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

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Date:   Fri, 31 May 2019 10:37:27 +0530
From:   Vinod Koul <vkoul@...nel.org>
To:     Kishon Vijay Abraham I <kishon@...com>
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 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?
If so I do not see a reason why this cannot be using dmaengine. The use
case would be memcpy for DMA right or mem to device (vice versa) transfers?

Btw many driver in sdhci do use dmaengine APIs and yes we are missing
support in framework than individual drivers

> If DMA within the PCIe controller cannot be registered in DMA subsystem, we
> should use something like what Alan has done in this patch with dma_read ops.
> The dma_read ops implementation in the EP controller can either use dmaengine
> APIs or use the DMA within the PCIe controller.
> 
> I'll review the patch separately.
> 
> Thanks
> Kishon

-- 
~Vinod