linux-kernel - Re: allow larger than require DMA masks

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Message-ID: <1569362368.5364.19.camel@HansenPartnership.com>
Date:   Tue, 24 Sep 2019 17:59:28 -0400
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Sven Schnelle <svens@...ckframe.org>, Helge Deller <deller@....de>,
        linux-parisc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: allow larger than require DMA masks

On Tue, 2019-09-24 at 23:25 +0200, Christoph Hellwig wrote:
> On Mon, Sep 23, 2019 at 08:59:42PM -0400, James Bottomley wrote:
> > > 	if (mask > ~0U)
> > > »     »       return 0;
> > > 
> > > Removing the if() makes the DMA mapping work. It's almost
> > > midnight here, so i won't look into that any further today. Does
> > > anyone have an opinion on this behaviour? Otherwise i will look a
> > > bit more into this in the next days.
> > 
> > The reason for the if was to kick the device into 32 bit
> > descriptors, which are usually more efficient, especially with
> > older dual descriptor format cards like we have on parisc systems.
> 
> These days we use the dma_get_required_mask API to query for that.

It looks like sym53c8xx didn't get the memo.  On the other hand, I'm
fairly certain it should be compiled in addressing mode zero on all
parisc systems (since all the 64 bit ones have iommus), so I think we
can take care of this ourselves.

> Svens patch looks right for how we are now using the DMA mask setting
> API.

Agreed.

James