linux-kernel - Re: [Linaro-mm-sig] [PATCH 2/4] staging: android: ion: Restrict cache maintenance to dma mapped memory

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Date:   Wed, 6 Feb 2019 23:31:04 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Ørjan Eide <Orjan.Eide@....com>
Cc:     Brian Starkey <Brian.Starkey@....com>,
        Liam Mark <lmark@...eaurora.org>,
        "devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
        "tkjos@...roid.com" <tkjos@...roid.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
        "arve@...roid.com" <arve@...roid.com>,
        "joel@...lfernandes.org" <joel@...lfernandes.org>, nd <nd@....com>,
        "maco@...roid.com" <maco@...roid.com>,
        "christian@...uner.io" <christian@...uner.io>
Subject: Re: [Linaro-mm-sig] [PATCH 2/4] staging: android: ion: Restrict
 cache maintenance to dma mapped memory

The CPU may only access DMA mapped memory if ownership has been
transferred back to the CPU using dma_sync_{single,sg}_to_cpu, and then
before the device can access it again ownership needs to be transferred
back to the device using dma_sync_{single,sg}_to_device.

> I've run some testing, and this patch does indeed fix the crash in
> dma_sync_sg_for_cpu when it tried to use the 0 dma_address from the sg
> list.
> 
> Tested-by: Ørjan Eide <orjan.eide@....com>
> 
> I tested this on an older kernel, v4.14, since the dma-mapping code
> moved, in v4.19, to ignore the dma_address and instead use sg_phys() to
> get a valid address from the page, which is always valid in the ion sg
> lists. While this wouldn't crash on newer kernels, it's still good to
> avoid the unnecessary work when no CMO is needed.

Can you also test is with CONFIG_DMA_API_DEBUG enabled, as that should
catch all the usual mistakes in DMA API usage, including the one found?