linux-kernel - Re: [PATCH V14 0/9] dma: add Qualcomm Technologies HIDMA driver

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Message-ID: <20160208170026.GA23324@cbox>
Date:	Mon, 8 Feb 2016 18:00:26 +0100
From:	Christoffer Dall <christoffer.dall@...aro.org>
To:	Sinan Kaya <okaya@...eaurora.org>
Cc:	dmaengine@...r.kernel.org, marc.zyngier@....com,
	mark.rutland@....com, timur@...eaurora.org,
	devicetree@...r.kernel.org, cov@...eaurora.org,
	vinod.koul@...el.com, jcm@...hat.com, vikrams@...eaurora.org,
	arnd@...db.de, eric.auger@...aro.org,
	linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, agross@...eaurora.org,
	shankerd@...eaurora.org
Subject: Re: [PATCH V14 0/9] dma: add Qualcomm Technologies HIDMA driver

On Mon, Feb 08, 2016 at 09:24:39AM -0500, Sinan Kaya wrote:
> Hi,
> 
> On 2/8/2016 5:14 AM, Christoffer Dall wrote:
> > Hi Sinan,
> > 
> > On Thu, Feb 04, 2016 at 11:34:31PM -0500, Sinan Kaya wrote:
> >> The Qualcomm Technologies HIDMA device has been designed
> >> to support virtualization technology. The driver has been
> >> divided into two to follow the hardware design.
> >>
> >> 1. HIDMA Management driver
> >> 2. HIDMA Channel driver
> >>
> >> Each HIDMA HW consists of multiple channels. These channels
> >> share some set of common parameters. These parameters are
> >> initialized by the management driver during power up.
> >> Same management driver is used for monitoring the execution
> >> of the channels. Management driver can change the performance
> >> behavior dynamically such as bandwidth allocation and
> >> prioritization in the future.
> >>
> >> The management driver is executed in host context and
> >> is the main management entity for all channels provided by
> >> the device.
> > 
> > I'm not at all familiar with this kind of hardware.
> > 
> 
> Let me help. 
> 
> > Once an entity (userspace or VM) has a HIDAM channel assigned via VFIO,
> > how does it use it?  
> 
> HIDMA is a HW accelerator for memory operations like memcpy and memset. It fits
> into the DMA engine framework.
> 
> There are two classes of DMA HW. These are slave DMA and standalone DMA. Slave DMA
> is generally related to another piece of HW that needs DMA for moving data. 
> Standalone DMA is intended for OS/CPU consumption not HW. These can be used to offload
> crypto operations (XOR) or other memory transactions (memcpy and memset) that are
> lengthy. 
> 
> Once the HIDMA channel is assigned to the guest machine, guest operating system uses the
> DMA channel to offload memory operations as HIDMA only supports memcpy. 
> 
> IOMMU HW provides the isolation from the rest of the host kernel or from other guest machines.
> 
> > Is it used in relation with another device to
> > perform DMA on its behalf or is a HIDMA channel something that's useful
> > on its own?
> 
> It is standalone only. The relationship you are referring to is known as slave DMA engine. 
> HIDMA does not support slave DMA. HIDMA only supports memcpy and memset in HW and only memcpy in SW
> as the memset support has been removed from the kernel long time ago.
> 
Thanks for the explanation.

-Christoffer