linux-kernel - Re: [PATCH 2/6] drivers/misc: Add Aspeed XDMA engine driver

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Message-Id: <e889a254-ec7e-49f5-bdb0-1d8d8a9063a6@www.fastmail.com>
Date:   Tue, 05 Mar 2019 19:00:05 -0500
From:   "Andrew Jeffery" <andrew@...id.au>
To:     "Eddie James" <eajames@...ux.ibm.com>,
        "Arnd Bergmann" <arnd@...db.de>
Cc:     "Mark Rutland" <mark.rutland@....com>,
        DTML <devicetree@...r.kernel.org>, linux-aspeed@...ts.ozlabs.org,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "OpenBMC Maillist" <openbmc@...ts.ozlabs.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        "Rob Herring" <robh+dt@...nel.org>
Subject: Re: [PATCH 2/6] drivers/misc: Add Aspeed XDMA engine driver



On Wed, 6 Mar 2019, at 08:15, Eddie James wrote:
> 
> On 3/5/19 2:01 AM, Arnd Bergmann wrote:
> > On Mon, Mar 4, 2019 at 10:37 PM Eddie James <eajames@...ux.ibm.com> wrote:
> >> The XDMA engine embedded in the AST2500 SOC performs PCI DMA operations
> >> between the SOC (acting as a BMC) and a host processor in a server.
> >>
> >> This commit adds a driver to control the XDMA engine and adds functions
> >> to initialize the hardware and memory and start DMA operations.
> >>
> >> Signed-off-by: Eddie James <eajames@...ux.ibm.com>
> > Hi Eddie,
> >
> > Thanks for your submission! Overall this looks well-implemented, but
> > I fear we already have too many ways of doing the same thing at
> > the moment, and I would hope to avoid adding yet another user space
> > interface for a specific hardware that does this.
> >
> > Your interface appears to be a fairly low-level variant, just doing
> > single DMA transfers through ioctls, but configuring the PCIe
> > endpoint over sysfs.
> 
> Hi, thanks for the quick response!
> 
> There is actually no PCIe configuration done in this driver. The two 
> sysfs entries control the system control unit (SCU) on the AST2500 
> purely to enable and disable entire PCIe devices. It might be possible 
> to control those devices more finely with a PCI endpoint driver, but 
> there is no need to do so. The XDMA engine does that by itself to 
> perform DMA fairly automatically.

I had a series a while back to expose random bits from devices in sysfs. It got
shot down pretty well, but the main contention was over the devicetree
bindings.

I think we could revive it as a library-type thing that drivers can use to expose
bits like what you're describing without putting the grubby details in the
devicetree. The we would have a consistent approach to exposing otherwise
hard to describe functions (which is what a lot of a BMC turns out to be).

Andrew