From mboxrd@z Thu Jan 1 00:00:00 1970 From: alexander.tarasikov@gmail.com (Alexander Tarasikov) Date: Tue, 17 Sep 2013 03:21:39 +0400 Subject: RFD on DMA_BIT_MASK and LPAE In-Reply-To: <20130916230440.GP12758@n2100.arm.linux.org.uk> References: <20130916230440.GP12758@n2100.arm.linux.org.uk> Message-ID: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Tue, Sep 17, 2013 at 3:04 AM, Russell King - ARM Linux wrote: > On Tue, Sep 17, 2013 at 02:52:28AM +0400, Alexander Tarasikov wrote: >> Hello, linux hackers! >> >> I've tried enabling LPAE (for the purpose of using KVM on A15) and have >> found out that many drivers break because of the DMA_BIT_MASK. I've found >> some patches and a discussion on the mailing list, but have not found what >> the final verdict on the problem was. > > The problem is that Linux insists that system memory will start at physical > address zero. > > That is not true on a lot of ARM platforms, especially with LPAE. This > assumption needs to be fixed. > > I have a large patch series which I posted a few months back - now that > -rc1 is out, I will be re-posting it and if it gets mostly ignored again, > I'll drop it into linux-next for more exposure and as a candidiate for > merging at the next window. Hello, Russell! Thanks for the explanation. > > Even with LPAE, even with memory all above the 4GB mark, if you have > 32-bit DMA controllers a 32-bit DMA mask should _still_ work. Does that mean that the of/platform file (which parses the DTS) will need no patching or that it should set the DMA mask based on what the ARM core supports? Either way, I hope in some time the vanilla linux will be usable across all those new A15 boards, and you guys have done a fantastic job with migrating to DTS and moving the code out of arch/arm. In the meantime, I am happy to use the vendor-supplied tree with custom patches because my proof-of-concept KVM port on OMAP5 is working fine. -- Regards, Alexander