Re: [PATCH] dmaengine: vdma: Add 64 bit addressing support to the driver

[Arnd Bergmann]

On Sunday 23 August 2015 19:09:33 Vinod Koul wrote:
> On Fri, Aug 21, 2015 at 02:01:59AM +0300, Laurent Pinchart wrote:
> > > > How is 64 bit DMA addressing implemented ? Can you use a 64-bit VDMA on
> > > > a 32- bit platform with LPAE ? Can you use a 32-bit VDMA on a 64-bit
> > > > platform ? Given that VDMA is an IP core you can instantiate in the
> > > > programmable logic I expect some level of flexibility to be possible, but
> > > > this patch doesn't seem to support it. Please provide more context to
> > > > allow a proper review (and please include it in the commit message of v2).
> > > 
> > > The VDMA core is a soft ip, which can be programmed to support both 32 bit
> > > and 64 bit addressing.When the VDMA core is configured for 32 bit address
> > > space , transfer start address is specified by a single register.
> > > 
> > > When the  VDMA core is configured for an address space greater than 32, each
> > > start address is specified by a combination of two registers.The first
> > > register specifies the LSB 32 bits of address, while the next register
> > > specifies the MSB 32 bits of address.For example,5Ch will specify the LSB
> > > bits while 60h will specify the MSB bits of the first start address. So we
> > > need to program two registers at a time.
> > > 
> > > Yes,64 bit vdma can be used on 32 bit platform and 32 bit vdma can also be
> > > used on 64 bit platform.As far as i know , there is no use case where 64
> > > bit dma can be used on 32 bit platform.Please correct me if i am wrong.
> > 
> > I'm not sure what the use cases would be, but it makes me feel uncomfortable 
> > to decide on whether the VDMA is 32 or 64 bits based on the type of CPU.
> > 
> > As the VDMA flavour is selected at synthesis time, how about specifying it in 
> > DT instead ? You could just add an address-width property.
> That would be saner thing to do. People wont check which IP and will mix and
> match. So you may have a 64 bit system with your 32 bit IP...

Note that you need two things here: an identification of whether the IP block
itself is configured as 64-bit or 32-bit (either using the compatible string,
or a separate property), and a dma-ranges property of the parent bus that
describes what the bus can do and how the address range of the device maps to
the address range of the parent bus.

The DT probe code will set the dma offset according to the dma-ranges, and
will prevent the device from setting a mask that does not get translated
properly, e.g. if you have a 64-bit capable device on a 32-bit bus.

	Arnd
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