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

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Date:   Fri, 31 May 2019 10:50:23 +0530
From:   Kishon Vijay Abraham I <kishon@...com>
To:     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>,
        "arnd@...db.de" <arnd@...db.de>,
        "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 Vinod,

On 31/05/19 10:37 AM, Vinod Koul wrote:
> Hi Kishon,
> 
> On 30-05-19, 11:16, Kishon Vijay Abraham I wrote:
>> +Vinod Koul
>>
>> Hi,
>>
>> On 30/05/19 4:07 AM, Alan Mikhak wrote:
>>> On Mon, May 27, 2019 at 2:09 AM Gustavo Pimentel
>>> <Gustavo.Pimentel@...opsys.com> wrote:
>>>>
>>>> On Fri, May 24, 2019 at 20:42:43, Alan Mikhak <alan.mikhak@...ive.com>
>>>> wrote:
>>>>
>>>> Hi Alan,
>>>>
>>>>> On Fri, May 24, 2019 at 1:59 AM Gustavo Pimentel
>>>>> <Gustavo.Pimentel@...opsys.com> wrote:
>>>>>>
>>>>>> Hi Alan,
>>>>>>
>>>>>> This patch implementation is very HW implementation dependent and
>>>>>> requires the DMA to exposed through PCIe BARs, which aren't always the
>>>>>> case. Besides, you are defining some control bits on
>>>>>> include/linux/pci-epc.h that may not have any meaning to other types of
>>>>>> DMA.
>>>>>>
>>>>>> I don't think this was what Kishon had in mind when he developed the
>>>>>> pcitest, but let see what Kishon was to say about it.
>>>>>>
>>>>>> I've developed a DMA driver for DWC PCI using Linux Kernel DMAengine API
>>>>>> and which I submitted some days ago.
>>>>>> By having a DMA driver which implemented using DMAengine API, means the
>>>>>> pcitest can use the DMAengine client API, which will be completely
>>>>>> generic to any other DMA implementation.
>>
>> 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.
> case would be memcpy for DMA right or mem to device (vice versa) transfers?

The device is memory mapped so it would be only memcopy.
> 
> Btw many driver in sdhci do use dmaengine APIs and yes we are missing
> support in framework than individual drivers

I think dmaengine APIs is used only when the platform uses system DMA and not
ADMA within the SDHCI controller. IOW there is no dma_async_device_register()
to register ADMA in SDHCI with DMA subsystem.

Thanks
Kishon