linux-kernel - Re: [PATCH 1/3] ARM/dma-mapping: move various helpers from dma-mapping.h to dma-direct.h

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Message-ID: <20200911062512.GC21597@lst.de>
Date:   Fri, 11 Sep 2020 08:25:12 +0200
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@....de>, iommu@...ts.linux-foundation.org,
        Russell King <linux@...linux.org.uk>,
        Santosh Shilimkar <ssantosh@...nel.org>,
        devicetree@...r.kernel.org,
        Florian Fainelli <f.fainelli@...il.com>,
        linux-sh@...r.kernel.org, Frank Rowand <frowand.list@...il.com>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-acpi@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Jim Quinlan <james.quinlan@...adcom.com>,
        linux-pci@...r.kernel.org,
        Nathan Chancellor <natechancellor@...il.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/3] ARM/dma-mapping: move various helpers from
 dma-mapping.h to dma-direct.h

On Thu, Sep 10, 2020 at 07:02:23PM +0100, Robin Murphy wrote:
> On 2020-09-10 06:40, Christoph Hellwig wrote:
>> Move the helpers to translate to and from direct mapping DMA addresses
>> to dma-direct.h.  This not only is the most logical place, but the new
>> placement also avoids dependency loops with pending commits.
>
> For the straightforward move as it should be,
>
> Reviewed-by: Robin Murphy <robin.murphy@....com>
>
> However I do wonder how much of this could be cleaned up further...
>> +
>> +#ifdef __arch_page_to_dma
>> +#error Please update to __arch_pfn_to_dma
>> +#endif
>
> This must be long, long dead by now.

Yeah.  I had a patch to remove this which lead me into the rabbit
hole your described later.  A few patches in I decided to give up
and just do the trivial move.  But it probably makes sense to pick
up at least the two trivial dead code removal patches..

>> +static inline unsigned long dma_to_pfn(struct device *dev, dma_addr_t addr)
>> +{
>> +	unsigned long pfn = __bus_to_pfn(addr);
>> +
>> +	if (dev)
>> +		pfn += dev->dma_pfn_offset;
>> +
>> +	return pfn;
>> +}
>
> These are only overridden for OMAP1510, and it looks like it wouldn't take 
> much for the platform code or ohci-omap driver to set up a generic DMA 
> offset for the relevant device.

I sent a ping to the omap maintainers earlier this week to ask for that :)

>> +static inline dma_addr_t virt_to_dma(struct device *dev, void *addr)
>> +{
>> +	if (dev)
>> +		return pfn_to_dma(dev, virt_to_pfn(addr));
>> +
>> +	return (dma_addr_t)__virt_to_bus((unsigned long)(addr));
>> +}
>
> And this is only used for some debug prints in dmabounce.
>
> Similarly the __bus_to_*()/__*_to_bus() calls themselves only appear 
> significant to mach-footbridge any more, and could probably also be evolved 
> into regular DMA offsets now that all API calls must have a non-NULL 
> device. I think I might come back and take a closer look at all this at 
> some point in future... :)

Yes,  pretty much all of this should eventually go away.  I just don't
want to bock the ranges work on all kinds of random arm cleanups..