linux-kernel - Re: [PATCH V13 07/10] mmc: block: blk-mq: Add support for direct completion

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Date:   Thu, 9 Nov 2017 09:27:38 +0200
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Ulf Hansson <ulf.hansson@...aro.org>,
        linux-mmc <linux-mmc@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Bough Chen <haibo.chen@....com>,
        Alex Lemberg <alex.lemberg@...disk.com>,
        Mateusz Nowak <mateusz.nowak@...el.com>,
        Yuliy Izrailov <Yuliy.Izrailov@...disk.com>,
        Jaehoon Chung <jh80.chung@...sung.com>,
        Dong Aisheng <dongas86@...il.com>,
        Das Asutosh <asutoshd@...eaurora.org>,
        Zhangfei Gao <zhangfei.gao@...il.com>,
        Sahitya Tummala <stummala@...eaurora.org>,
        Harjani Ritesh <riteshh@...eaurora.org>,
        Venu Byravarasu <vbyravarasu@...dia.com>,
        Shawn Lin <shawn.lin@...k-chips.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH V13 07/10] mmc: block: blk-mq: Add support for direct
 completion

On 08/11/17 11:28, Linus Walleij wrote:
> On Fri, Nov 3, 2017 at 2:20 PM, Adrian Hunter <adrian.hunter@...el.com> wrote:
> 
>> For blk-mq, add support for completing requests directly in the ->done
>> callback. That means that error handling and urgent background operations
>> must be handled by recovery_work in that case.
>>
>> Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
> 
> I tried enabling this on my MMC host (mmci) but I got weird
> DMA error messages when I did.
> 
> I guess this has not been tested on a non-DMA-coherent
> system?

I don't see what DMA-coherence has to do with anything.

Possibilities:
	- DMA unmapping doesn't work in an atomic context
	- requests' DMA operations have to be synchronized with each other

> I think I might be seeing this because the .pre and .post
> callbacks need to be strictly sequenced, and this is
> maybe not taken into account here?

I looked at mmci but that did not seem to be the case.

> Isn't there as risk
> that the .post callback of the next request is called before
> the .post callback of the previous request has returned
> for example?

Of course, the requests are treated as independent.  If the separate DMA
operations require synchronization, that is for the host driver to fix.