linux-kernel - Re: [PATCH] PCI: endpoint: Add DMA to Linux PCI EP Framework

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Date:   Mon, 3 Jun 2019 09:59:16 +0530
From:   Kishon Vijay Abraham I <kishon@...com>
To:     Arnd Bergmann <arnd@...db.de>, Vinod Koul <vkoul@...nel.org>
CC:     Alan Mikhak <alan.mikhak@...ive.com>,
        Gustavo Pimentel <Gustavo.Pimentel@...opsys.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "lorenzo.pieralisi@....com" <lorenzo.pieralisi@....com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "jingoohan1@...il.com" <jingoohan1@...il.com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "wen.yang99@....com.cn" <wen.yang99@....com.cn>,
        "kjlu@....edu" <kjlu@....edu>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "palmer@...ive.com" <palmer@...ive.com>,
        "paul.walmsley@...ive.com" <paul.walmsley@...ive.com>
Subject: Re: [PATCH] PCI: endpoint: Add DMA to Linux PCI EP Framework

Hi,

On 31/05/19 1:19 PM, Arnd Bergmann wrote:
> On Fri, May 31, 2019 at 8:32 AM Vinod Koul <vkoul@...nel.org> wrote:
>> On 31-05-19, 10:50, Kishon Vijay Abraham I wrote:
>>> On 31/05/19 10:37 AM, Vinod Koul wrote:
>>>> On 30-05-19, 11:16, Kishon Vijay Abraham I wrote:
>>>>>
>>>>> right, my initial thought process was to use only dmaengine APIs in
>>>>> pci-epf-test so that the system DMA or DMA within the PCIe controller can be
>>>>> used transparently. But can we register DMA within the PCIe controller to the
>>>>> DMA subsystem? AFAIK only system DMA should register with the DMA subsystem.
>>>>> (ADMA in SDHCI doesn't use dmaengine). Vinod Koul can confirm.
>>>>
>>>> So would this DMA be dedicated for PCI and all PCI devices on the bus?
>>>
>>> Yes, this DMA will be used only by PCI ($patch is w.r.t PCIe device mode. So
>>> all endpoint functions both physical and virtual functions will use the DMA in
>>> the controller).
>>>> If so I do not see a reason why this cannot be using dmaengine. The use
>>>
>>> Thanks for clarifying. I was under the impression any DMA within a peripheral
>>> controller shouldn't use DMAengine.
>>
>> That is indeed a correct assumption. The dmaengine helps in cases where
>> we have a dma controller with multiple users, for a single user case it
>> might be overhead to setup dma driver and then use it thru framework.
>>
>> Someone needs to see the benefit and cost of using the framework and
>> decide.
> 
> I think the main question is about how generalized we want this to be.
> There are lots of difference PCIe endpoint implementations, and in
> case of some licensable IP cores like the designware PCIe there are
> many variants, as each SoC will do the implementation in a slightly
> different way.
> 
> If we can have a single endpoint driver than can either have an
> integrated DMA engine or use an external one, then abstracting that
> DMA engine helps make the driver work more readily either way.
> 
> Similarly, there may be PCIe endpoint implementations that have
> a dedicated DMA engine in them that is not usable for anything else,
> but that is closely related to an IP core we already have a dmaengine
> driver for. In this case, we can avoid duplication.

right. Either way it makes more sense to register DMA embedded within the PCIe
endpoint controller instead of creating epc_ops for DMA transfers.

Thanks
Kishon