Return-path: Received: from mail-gw3-out.broadcom.com ([216.31.210.64]:35687 "EHLO mail-gw3-out.broadcom.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752073AbaLEJW2 (ORCPT ); Fri, 5 Dec 2014 04:22:28 -0500 Message-ID: <5481794E.4050406@broadcom.com> (sfid-20141205_102249_829447_030AFBAD) Date: Fri, 5 Dec 2014 10:22:22 +0100 From: Arend van Spriel MIME-Version: 1.0 To: Russell King CC: "linux-arm-kernel@lists.infradead.org" , David Miller , "linux-kernel@vger.kernel.org" , "brcm80211-dev-list@broadcom.com" , linux-wireless Subject: using DMA-API on ARM Content-Type: text/plain; charset="utf-8" Sender: linux-wireless-owner@vger.kernel.org List-ID: Hi Russell, For our brcm80211 development we are working on getting brcmfmac driver up and running on a Broadcom ARM-based platform. The wireless device is a PCIe device, which is hooked up to the system behind a PCIe host bridge, and we transfer information between host and device using a descriptor ring buffer allocated using dma_alloc_coherent(). We mostly tested on x86 and seen no issue. However, on this ARM platform (single-core A9) we detect occasionally that the descriptor content is invalid. When this occurs we do a dma_sync_single_for_cpu() and this is retried a number of times if the problem persists. Actually, found out that someone made a mistake by using virt_to_dma(va) to get the dma_handle parameter. So probably we only provided a delay in the retry loop. After fixing that a single call to dma_sync_single_for_cpu() is sufficient. The DMA-API-HOWTO clearly states that: """ the hardware should guarantee that the device and the CPU can access the data in parallel and will see updates made by each other without any explicit software flushing. """ So it seems incorrect that we would need to do a dma_sync for this memory. That we do need it seems like this memory can end up in cache(?), or whatever happens, in some rare condition. Is there anyway to investigate this situation either through DMA-API or some low-level ARM specific functions. Regards, Arend