Re: [PATCH v2 0/7] of: setup dma parameters using dma-ranges and dma-coherent

[Thomas Petazzoni]

Dear Santosh Shilimkar,

On Sat, 19 Apr 2014 10:32:45 -0400, Santosh Shilimkar wrote:
> Here is an updated version of [2] based on discussion. Series introduces
> support for setting up dma parameters based on device tree properties
> like 'dma-ranges' and 'dma-coherent' and also update to ARM 32 bit port.
> Earlier version of the same series is here [1].
> 
> The 'dma-ranges' helps to take care of few DMAable system memory restrictions
> by use of dma_pfn_offset which we maintain now per device. Arch code then
> uses it for dma address translations for such cases. We update the
> dma_pfn_offset accordingly during DT the device creation process.The
> 'dma-coherent' property is used to setup arch's coherent dma_ops.
> 
> After some off-list discussion with RMK and Arnd, I have now dropped the
> controversial dma_mask setup code from the series which actually isn't blocking
> me as such. Considering rest of the parts of the series are already aligned,
> am hoping to get this version merged for 3.16 merge window.
> 
> We agreed in last discussion that drivers have the ultimate
> responsibility to setup the correct dma mask but then we need to have some
> means to see if bus can support what driver has requested for a case where
> driver request for bigger mask than what bus supports. I can follow up on
> the mask topic if we have broken drivers.

I am not sure whether there is an intersection or not, but I wanted to
mention that the mvebu platform (in mach-mvebu) supports hardware I/O
coherency, which makes it a coherent DMA platform. However, we are not
able to use arm_coherent_dma_ops for this platform, because when a
transfer is being made DMA_FROM_DEVICE, at the end of the transfer, we
need to perform an I/O barrier to wait for the snooping unit to
complete its coherency work. So we're coherent, but not with
arm_coherent_dma_ops: we have our own dma operation implementation (see
arch/arm/mach-mvebu/coherency.c).

However, it seems that your patch series, at least in PATCH 6/7 makes
the assumption that for all DMA coherent platforms,
arm_coherent_dma_ops is going to be OK.

Also, I haven't followed all the discussions, but what is the intended
usage of of_dma_is_coherent() and set_arch_dma_coherent_ops() (device
drivers? platform code?).

In mach-mvebu, what we do is that we register a bus notifier on the
platform bus, so that we can set our custom DMA operations for all
platform devices in the system. Should this be done in a different way
after your series?

Thanks,

Thomas
-- 
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
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