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Date:   Tue, 1 Sep 2020 19:38:10 +0200
From:   Thomas Bogendoerfer <tsbogend@alpha.franken.de>
To:     Christoph Hellwig <hch@lst.de>
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Subject: Re: [PATCH 22/28] sgiseeq: convert from dma_cache_sync to
 dma_sync_single_for_device
Message-ID: <20200901173810.GA25282@alpha.franken.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>
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On Tue, Sep 01, 2020 at 07:16:27PM +0200, Christoph Hellwig wrote:
> Well, if IP22 doesn't speculate (which I'm pretty sure is the case),
> dma_sync_single_for_cpu should indeeed be a no-op.  But then there
> also shouldn't be anything in the cache, as the previous
> dma_sync_single_for_device should have invalidated it.  So it seems like
> we are missing one (or more) ownership transfers to the device.  I'll
> try to look at the the ownership management in a little more detail
> tomorrow.

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. 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 ]