From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Message-ID: <52DFB1BC.7080000@huawei.com> Date: Wed, 22 Jan 2014 19:55:40 +0800 From: Wang Nan MIME-Version: 1.0 Subject: Re: [PATCH 1/3] ARM: Premit ioremap() to map reserved pages References: <1390389916-8711-1-git-send-email-wangnan0@huawei.com> <1390389916-8711-2-git-send-email-wangnan0@huawei.com> <20140122114215.GZ15937@n2100.arm.linux.org.uk> In-Reply-To: <20140122114215.GZ15937@n2100.arm.linux.org.uk> List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "kexec" Errors-To: kexec-bounces+dwmw2=twosheds.infradead.org@lists.infradead.org To: Russell King - ARM Linux Cc: kexec@lists.infradead.org, linux-kernel@vger.kernel.org, Geng Hui , linux-mm@kvack.org, Eric Biederman , Andrew Morton , linux-arm-kernel@lists.infradead.org On 2014/1/22 19:42, Russell King - ARM Linux wrote: > On Wed, Jan 22, 2014 at 07:25:14PM +0800, Wang Nan wrote: >> This patch relaxes the restriction set by commit 309caa9cc, which >> prohibit ioremap() on all kernel managed pages. >> >> Other architectures, such as x86 and (some specific platforms of) powerpc, >> allow such mapping. >> >> ioremap() pages is an efficient way to avoid arm's mysterious cache control. >> This feature will be used for arm kexec support to ensure copied data goes into >> RAM even without cache flushing, because we found that flush_cache_xxx can't >> reliably flush code to memory. > > Yes, let's bypass the check and allow this in violation of the > architecture specification by allowing mapping the same memory with > different types, which leads to unpredictable behaviour. Yes, that's > a very good idea, because what we want to do is far more important than > following the requirements of the architecture. > > So... NAK. > > Yes, flush_cache_xxx() doesn't flush back to physical RAM, that's not > what it's defined to do - it's defined that it flushes enough of the > cache to ensure that page table updates are safe (such as when tearing > down a page mapping.) So it's hardly surprising that doesn't work. > > If you want to be able to have DMA access to memory, then you need to > use an API which has been designed for that purpose, and if there isn't > one, then you need to discuss your requirements, rather than trying to > hack around the problem. So what is correct API which is designed for this propose? > > The issue here will be that the APIs we currently have for DMA become > extremely expensive when you want to deal with (eg) all system RAM. > Or, there's flush_cache_all() which should flush all levels of cache > in the system, and thus push all data back to RAM. > > Now, why are you copying your patches to the stable people? That makes > no sense - they haven't been reviewed and they haven't been integrated > into an existing kernel. So, they don't meet the basic requirements > for stable tree submission... > _______________________________________________ kexec mailing list kexec@lists.infradead.org http://lists.infradead.org/mailman/listinfo/kexec