From mboxrd@z Thu Jan 1 00:00:00 1970 From: msalter@redhat.com (Mark Salter) Date: Thu, 01 Sep 2011 08:36:34 -0400 Subject: [PATCH 1/3] add dma_coherent_write_sync to DMA API In-Reply-To: References: <1314826214-22428-1-git-send-email-msalter@redhat.com> <1314826214-22428-2-git-send-email-msalter@redhat.com> Message-ID: <1314880595.1439.13.camel@deneb.redhat.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Thu, 2011-09-01 at 11:57 +0200, Micha? Miros?aw wrote: > BTW, if there's no time limit on write buffers flushing, or if write > buffers can cause reordering of the writes, then the memory accesses > need to be managed just like non-DMA-coherent memory. So what differs > then in DMA-coherent vs non-DMA-coherent mappings then? My understanding is that ordering is preserved, but an ARM guy should probably verify that. IIUC, the write buffers could hold data indefinitely. As a practical matter other writes needing to go out to memory will force buffered data out eventually. Again, this is my understanding which may be faulty. My feeling is that this extended write buffering makes it hard to call the dma memory fully coherent, but other limitations on ARMv7 make the buffering hard to avoid. --Mark