linux-kernel - Re: ARM: OMPA4+: is it expected dma_coerce_mask_and_coherent(dev, DMA_BIT

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Message-ID: <2886917.pqK9QloHOD@wuerfel>
Date:	Mon, 09 Mar 2015 22:33:45 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	"Grygorii.Strashko@...aro.org" <grygorii.strashko@...aro.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux@....linux.org.uk,
	Tejun Heo <tj@...nel.org>, Tony Lindgren <tony@...mide.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	linux-arm <linux-arm-kernel@...ts.infradead.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	Laura Abbott <lauraa@...eaurora.org>,
	open list <linux-kernel@...r.kernel.org>,
	Santosh Shilimkar <ssantosh@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Peter Ujfalusi <peter.ujfalusi@...com>
Subject: Re: ARM: OMPA4+: is it expected dma_coerce_mask_and_coherent(dev, DMA_BIT_MASK(64)); to fail?

On Thursday 05 March 2015 20:55:07 Grygorii.Strashko@...aro.org wrote:
> Hi All,
> 
> Now I can see very interesting behavior related to dma_coerce_mask_and_coherent()
> and friends which I'd like to explain and clarify.
> 
> Below is set of questions I have (why - I explained below):
> - Is expected dma_coerce_mask_and_coherent(DMA_BIT_MASK(64)) and friends to fail on 32 bits HW?

No. dma_coerce_mask_and_coherent() is meant to ignore the actual mask. It's
usually considered a bug to use this function for that reason.

> - What is expected value for max_pfn: max_phys_pfn or max_phys_pfn + 1?
> 
> - What is expected value for struct memblock_region->size: mem_range_size or mem_range_size - 1?
> 
> - What is expected value to be returned by memblock_end_of_DRAM():
>   @base + @size(max_phys_addr + 1) or @base + @size - 1(max_phys_addr)?
> 
> 
> I'm working with BeaglBoard-X15 (AM572x/DRA7xx) board and have following code in OMAP ASOC driver
> which is failed SOMETIMES during the boot with error -EIO.
> === to omap-pcm.c:
> omap_pcm_new() {
> ...
> 	ret = dma_coerce_mask_and_coherent(card->dev, DMA_BIT_MASK(64));
> ^^ failed sometimes
> 	if (ret)
> 		return ret;
> }

The code should be fixed to use dma_set_mask_and_coherent(), which is expected to
fail if the bus is incapable of addressing all RAM within the mask.

> I'd be very appreciated for any comments/clarification on questions I've listed at the
> beginning of my e-mail - there are no patches from my side as I'd like to understand 
> expected behavior of the kernel first (especially taking into account that any
> memblock changes might affect on at least half of arches). 

Is the device you have actually 64-bit capable?

Is the bus it is connected to 64-bit wide?

Does the dma-ranges property of the parent bus reflect the correct address width?

	Arnd
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