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Date:   Thu, 3 Sep 2020 10:43:02 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc:     Christoph Hellwig <hch@lst.de>, alsa-devel@alsa-project.org,
        linux-ia64@vger.kernel.org, linux-doc@vger.kernel.org,
        nouveau@lists.freedesktop.org, linux-nvme@lists.infradead.org,
        linux-kernel@vger.kernel.org,
        "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>,
        linux-mm@kvack.org, Marek Szyprowski <m.szyprowski@samsung.com>,
        linux-samsung-soc@vger.kernel.org,
        Joonyoung Shim <jy0922.shim@samsung.com>,
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        Kyungmin Park <kyungmin.park@samsung.com>,
        Ben Skeggs <bskeggs@redhat.com>,
        Matt Porter <mporter@kernel.crashing.org>,
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        Tom Lendacky <thomas.lendacky@amd.com>,
        Pawel Osciak <pawel@osciak.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
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        netdev@vger.kernel.org, Seung-Woo Kim <sw0312.kim@samsung.com>,
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Subject: Re: [PATCH 22/28] sgiseeq: convert from dma_cache_sync to
 dma_sync_single_for_device
Message-ID: <20200903084302.GB24410@lst.de>
References: <20200819065555.1802761-1-hch@lst.de> <20200819065555.1802761-23-hch@lst.de> <20200901152209.GA14288@alpha.franken.de> <20200901171241.GA20685@alpha.franken.de> <20200901171627.GA8255@lst.de> <20200901173810.GA25282@alpha.franken.de>
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On Tue, Sep 01, 2020 at 07:38:10PM +0200, Thomas Bogendoerfer wrote:
> this is the problem:
> 
>        /* Always check for received packets. */
>         sgiseeq_rx(dev, sp, hregs, sregs);
> 
> so the driver will look at the rx descriptor on every interrupt, so
> we cache the rx descriptor on the first interrupt and if there was
> $no rx packet, we will only see it, if cache line gets flushed for
> some other reason.

That means a transfer back to device ownership is missing after a
(negative) check.

> kick_tx() does a busy loop checking tx descriptors,
> with just sync_desc_cpu...
> 
> Thomas.
> 
> -- 
> Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
> good idea.                                                [ RFC1925, 2.3 ]
---end quoted text---