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What's up with cpu_is_noncoherent_r10000() ?

To: MIPS Linux List <linux-mips@linux-mips.org>
Subject: What's up with cpu_is_noncoherent_r10000() ?
From: David Daney <ddaney@avtrex.com>
Date: Mon, 25 Aug 2008 09:34:29 -0700
Original-recipient: rfc822;linux-mips@linux-mips.org
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I am bringing up the git HEAD on an old ATI Xilleon X226. This nice system claims to be 4KEc, but for some reason doesn't support mips32r2, but I digress.

Among its other problems this is a CONFIG_DMA_NONCOHERENT system, so drivers like net/e100.c do not function properly if the cache is not appropriately flushed/invalidated when they are doing DMA. Fortunately the authors of said driver have used pci_dma_sync_single_for_{cpu,device} in what seems like the appropriate manner.

pci_dma_sync_single_for_device() ends up in dma_sync_single_for_device() (in mm/dme-default.c) and is does the cache flush as expected. The problem is with dma_sync_single_for_cpu() which for some reason only does the cache flush/invalidate if cpu_is_noncoherent_r10000() returns true (which it does only for R10K CPUs). When I hack it up so that it returns true unconditionally, e100 starts functioning normally for me. This leads me to think that the cache operation should be done for all CONFIG_DMA_NONCOHERENT systems not just R10K based systems.

What is the reasoning for only doing the cache operation on R10K based systems?

David Daney

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