> 
> Right.

Thanks
Kishon