Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Thu, 20 Aug 2020 08:52:21 +0200
From:   Sascha Hauer <s.hauer@pengutronix.de>
To:     Lars-Peter Clausen <lars@metafoo.de>
Cc:     Benjamin Bara - SKIDATA <Benjamin.Bara@skidata.com>,
        Robin Gong <yibin.gong@nxp.com>,
        "alsa-devel@alsa-project.org" <alsa-devel@alsa-project.org>,
        "timur@kernel.org" <timur@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "nicoleotsuka@gmail.com" <nicoleotsuka@gmail.com>,
        "vkoul@kernel.org" <vkoul@kernel.org>,
        dl-linux-imx <linux-imx@nxp.com>,
        "kernel@pengutronix.de" <kernel@pengutronix.de>,
        "dmaengine@vger.kernel.org" <dmaengine@vger.kernel.org>,
        "dan.j.williams@intel.com" <dan.j.williams@intel.com>,
        "shawnguo@kernel.org" <shawnguo@kernel.org>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        Richard Leitner - SKIDATA <Richard.Leitner@skidata.com>
Subject: Re: pcm|dmaengine|imx-sdma race condition on i.MX6
Message-ID: <20200820065221.GF19745@pengutronix.de>
References: <20200813112258.GA327172@pcleri>
 <VE1PR04MB6638EE5BDBE2C65FF50B7DB889400@VE1PR04MB6638.eurprd04.prod.outlook.com>
 <61498763c60e488a825e8dd270732b62@skidata.com>
 <16942794-1e03-6da0-b8e5-c82332a217a5@metafoo.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <16942794-1e03-6da0-b8e5-c82332a217a5@metafoo.de>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Aug 19, 2020 at 01:08:29PM +0200, Lars-Peter Clausen wrote:
> > For the first option, which is potentially more performant, we have to leave the atomic PCM context
> > and we are not sure if we are allowed to.
> > For the second option, we would have to divide the dma_device terminate_all into an atomic sync and
> > an async one, which would align with the dmaengine API, giving it the option to ensure termination
> > in an atomic context.
> > Based on my understanding, most of them are synchronous anyways, for the currently async ones we
> > would have to implement busy waits.
> > However, with this approach, we reach the WARN_ON [6] inside of an atomic context,
> > indicating we might not do the right thing.
> 
> I don't know how feasible this is to implement in the SDMA dmaengine driver.
> But I think what is should do is to have some flag to indicate if a
> terminate is in progress. If a new transfer is issued while terminate is in
> progress the transfer should go on a list. Once terminate finishes it should
> check the list and start the transfer if there are any on the list.

The list is already there in form of the vchan helpers the driver uses.

I think the big mistake the driver makes is to configure fields in
struct sdma_channel and also the hardware directly in
sdma_prep_memcpy(), sdma_prep_slave_sg() and sdma_prep_dma_cyclic(). All
information should be stored in the struct sdma_desc allocated in the
prep functions and only be used when it's time to fire that specific
descriptor.

More specifically sdma_config_write() may not be called from
sdma_prep_slave_sg() or sdma_prep_dma_cyclic(), but instead must be
called from sdma_start_desc().  sdma_config_ownership() also must be
called later in sdma_start_desc(). 'direction' must be a member of
struct sdma_desc, not of struct sdma_channel.

Overall this sounds like a fair amount of work to do, but should be
feasible and IMO is a step in the right direction.

Sascha

-- 
Pengutronix e.K.                           |                             |
Steuerwalder Str. 21                       | http://www.pengutronix.de/  |
31137 Hildesheim, Germany                  | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |