Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756365AbbDOSxZ (ORCPT ); Wed, 15 Apr 2015 14:53:25 -0400 Received: from smtp.domeneshop.no ([194.63.252.55]:43361 "EHLO smtp.domeneshop.no" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752287AbbDOSxR (ORCPT ); Wed, 15 Apr 2015 14:53:17 -0400 Message-ID: <552EB393.4050609@tronnes.org> Date: Wed, 15 Apr 2015 20:53:07 +0200 From: =?UTF-8?B?Tm9yYWxmIFRyw7hubmVz?= User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0 MIME-Version: 1.0 To: Martin Sperl CC: dmaengine@vger.kernel.org, vinod.koul@intel.com, linux-kernel@vger.kernel.org, jonathan@raspberrypi.org, linux-rpi-kernel@lists.infradead.org, dan.j.williams@intel.com Subject: Re: [PATCH] dmaengine: bcm2835: Add slave dma support References: <1429091778-26350-1-git-send-email-noralf@tronnes.org> <75BEAE20-81A9-4567-8AB2-1C1865A0B72D@sperl.org> In-Reply-To: <75BEAE20-81A9-4567-8AB2-1C1865A0B72D@sperl.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1738 Lines: 40 Den 15.04.2015 16:37, skrev Martin Sperl: >> On 15.04.2015, at 11:56, Noralf Trønnes wrote: >> +#define MAX_LITE_TRANSFER (SZ_64K - 1) >> +#define MAX_NORMAL_TRANSFER SZ_1G > ... >> + if (c->ch >= 8) /* LITE channel */ >> + max_size = MAX_LITE_TRANSFER; >> + else >> + max_size = MAX_NORMAL_TRANSFER; >> + period_len = min(period_len, max_size); >> + d->frames = (buf_len - 1) / (period_len + 1); > I wonder if it is wise to split the transfers on 65535 bytes for the > Lite DMA-channels - especially if you are transferring to word size > registers (like SPI_FIFO), you still push 16384 words into the register > and the last word of this transfer (word 16384) still is assumed 4 valid > bytes by the device and thus gets operated upon - even if the last byte > contains garbage from the DMA-transfer point of view. > > So maybe it is better to separate on SZ_64K-4 or better still SZ_32K to > be on a power of 2 address boundary. The datasheet is contradictory: BCM2835 ARM Peripherals - 4.5 DMA LITE Engines 3. The DMA length register is now 16 bits, limiting the maximum transferrable length to 65536 bytes. A 16-bit register can't hold a value of 65536. Either the max value is 65535 or the register is 17-bits wide. There is currently no driver that we can use to test >32k buffers. Unless someone disagrees, I will change this back to the SZ_32K value from the original driver (and add a comment to explain that 32k is chosen to stay on the safe side). -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/