From mboxrd@z Thu Jan 1 00:00:00 1970 From: Antoine Tenart Subject: Re: [PATCH net 1/3] net: mvpp2: fix the dma_mask and coherent_dma_mask settings for PPv2.2 Date: Thu, 21 Sep 2017 16:24:13 +0200 Message-ID: <20170921142413.GH3568@kwain> References: <20170918130408.23114-1-antoine.tenart@free-electrons.com> <20170918130408.23114-2-antoine.tenart@free-electrons.com> <20170918.171858.667084185425779267.davem@davemloft.net> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8BIT Cc: antoine.tenart@free-electrons.com, andrew@lunn.ch, gregory.clement@free-electrons.com, thomas.petazzoni@free-electrons.com, miquel.raynal@free-electrons.com, nadavh@marvell.com, linux@armlinux.org.uk, linux-kernel@vger.kernel.org, mw@semihalf.com, stefanc@marvell.com, netdev@vger.kernel.org To: David Miller Return-path: Content-Disposition: inline In-Reply-To: <20170918.171858.667084185425779267.davem@davemloft.net> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org Hi David, On Mon, Sep 18, 2017 at 05:18:58PM -0700, David Miller wrote: > From: Antoine Tenart > Date: Mon, 18 Sep 2017 15:04:06 +0200 > > > The dev->dma_mask usually points to dev->coherent_dma_mask. This is an > > issue as setting both of them will override the other. This is > > problematic here as the PPv2 driver uses a 32-bit-mask for coherent > > accesses (txq, rxq, bm) and a 40-bit mask for all other accesses due to > > an hardware limitation. > > > > This can lead to a memory remap for all dma_map_single() calls when > > dealing with memory above 4GB. > > > > Fixes: 2067e0a13cfe ("net: mvpp2: set dma mask and coherent dma mask on PPv2.2") > > Reported-by: Stefan Chulski > > Signed-off-by: Antoine Tenart > > I surrmise that if the platform has made dev->dma_mask point to > &dev->coherent_dma_mask, it is because it does not allow the two > settings to be set separately. That's also the default when the platform does not allocate dma_mask. You have a point, that could be because it's not supported. But I don't know what would be a good check then. > By rearranging the pointer, you are bypassing that, and probably > breaking things or creating a situation that the DMA mapping > layer is not expecting. > > I want to know more about the situations where dma_mask is set to > point to &coherent_dma_mask and how that is supposed to work. >>From what I see in other parts of the kernel, dma_mask points to &coherent_dma_mask by default and having two different values for these two masks isn't a use case for many drivers. Antoine -- Antoine Ténart, Free Electrons Embedded Linux and Kernel engineering http://free-electrons.com