From mboxrd@z Thu Jan 1 00:00:00 1970 From: Joel A Fernandes Subject: [PATCH 2/2] DMA: EDMA: Add comments for A-sync case calculations Date: Tue, 25 Jun 2013 20:32:04 -0500 Message-ID: <1372210324-7181-1-git-send-email-joelagnel@ti.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: Errors-To: davinci-linux-open-source-bounces-VycZQUHpC/PFrsHnngEfi1aTQe2KTcn/@public.gmane.org To: Tony Lindgren , Sekhar Nori , Matt Porter , Grant Likely , Rob Herring , Vinod Koul , Mark Brown , Benoit Cousson , Russell King , Balaji TK , Chris Ball Cc: Linux DaVinci Kernel List , Arnd Bergmann , Linux Documentation List , Devicetree Discuss , Linux MMC List , Linux Kernel Mailing List , Linux SPI Devel List , Linux OMAP List , Linux ARM Kernel List List-Id: devicetree@vger.kernel.org From: Joel A Fernandes A-sync case in EDMA driver is tricky and not so obvious. Document the reasons for the calculations and the scenarious they are used. Signed-off-by: Joel A Fernandes --- drivers/dma/edma.c | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+) diff --git a/drivers/dma/edma.c b/drivers/dma/edma.c index e008ed2..a1d9f3785 100644 --- a/drivers/dma/edma.c +++ b/drivers/dma/edma.c @@ -284,8 +284,24 @@ static struct dma_async_tx_descriptor *edma_prep_slave_sg( */ if (burst == 1) { edesc->absync = false; + /* + * For the A-sync case, bcnt and ccnt are the remainder + * and quotient respectively of the division of: + * (sg_dma_len(sg) / acnt) by (SZ_64K -1). This is so + * that in case bcnt over flows, we have ccnt to use. + * Note: In A-sync tranfer only, bcntrld is used, but it + * only applies for sg_dma_len(sg) >= SZ_64K. + * In this case, the best way adopted is- bccnt for the + * first frame will be the remainder below. Then for + * every successive frame, bcnt will be SZ_64K-1. This + * is assured as bcntrld = 0xffff in end of function. + */ ccnt = sg_dma_len(sg) / acnt / (SZ_64K - 1); bcnt = sg_dma_len(sg) / acnt - ccnt * (SZ_64K - 1); + /* + * If bcnt is non-zero, we have a remainder and hence an + * extra frame to transfer, so increment ccnt. + */ if (bcnt) ccnt++; else @@ -343,6 +359,12 @@ static struct dma_async_tx_descriptor *edma_prep_slave_sg( edesc->pset[i].a_b_cnt = bcnt << 16 | acnt; edesc->pset[i].ccnt = ccnt; + /* + * Only time when (bcntrld) auto reload is required is for + * A-sync case, and in this case, a requirement of reload value + * of SZ_64K-1 only is assured. 'link' is initially set to NULL + * and then later will be populated by edma_execute. + */ edesc->pset[i].link_bcntrld = 0xffffffff; } -- 1.7.9.5