linux-kernel - Re: [PATCH v5 19/19] iommu/dma: Add support for mapping MSIs

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Date:   Wed, 24 Aug 2016 11:06:59 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     joro@...tes.org, will.deacon@....com,
        iommu@...ts.linux-foundation.org, devicetree@...r.kernel.org,
        lorenzo.pieralisi@....com, jean-philippe.brucker@....com,
        punit.agrawal@....com, thunder.leizhen@...wei.com,
        eric.auger@...hat.com, Jason Cooper <jason@...edaemon.net>,
        Marc Zyngier <marc.zyngier@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 19/19] iommu/dma: Add support for mapping MSIs

On 24/08/16 09:16, Thomas Gleixner wrote:
> On Tue, 23 Aug 2016, Robin Murphy wrote:
>> +	cookie = domain->iova_cookie;
>> +	iovad = &cookie->iovad;
>> +
>> +	spin_lock(&cookie->msi_lock);
>> +	list_for_each_entry(msi_page, &cookie->msi_page_list, list)
>> +		if (msi_page->phys_hi == msg->address_hi &&
>> +		    msi_page->phys_lo - msg->address_lo < iovad->granule)
>> +			goto unlock;
>> +
>> +	ret = __iommu_dma_map_msi_page(dev, msg, domain, &msi_page);
>> +unlock:
>> +	spin_unlock(&cookie->msi_lock);
>> +
>> +	if (!ret) {
>> +		msg->address_hi = upper_32_bits(msi_page->iova);
>> +		msg->address_lo &= iova_mask(iovad);
>> +		msg->address_lo += lower_32_bits(msi_page->iova);
>> +	} else {
>> +		/*
>> +		 * We're called from a void callback, so the best we can do is
>> +		 * 'fail' by filling the message with obviously bogus values.
>> +		 * Since we got this far due to an IOMMU being present, it's
>> +		 * not like the existing address would have worked anyway...
>> +		 */
>> +		msg->address_hi = ~0U;
>> +		msg->address_lo = ~0U;
>> +		msg->data = ~0U;
>> +	}
> 
> The above is really horrible to parse. I had to read it five times to
> understand the logic.

Yeah, on reflection it is needlessly hideous. I think we should take
this as a clear lesson that whenever you find yourself thinking "Man, I
wish I had Python's for...else construct here", you're doing it wrong ;)

> static struct iommu_dma_msi_page *
> find_or_map_msi_page(struct iommu_dma_cookie *cookie, struct msi_msg *msg)
> {
> 	struct iova_domain *iovad = &cookie->iovad;
>        	struct iommu_dma_msi_page *page;
> 
> 	list_for_each_entry(*page, &cookie->msi_page_list, list) {
> 		if (page->phys_hi == msg->address_hi &&
> 		    page->phys_lo - msg->address_lo < iovad->granule)
> 		    	return page;
> 	}
> 
> 	/*
> 	 * FIXME: __iommu_dma_map_msi_page() should return a page or NULL.
> 	 * The integer return value is pretty pointless. If seperate error
> 	 * codes are required that's what ERR_PTR() is for ....
> 	 */
> 	ret = __iommu_dma_map_msi_page(dev, msg, domain, &page);
> 	return ret ? ERR_PTR(ret) : page;
> }
> 
> So now the code in iommu_dma_map_msi_msg() becomes:
> 
> 	spin_lock(&cookie->msi_lock);
> 	msi_page = find_or_map_msi_page(cookie, msg);
> 	spin_unlock(&cookie->msi_lock);
> 
> 	if (!IS_ERR_OR_NULL(msi_page)) {
> 		msg->address_hi = upper_32_bits(msi_page->iova);
> 		msg->address_lo &= iova_mask(iovad);
> 		msg->address_lo += lower_32_bits(msi_page->iova);
> 	} else {
> 		/*
> 		 * We're called from a void callback, so the best we can do is
> 		 * 'fail' by filling the message with obviously bogus values.
> 		 * Since we got this far due to an IOMMU being present, it's
> 		 * not like the existing address would have worked anyway...
> 		 */
> 		msg->address_hi = ~0U;
> 		msg->address_lo = ~0U;
> 		msg->data = ~0U;
> 	}
> 
> Hmm? 

OK, I've turned map_msi_page into get_msi_page (returning a page) and
just hoisted the list lookup into that, which leads to knock-on
simplifications throughout and is _much_ nicer. I now can't imagine why
I didn't get that far in the first place - thanks for the reality check!

Robin.

> 
> Thanks,
> 
> 	tglx
>