From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stanimir Varbanov Subject: Re: [PATCH v2] spi: qup: Add DMA capabilities Date: Tue, 24 Feb 2015 18:08:54 +0200 Message-ID: <54ECA216.306@linaro.org> References: <1424782803-13103-1-git-send-email-stanimir.varbanov@linaro.org> <20150224135622.GH6236@finisterre.sirena.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-arm-msm-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-spi-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Rob Herring , Mark Rutland , Kumar Gala , Andy Gross , Sagar Dharia , Daniel Sneddon To: Mark Brown Return-path: In-Reply-To: <20150224135622.GH6236-bheZrs9scGb3/WHNxyQH9YN0K6Il/+VY@public.gmane.org> Sender: linux-spi-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: On 02/24/2015 03:56 PM, Mark Brown wrote: > On Tue, Feb 24, 2015 at 03:00:03PM +0200, Stanimir Varbanov wrote: > >> +static void spi_qup_dma_done(void *data) >> +{ >> + struct spi_qup *qup = data; >> + >> + if (atomic_dec_and_test(&qup->dma_outstanding)) >> + complete(&qup->done); >> +} > > I'm finding it hard to be thrilled about the use of atomics for > synchronization (they're just generally hard to work with) and... > >> + cookie = dmaengine_submit(desc); >> + ret = dma_submit_error(cookie); >> + if (ret) >> + return ret; > >> + atomic_inc(&qup->dma_outstanding); > > ..don't we have two potential races here: one if somehow the DMA manages > to complete prior to the atomic_inc() (unlikely but that's what race > conditions are all about really) and one if we are issuing multiple DMAs > and the early ones complete before the later ones are issued? > yes, there is a potential race between atomic_inc and dma callback. I reordered these calls to save few checks, and now it returns to me. I imagine few options here: - reorder the dmaengine calls and atomic operations, i.e. call atomic_inc for rx and tx channels before corresponding dmaengine_submit and dmaengine_issue_pending. - have two different dma callbacks and two completions and waiting for the two. - manage to receive only one dma callback, i.e. the last transfer in case of presence of the rx_buf and tx_buf at the same time. - let me see for better solution. Thanks for the comments. regards, Stan -- To unsubscribe from this list: send the line "unsubscribe linux-spi" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html