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

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Date:   Mon, 3 Jun 2019 10:42:34 -0700
From:   Alan Mikhak <alan.mikhak@...ive.com>
To:     Kishon Vijay Abraham I <kishon@...com>
Cc:     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>,
        Vinod Koul <vkoul@...nel.org>
Subject: Re: [PATCH] PCI: endpoint: Add DMA to Linux PCI EP Framework

On Sun, Jun 2, 2019 at 9:43 PM Kishon Vijay Abraham I <kishon@...com> wrote:
> Hi Alan,
> On 31/05/19 11:46 PM, Alan Mikhak wrote:
> > On Thu, May 30, 2019 at 10:08 PM Kishon Vijay Abraham I <kishon@...com> wrote:
> >> Hi Alan,
> >>> Hi Kishon,
> >>
> >> I still have to look closer into your DMA patch but linked-list mode or single
> >> block mode shouldn't be an user select-able option but should be determined by
> >> the size of transfer.
> >
> > Please consider the following when taking a closer look at this patch.
>
> After seeing comments from Vinod and Arnd, it looks like the better way of
> adding DMA support would be to register DMA within PCI endpoint controller to
> DMA subsystem (as dmaengine) and use only dmaengine APIs in pci_epf_test.

Thanks Kishon. That makes it clear where these pieces should go.

> > In my specific use case, I need to verify that any valid block size,
> > including a one byte transfer, can be transferred across the PCIe bus
> > by memcpy_toio/fromio() or by DMA either as a single block or as
> > linked-list. That is why, instead of deciding based on transfer size,
> > this patch introduces the '-L' flag for pcitest to communicate the
> > user intent across the PCIe bus to pci-epf-test so the endpoint can
> > initiate the DMA transfer using a single block or in linked-list mode.
> The -L option seems to select an internal DMA configuration which might be
> specific to one implementation. As Gustavo already pointed, we should have only
> generic options in pcitest. This would no longer be applicable when we move to
> dmaengine.

Single-block DMA seemed as generic as linked-list DMA and
memcpy_toio/fromio. It remains unclear how else to communicate that
intent to pci_epf_test each time I invoke pcitest.

Regards,
Alan