Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753001AbbF3NBB (ORCPT ); Tue, 30 Jun 2015 09:01:01 -0400 Received: from eusmtp01.atmel.com ([212.144.249.243]:34647 "EHLO eusmtp01.atmel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750913AbbF3NAy (ORCPT ); Tue, 30 Jun 2015 09:00:54 -0400 Date: Tue, 30 Jun 2015 15:01:34 +0200 From: Ludovic Desroches To: Cyrille Pitchen CC: , , , , , , Subject: Re: [PATCH v2 1/1] dmaengine: at_xdmac: fix transfer data width in at_xdmac_prep_slave_sg() Message-ID: <20150630130134.GN18384@odux.rfo.atmel.com> Mail-Followup-To: Cyrille Pitchen , nicolas.ferre@atmel.com, dan.j.williams@intel.com, vinod.koul@intel.com, dmaengine@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <32f90f9820ddf6412a637cdf3fda4e284a7854d1.1435667600.git.cyrille.pitchen@atmel.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <32f90f9820ddf6412a637cdf3fda4e284a7854d1.1435667600.git.cyrille.pitchen@atmel.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4018 Lines: 88 On Tue, Jun 30, 2015 at 02:36:57PM +0200, Cyrille Pitchen wrote: > This patch adds the missing update of the transfer data width in > at_xdmac_prep_slave_sg(). > > Indeed, for each item in the scatter-gather list, we check whether the > transfer length is aligned with the data width provided by > dmaengine_slave_config(). If so, we directly use this data width for the > current part of the transfer we are preparing. Otherwise, the data width > is reduced to 8 bits (1 byte). Of course, the actual number of register > accesses must also be updated to match the new data width. > > So one chunk was missing in the original patch (see Fixes tag below): the > number of register accesses was correctly set to (len >> fixed_dwidth) in > mbr_ubc but the real data width was not updated in mbr_cfg. Since mbr_cfg > may change for each part of the scatter-gather transfer this also explains > why the original patch used the Descriptor View 2 instead of the > Descriptor View 1. > > Let's take the example of a DMA transfer to write 8bit data into an Atmel > USART with FIFOs. When FIFOs are enabled in the USART, its Transmit > Holding Register (THR) works in multidata mode, that is to say that up to > 4 8bit data can be written into the THR in a single 32bit access and it is > still possible to write only one data with a 8bit access. To take > advantage of this new feature, the DMA driver was modified to allow > multiple dwidths when doing slave transfers. > For instance, when the total length is 22 bytes, the USART driver splits > the transfer into 2 parts: > > First part: 20 bytes transferred through 5 32bit writes into THR > Second part: 2 bytes transferred though 2 8bit writes into THR > > For the second part, the data width was first set to 4_BYTES by the USART > driver thanks to dmaengine_slave_config() then at_xdmac_prep_slave_sg() > reduces this data width to 1_BYTE because the 2 byte length is not aligned > with the original 4_BYTES data width. Since the data width is modified, > the actual number of writes into THR must be set accordingly. > > Signed-off-by: Cyrille Pitchen Acked-by: Ludovic Desroches Thanks Cyrille > Fixes: 6d3a7d9e3ada ("dmaengine: at_xdmac: allow muliple dwidths when doing slave transfers") > Cc: stable@vger.kernel.org #4.0 and later > --- > ChangeLog > > v2: > - reword commit message: add Fixes and Cc tags > > v1: > > drivers/dma/at_xdmac.c | 7 ++++--- > 1 file changed, 4 insertions(+), 3 deletions(-) > > diff --git a/drivers/dma/at_xdmac.c b/drivers/dma/at_xdmac.c > index cf1213de7865..931b2183e2a7 100644 > --- a/drivers/dma/at_xdmac.c > +++ b/drivers/dma/at_xdmac.c > @@ -681,15 +681,16 @@ at_xdmac_prep_slave_sg(struct dma_chan *chan, struct scatterlist *sgl, > desc->lld.mbr_sa = mem; > desc->lld.mbr_da = atchan->sconfig.dst_addr; > } > - desc->lld.mbr_cfg = atchan->cfg; > - dwidth = at_xdmac_get_dwidth(desc->lld.mbr_cfg); > + dwidth = at_xdmac_get_dwidth(atchan->cfg); > fixed_dwidth = IS_ALIGNED(len, 1 << dwidth) > - ? at_xdmac_get_dwidth(desc->lld.mbr_cfg) > + ? dwidth > : AT_XDMAC_CC_DWIDTH_BYTE; > desc->lld.mbr_ubc = AT_XDMAC_MBR_UBC_NDV2 /* next descriptor view */ > | AT_XDMAC_MBR_UBC_NDEN /* next descriptor dst parameter update */ > | AT_XDMAC_MBR_UBC_NSEN /* next descriptor src parameter update */ > | (len >> fixed_dwidth); /* microblock length */ > + desc->lld.mbr_cfg = (atchan->cfg & ~AT_XDMAC_CC_DWIDTH_MASK) | > + AT_XDMAC_CC_DWIDTH(fixed_dwidth); > dev_dbg(chan2dev(chan), > "%s: lld: mbr_sa=%pad, mbr_da=%pad, mbr_ubc=0x%08x\n", > __func__, &desc->lld.mbr_sa, &desc->lld.mbr_da, desc->lld.mbr_ubc); > -- > 1.8.2.2 > -- 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/