Return-path: Received: from mail-gw2-out.broadcom.com ([216.31.210.63]:8578 "EHLO mail-gw2-out.broadcom.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750893AbaLETWJ (ORCPT ); Fri, 5 Dec 2014 14:22:09 -0500 Message-ID: <548205DD.4080101@broadcom.com> (sfid-20141205_202227_981642_54594DD9) Date: Fri, 5 Dec 2014 20:22:05 +0100 From: Arend van Spriel MIME-Version: 1.0 To: Catalin Marinas CC: Russell King - ARM Linux , Hante Meuleman , linux-wireless , brcm80211-dev-list , "linux-kernel@vger.kernel.org" , Will Deacon , David Miller , "linux-arm-kernel@lists.infradead.org" , Marek Szyprowski Subject: Re: using DMA-API on ARM References: <5481794E.4050406@broadcom.com> <20141205094507.GP11285@n2100.arm.linux.org.uk> <20141205122423.GK1630@arm.com> <20141205132332.GS11285@n2100.arm.linux.org.uk> <20141205150648.GT11285@n2100.arm.linux.org.uk> <20141205182851.GC31222@e104818-lin.cambridge.arm.com> In-Reply-To: <20141205182851.GC31222@e104818-lin.cambridge.arm.com> Content-Type: text/plain; charset="ISO-8859-1"; format=flowed Sender: linux-wireless-owner@vger.kernel.org List-ID: On 12/05/14 19:28, Catalin Marinas wrote: > On Fri, Dec 05, 2014 at 03:06:48PM +0000, Russell King - ARM Linux wrote: >> I've been doing more digging into the current DMA code, and I'm dismayed >> to see that there's new bugs in it... >> >> commit 513510ddba9650fc7da456eefeb0ead7632324f6 >> Author: Laura Abbott >> Date: Thu Oct 9 15:26:40 2014 -0700 >> >> common: dma-mapping: introduce common remapping functions >> >> This uses map_vm_area() to achieve the remapping of pages allocated inside >> dma_alloc_coherent(). dma_alloc_coherent() is documented in a rather >> round-about way in Documentation/DMA-API.txt: >> >> | Part Ia - Using large DMA-coherent buffers >> | ------------------------------------------ >> | >> | void * >> | dma_alloc_coherent(struct device *dev, size_t size, >> | dma_addr_t *dma_handle, gfp_t flag) >> | >> | void >> | dma_free_coherent(struct device *dev, size_t size, void *cpu_addr, >> | dma_addr_t dma_handle) >> | >> | Free a region of consistent memory you previously allocated. dev, >> | size and dma_handle must all be the same as those passed into >> | dma_alloc_coherent(). cpu_addr must be the virtual address returned by >> | the dma_alloc_coherent(). >> | >> | Note that unlike their sibling allocation calls, these routines >> | may only be called with IRQs enabled. >> >> Note that very last paragraph. What this says is that it is explicitly >> permitted to call dma_alloc_coherent() with IRQs disabled. > > This is solved by using a pre-allocated, pre-mapped atomic_pool which > avoids any further mapping. __dma_alloc() calls __alloc_from_pool() when > !__GFP_WAIT. So we are actually calling dma_alloc_coherent() with GFP_KERNEL during device probe. That last paragraph Russell pointed out seems to suggest this is not allowed. > This code got pretty complex and we may find bugs. It can be simplified > by a pre-allocated non-cacheable region that is safe in atomic context > (how big you allocate this is hard to say). > >> If the problem which you (Broadcom) are suffering from is down to the >> issue I suspect (that being having mappings with different cache >> attributes) then I'm not sure that there's anything we can realistically >> do about that. There's a number of issues which make it hard to see a >> way forward. > > I'm still puzzled by this problem, so I don't have any suggestion yet. I > wouldn't blame the mismatched attributes yet as I haven't seen such > problem in practice (but you never know). > > How does the DT describe this device? Could it have some dma-coherent > property in there that causes dma_alloc_coherent() to create a cacheable > memory? Ok. Will add it to our todo list: check DTS files for dma-coherent property. Thanks, Arend > The reverse could also cause problems: the device is coherent but the > CPU creates a non-cacheable mapping. >